Virulence and antimicrobial resistance mechanisms in Pseudomonas aeruginosa in dogs and cats: a review

1 Background

Pseudomonas aeruginosa is a Gram-negative, oxidase- and catalase-positive γ-proteobacterium with a rod-shaped morphology and belongs to the family Pseudomonadaceae (Abdullahi et al., 2025). It is primarily an aerobic bacterium but can also grow under anaerobic conditions via nitrate respiration (Targońska-Karasek et al., 2025). Members of the genus Pseudomonas are ubiquitous due to remarkable genetic plasticity and metabolic versatility, enabling utilisation of a wide range of organic and inorganic compounds (Moradali et al., 2017; Moore et al., 2006). P. aeruginosa is widely distributed in natural and anthropogenic environments, particularly in soil, water systems, plant rhizospheres, and animal hosts (Weller, 2007; Pye, 2018; Schauer et al., 2021).

In companion animals, P. aeruginosa acts as an opportunistic pathogen that can occasionally colonise skin and mucosal surfaces without causing disease but may act as an opportunistic pathogen when host defenses are compromised (Paterson and Matyskiewicz, 2018; Pye, 2018). Clinical manifestations include otitis externa, the most commonly reported condition in dogs, as well as infections of the skin and soft tissues, urinary tract, eyes, and respiratory tract (Dégi et al., 2021; Badis et al., 2025; Newstead et al., 2025). Chronic and recalcitrant infections are common due to biofilm formation and intrinsic or acquired antimicrobial resistance (Flemming and Wingender, 2010; Płókarz et al., 2022).

The pathogen’s virulence is mediated by pili, flagella, lipopolysaccharides, exoenzymes, and toxins delivered via multiple secretion systems (types I, II, III, IV, and VI), while quorum sensing coordinates biofilm maturation, toxin production, and motility (Jurado-Martín et al., 2021; Ridha et al., 2023; Shrout et al., 2006; Yan and Wu, 2019).

Antimicrobial resistance, disinfectant tolerance, and biofilm-mediated persistence pose significant diagnostic and therapeutic challenges in veterinary medicine (Dégi et al., 2021; Shepherd et al., 2018; Lysitsas et al., 2025). Despite growing knowledge, important gaps remain in understanding host-specific adaptations, the distribution and clinical impact of high-risk clones, and the development of preventive and alternative therapeutic strategies.

This review provides a comprehensive overview of P. aeruginosa in companion animals, summarising prevalence, virulence traits, and resistance patterns, while highlighting knowledge gaps that are critical for improving clinical management and contributing to One Health approach.

2 Taxonomy, structure, and morphology

Pseudomonas is an oxidase- and catalase-positive, lactose-non-fermenting, Gram-negative bacterium in the class Gammaproteobacteria and the family Pseudomonadaceae (Abdullahi et al., 2025). It possesses polar flagella and type IV pili, which contribute to motility, surface attachment, and biofilm formation (Burrows, 2012), as well as lipopolysaccharides in the outer membrane that provide intrinsic resistance to antimicrobials and modulate host immune responses (Huszczynski et al., 2019).

Pseudomonas is an obligate aerobe that can also grow anaerobically when nitrate is available (Targońska-Karasek et al., 2025) as an alternative electron acceptor, enabling its survival in diverse environments, including clinical, environmental, and host-associated niches.

It produces a range of pigments, including pyocyanin, fluorescein, pyorubin and pyomelanin (Abdelaziz et al., 2023). These pigments contribute to virulence and ecological fitness by facilitating iron acquisition, inducing oxidative stress in host tissues, protecting against environmental stressors, and enhancing microbial competition.

There are currently more than 100 species in the genus Pseudomonas (Peix et al., 2018). Within the genus, the most predominant species are Pseudomonas aeruginosa, P. syringae, P. putida, and P. fluorescens.

Members of this genus are ubiquitous in a wide variety of environments, which is attributed to their genetic plasticity and metabolic versatility (Moradali et al., 2017), enabling the utilisation of a vast array of organic and inorganic compounds, and a broad ability to adapt successfully under diverse and challenging environmental conditions (Moore et al., 2006). It can be found in agricultural soils and is well adapted to grow in the rhizosphere (Weller, 2007) and a wide range of aquatic environments, such as drinking water (Vaz-Moreira et al., 2012), wastewaters (Vélez et al., 2021), spring, stream and lake water (Duman et al., 2021). It can also be found in plants (Xin et al., 2018) companion animals (Pye, 2018), and farm animals (Schauer et al., 2021).

P. aeruginosa, an opportunistic pathogen, can asymptomatically colonise skin and mucosal surfaces, however, under favourable conditions, cause severe infections; its intrinsic resistance and remarkable capacity to acquire additional antimicrobial resistance mechanisms underscore its significant clinical relevance in both human and veterinary medicine.

3 Relevance in companion animals

3.1 Clinical syndromes associated with Pseudomonas aeruginosa

P. aeruginosa commonly causes a variety of acute and chronic infections (Hayashi et al., 2021) in humans and animals (Li et al., 2021; Płókarz et al., 2022) worldwide (Figure 1), and its clinical prevalence varies between species and disease conditions. In companion animals, P. aeruginosa is associated with a wide range of clinical syndromes, primarily affecting dogs, with a prevalence ranging from approximately 2.0 to 40.9%. In cats, prevalence is generally lower, ranging from 0.8% to 25.0%, with higher rates observed in chronic or recurrent infections. These variations in reported prevalence estimates likely reflects heterogeneity among studies, including differences in animal populations, health status, sample size, breed distribution and diagnostic approaches (Dégi et al., 2021). Table 1 summarises current studies in companion animals in which Pseudomonas species were isolated, along with reported resistance patterns and identified resistance determinants, when available.

Table 1. Main findings of studies investigating Pseudomonas spp. in clinical and non-clinical companion animal samples.

AK, Amikacin; AZT, Aztreonam; CAZ, Ceftazidime; CIP, Ciprofloxacin; CN, Gentamicin; Colistin, Colistin (Polymyxin E); CVN, Cefovecin; ENR, Enrofloxacin; F, Nitrofurantoin; FEP, Cefepime; FOS, Fosfomycin; IMP, Imipenem; LEV, Levofloxacin; MAR, Marbofloxacin; MEM, Meropenem; MDR, Multidrug-resistant; N, Neomycin; ND, Not Defined; OFX, Ofloxacin; PIP/PIT, Piperacillin–Tazobactam; PMB, Polymyxin B; TIC, Ticarcillin; TOB, Tobramycin; TZP, Piperacillin–Tazobactam; VA, Vancomycin.

While P. aeruginosa is the most frequently isolated Pseudomonas species from companion animals, other species, such as P. fluorescens have occasionally been reported in dogs and cats (Li et al., 2021). Futhermore, Pseudomonas luteola and Pseudomonas aestus, have also been associated with cases of pyogranulomatous panniculitis and chronic rhinitis in cats, respectively (Milliron et al., 2021; Abreu et al., 2024), highlighting the diversity of Pseudomonas spp. affecting companion animals.

Infections are typically opportunistic, occurring in the presence of underlying conditions, disruption of host barriers, immunosuppression, or prior antimicrobial exposure (Pereira et al., 2025; Paterson and Matyskiewicz, 2018). The pathogen is particularly notable for its role in chronic, recurrent, and treatment-refractory infections, largely due to its capacity for biofilm formation and multidrug resistance (Dégi et al., 2021; Płókarz et al., 2022).

A variety of predisposing factors can increase the risk of clinical disease, including underlying health conditions, environmental stressors, and anatomical characteristics that promote bacterial growth (Badis et al., 2025) and subsequent infections. The most clinically relevant manifestations include OE, UTI, ocular infections, and skin and soft tissue infections.

3.1.1 Otitis externa

OE represents one of the most frequent and clinically significant presentations associated with P. aeruginosa in dogs and represents a major therapeutic challenge in veterinary practice (Pye, 2018). Canine OE is a common condition, affecting approximately 20 - 29% of dogs presented to veterinary practices in some populations (Hill et al., 2006) (O’Neill et al., 2021). While OE is typically initiated by primary factors such as allergic dermatitis, endocrine disorders, or ectoparasites, P. aeruginosa frequently emerges as a secondary pathogen, particularly in chronic or recurrent cases (Paterson and Matyskiewicz, 2018). Anatomical features, including excessive hair in the ear canal and pendulous pinnae, may increase humidity and reduce ventilation, creating conditions favourable for microbial proliferation (Secker et al., 2023).

Primary factors often initiate an inflammatory response, that creates a favourable environment for bacterial colonisation. These include allergic skin diseases, endocrinopathies, ectoparasites infestations (fleas, mites, and lice), keratinisation disorders, and immune-mediated conditions, all of which promote inflammation and/or infection of the skin and ear canal (Dubin et al., 2024). In addition, local factors, such as foreign bodies, may directly affect the ear canal and act as a predisposing factor for otitis externa (O’Neill et al., 2021). The most frequently reported primary causes include environmental or food-related allergies, endocrine disorders, keratinisation abnormalities, foreign bodies and autoimmune diseases (Paterson and Matyskiewicz, 2018; Tanveer et al., 2024). In chronic or recurrent cases of canine otitis externa, P. aeruginosa is the predominant bacterial pathogen, accounting for approximately 28–37% of infections (Secker et al., 2025; Newstead et al., 2025). The aetiology of OE is often multifactorial and can be classified according to primary, secondary, predisposing, and perpetuating factors (O’Neill et al., 2021). In this context, P. aeruginosa frequently emerges as a dominant pathogen, often displacing commensal microbiota and contributing to disease chronicity. Its ability to adhere to the epithelium of the ear canal, produce proteases and toxins, and form robust biofilms promotes persistence and reduces susceptibility to both host immune responses and antimicrobial therapy (Secker et al., 2023) (Figure 2). Biofilm formation, in particular, is a key factor underlying treatment failure, as it significantly reduces antibiotic penetration and enhances bacterial tolerance (Seres-Steinbach et al., 2025; Scott et al., 2019).

Clinically, infections are characterised by purulent exudate, malodour, pain, and inflammation (Secker et al., 2023), often accompanied by epithelial hyperplasia and canal stenosis in advanced cases (Bajwa, 2019).

The high prevalence of MDR strains (Table 1) further complicates treatment, frequently necessitating culture and susceptibility testing and, in some cases, the use of topical or systemic antimicrobials with limited therapeutic options (Elfadadny et al., 2023; Pye, 2018).

3.1.2 Urinary tract infections

Although less commonly reported than otitis externa, P. aeruginosa is an important opportunistic pathogen in urinary tract infections (UTI) in dogs and cats (Scott et al., 2019), particularly in complicated or recurrent cases (Aurich et al., 2022; Chan et al., 2022). Reported prevalences ranges from 3.3% - 13.1% in dogs and from 1.3% - 25% in cats. These infections are commonly associated with predisposing factors such as urinary catheterisation, urolithiasis, anatomical abnormalities, and prior antimicrobial therapy, which facilitate bacterial colonisation and persistence (Badis et al., 2025). In cats, underlying chronic conditions, including chronic kidney disease, hyperthyroidism, and diabetes mellitus, further increase susceptibility to infection (Weese et al., 2019).

UTI caused by P. aeruginosa are often persistent and prone to recurrence, largely due to the organism’s intrinsic resistance mechanisms and its capacity to form biofilms on uroepithelial surfaces and indwelling medical devices (Cole et al., 2014). Within these biofilms, bacterial cells are embedded in a protective matrix that enhances survival by limiting antimicrobial penetration and reducing susceptibility to host immune responses, thereby acting as a reservoir for ongoing or recurrent infection (Tegegne et al., 2025).

Clinically, affected animals may present with dysuria, pollakiuria, hematuria, or may be asymptomatic in cases of subclinical bacteriuria (Bartges, 2004). The presence of MDR strains is common and poses significant therapeutic challenges, often requiring tailored antimicrobial therapy guided by susceptibility testing (Pereira et al., 2025; Weese et al., 2019). In some cases, treatment failure may occur despite appropriate therapy, emphasising the importance of addressing underlying predisposing factors.

3.1.3 Ocular infections

P. aeruginosa is a well-known cause of severe ocular infections in companion animals, particularly ulcerative keratitis in dogs (Casemiro et al., 2025), accounting for approximately 15.5-19.5% of cases (Goss et al., 2024; Hewitt et al., 2020). These infections are typically acute, rapidly progressive, and potentially vision-threatening (Ollivier, 2003). Although P. aeruginosa is the predominant species, other organisms, such as P. fluorescens, have also been isolated (Hewitt et al., 2020; Goss et al., 2024). Ulcerative keratitis is associated with multiple predisposing factors, including systemic diseases, eyelid abnormalities, ciliary dysfunction, reduced tear production or tear film quality, corneal trauma, and foreign bodies (Gelatt et al., 2021). Additional risk factors include breed, age, and body weight (Maggs et al., 2018). Brachycephalic breeds have a higher incidence and severity of the condition (Casemiro et al., 2025), likely due to anatomical and functional alterations of periocular and ocular structures (Ledbetter et al., 2009; Packer et al., 2015).

The pathogenicity of P. aeruginosa in the cornea is largely driven by its production of proteases, exotoxins, and other virulence factors that contribute to rapid tissue destruction (Marquart and O’Callaghan, 2013). Strains expressing cytotoxic factors, have been associated with more aggressive disease and extensive corneal damage (Ramirez et al., 2012). Loss of the corneal epithelium, the physical barrier to microbial invasion, results in an increased susceptibility to infection from opportunistic bacteria. This can lead to a rapid loss of corneal integrity due to bacterial toxins and enzymes that mediate host cell death and degradation of stromal collagen (Marquart and O’Callaghan, 2013; Ollivier et al., 2007). Additionally, bacterial invasion may induce and activate latent corneal proteases, including matrix metalloproteinases (Matsumoto et al., 1993) As a consequence of proteinase and proteinase inhibitor imbalance, dramatic liquefaction of the cornea can occur, known as keratomalacia (Ollivier et al., 2007).

Clinically, affected animals may present with corneal opacity, ulceration, purulent discharge, blepharospasm, and ocular pain. The rapid progression of infection requires prompt diagnosis and aggressive therapy (Casemiro et al., 2025), often involving intensive topical antimicrobial therapy administered at frequent intervals to control bacterial proliferation and prevent corneal perforation (Maggs et al., 2018; Gelatt et al., 2021). However, treatment can be complicated by antimicrobial resistance and the biofilms, which may contribute to delayed healing or corneal perforation in severe cases (Goss et al., 2024).

3.1.4 Other infections

P. aeruginosa can also cause skin and soft tissue infections as well as respiratory tract infections, including rhinosinusitis in companion animals (Dégi et al., 2021; Badis et al., 2025; Płókarz et al., 2022; Targońska-Karasek et al., 2025).

Skin and soft tissue infections include wound infections, pyoderma, and post-surgical infections (Nadăş et al., 2025; Nocera et al., 2021; Scott et al., 2019). These infections typically occur following disruption of the skin barrier, such as trauma, burns, or underlying dermatological conditions that predispose to bacterial colonisation. In clinical studies, P. aeruginosa has been reported as a secondary opportunistic pathogen in association with conditions such as demodicosis, pemphigus foliaceus, and acral lick dermatitis (Hillier et al., 2006).

The clinical presentation may resemble that of superficial and deep staphylococcal pyoderma, with often malodorous lesions including papules and haemorrhagic bullae often surrounded by moderate to severe erythema (Hillier et al., 2006; Tanveer et al., 2024). As lesions progress, rupture may lead to ulceration, draining tracts, and haemorrhagic crust formation, while coalescing lesions can resemble extensive cellulitis. These lesions are typically painful on palpation and may require sedation or general anaesthesia to permit thorough clinical examination (Hillier et al., 2006).

The ability of P. aeruginosa to survive in moist environments and colonise damaged tissues facilitates infection, particularly in chronic or poorly managed wounds (Di Lodovico et al., 2024; Serra et al., 2015). In veterinary patients, the organism has been isolated from both acute and chronic wound infections, where it contributes to polymicrobial communities and establishes biofilms within the wound environment (Padhy et al., 2014). These biofilms play a key role in persistence, reducing antimicrobial efficacy and contributing to chronic infection and impaired healing (Metcalf and Bowler, 2013; Swanson et al., 2014). Also, MDR strains are frequently encountered, limiting therapeutic options and necessitating culture-based treatment strategies.

In addition to P. aeruginosa, other Pseudomonas species have also been implicated in skin infections in companion animals. A case report described P. luteola associated with pyogranulomatous panniculitis in a cat, presenting as a raised, ulcerated abdominal mass. Histopathological examination revealed supurative and ulcerative dermatitis accompanied by panniculitis (Milliron et al., 2021).

Although less commonly reported, P. aeruginosa has been implicated in respiratory infections in companion animals, including rhinitis and pneumonia (Badis et al., 2025; Płókarz et al., 2022; Jangsangthong et al., 2024).

Other Pseudomonas species have also been identified in respiratory disease. A case report study (Abreu et al., 2024) identified a MDR Pseudomonas aestus from the upper airways of a cat with chronic rhinitis. In addition, sporadic infections involving the reproductive tract, oral cavity, and other anatomical sites have been described (Table 1).

In cats and dogs, bacterial respiratory infections are often opportunistic and occur secondary to underlying conditions that compromise mucosal defences or immune function, facilitating colonisation by organisms (Abreu et al., 2024) and contributing to persistence and treatment difficulty.

4 Diagnostic considerations in companion-animal Pseudomonas spp. infections

The effective management of P. aeruginosa infections in companion animals relies on a structured diagnostic approach aimed not only at identifying the pathogen, but also at understanding the clinical context of infection, assessing lesion severity, and guiding targeted therapy (EFSA, 2022; Pereira et al., 2025). Because P. aeruginosa is frequently associated with chronic, recurrent, or treatment-refractory disease, the diagnosis should extend beyond simple organism detection and includes evaluation of the underlying predisposing factors, extent of tissue involvement, and antimicrobial resistance (Elfadadny et al., 2023; Secker et al., 2023).

4.1 Clinical examination and assessment of predisposing factors

Diagnosis begins with a thorough history and comprehensive clinical examination. Particular attention should be directed toward identifying underlying conditions that may predispose animals to opportunistic infection, including allergic skin disease, endocrinopathies, chronic systemic illness, immunosuppression, and previous antimicrobial exposure (Dubin et al., 2024; Paterson and Matyskiewicz, 2018; Secker et al., 2023). Clinical assessment should also identify site-specific factors that facilitate the persistence of infection, such as ear canal stenosis in OE, corneal ulceration in ocular disease (Gelatt et al., 2021), indwelling urinary catheters in UTI (Badis et al., 2025; Weese et al., 2019), and tissue devitalisation in wounds or post-surgical infections (Nadăş et al., 2025; Nocera et al., 2021; Scott et al., 2019). In many cases, P. aeruginosa infection occurs as a secondary complication following disruption of host protective barriers or the development of chronic inflammation, highlighting the importance of identifying and addressing underlying predisposing factors.

4.2 Cytology and direct microscopic evaluation

Cytological examination is often the most rapid and informative first-line diagnostic test, particularly in OE, wound infections, and exudative skin lesions. In these settings, direct microscopic examination can reveal inflammatory cell populations and bacterial morphology, providing immediate guidance for empirical management (Beco et al., 2013). In otitis, the presence of numerous rod-shaped bacteria, often associated with degenerate neutrophils, is highly suggestive of Gram-negative suppurative infection and should raise suspicion for Pseudomonas spp (Shaw, 2016). Similar principles apply to wound exudates and cutaneous lesions, where cytology may support early recognition of severe bacterial infection. Although cytology cannot confirm species identity, it is valuable for identifying cases that warrant culture and susceptibility testing (Loeffler et al., 2025).

4.3 Bacterial culture and antimicrobial susceptibility testing

Bacterial culture with antimicrobial susceptibility testing is particularly important in chronic, recurrent, deep, or treatment-resistant infections (Pereira et al., 2025; Płókarz et al., 2022). Because P. aeruginosa frequently exhibits intrinsic, acquired resistance and multidrug antimicrobial resistance, culture-based diagnosis is essential whenever empirical therapy fails, when systemic antimicrobial treatment is being considered (Pye, 2018), or when lesions occur at sites where treatment options are limited, such as the middle ear, urinary tract (Weese et al., 2019), cornea (Hewitt et al., 2020), or deep wounds (Loeffler et al., 2025). Accurate diagnosis depends on appropriate and aseptic collection of samples from the most representative lesion site. Recommended specimens include deep ear canal exudate or swabs for otitis, cystocentesis urine samples for UTI (Weese et al., 2019), corneal scrapings or conjunctival swabs from wounds (Hewitt et al., 2020), and tissue biopsies or deep exudate samples from wounds (Loeffler et al., 2025). In cases of suspected otitis media, myringotomy may be required to obtain a representative samples from the middle ear cavity (Mohammaddavoodi et al., 2021; Palmeiro et al., 2004).

4.4 Imaging and evaluation of lesion extent

Diagnostic imaging is indicated when there is concern for deep extension of infection, chronic structural damage, or involvement of inaccessible anatomical sites. In otitis, computed tomography is particularly useful for assessment of the tympanic bullae with evaluating the tympanic bullae canal mineralisation, proliferative change, and possible bone lysis, whereas magnetic resonance imaging may be preferable when soft tissue extension or neurological involvement is suspected (Lanz and Wood, 2004). In other syndromes, imaging may also assist in assessing disease severity and identifying underlying complications, such as ultrasonography in complicated UTI (Robotti and Lanfranchi, 2013) or advanced imaging in destructive sinonasal disease (Henderson et al., 2004). The role of imaging is therefore not to identify P. aeruginosa directly, but to characterise the anatomical extent and consequences of infection and to guide clinical decision-making.

4.5 Advanced diagnostics and epidemiological characterisation

In addition to routine culture and antimicrobial susceptibility testing, molecular approaches are increasingly important in the investigation of Pseudomonas spp. infections in companion animals. Techniques such as polymerase chain reaction (PCR), whole-genome sequencing, multilocus sequence typing (MLST), and resistance gene profiling provide valuable information on pathogen epidemiology and antimicrobial resistance. These methods facilitate the identification of high-risk clones, carbapenemase-producing strains (Badis et al., 2025; Lysitsas et al., 2025), virulence-associated determinants (Hayashi et al., 2021), and potential transmission pathways between animals, humans, and the environmental reservoirs. Although advanced molecular diagnosis are not routinely available in most veterinary clinical settings, they play a critical role in antimicrobial resistance surveillance, outbreak investigations, and One Health research initiatives aimed at understanding the emergence and dissemination of multidrug-resistant P. aeruginosa (Newstead et al., 2025).

4.6 A rational and hierarchical diagnostic strategy

Overall, diagnosis of P. aeruginosa infection in companion animals should follow a rational and hierarchical approach in which each diagnostic step answers a specific clinical question (Pye, 2018). Initial examination and cytology provide rapid orientation, culture and susceptibility testing define antimicrobial options, and imaging or molecular methods are reserved for cases requiring deeper anatomical or epidemiological investigation (Loeffler et al., 2025). This approach helps avoid unnecessary empirical escalation of therapy and supports more judicious use of antimicrobials in veterinary medicine (Newstead et al., 2025; Weese et al., 2019).

5 Clinical and therapeutic challenges

5.1 Antimicrobial resistance patterns and mechanisms

P. aeruginosa is a major opportunistic pathogen with substantial clinical and epidemiological impact and has been designated by the World Health Organization (WHO) as a high priority bacterium requiring urgent development of new antimicrobial therapies (World Health Organization, 2024). It is also a member of the ESKAPE group of pathogens, comprising Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp., which are collectively recognised for their ability to evade antimicrobial treatment and their critical importance in global public health (Boucher et al., 2009). This classification highlights pathogens that pose serious public health threats, particularly those exhibiting MDR phenotypes, which severely limit treatment options. The emergence of MDR and extensively drug-resistant (XDR) Pseudomonas spp. strains represents a major challenge in the clinical management of infections in companion animals. In dogs and cats, MDR isolates are increasingly reported across a range of clinical conditions, including otitis and UTI, reflecting both the organism’s intrinsic resistance and its capacity to acquire additional resistance determinants (Elfadadny et al., 2023; Pereira et al., 2025). In some cases, XDR phenotypes have been identified, with isolates exhibiting resistance to nearly all available antimicrobial classes and carrying multiple resistance genes (Lysitsas et al., 2025). These resistance mechanisms significantly limit therapeutic options and are frequently associated with persistent infection, treatment failure, and increased risk of recurrence (Jańczak et al., 2026).

P. aeruginosa demonstrates extensive intrinsic resistance to multiple classes of antimicrobials. It is naturally resistant to β-lactams, including ampicillin, amoxicillin, and their β-lactamase inhibitor combinations (such as clavulanic acid or sulbactam), third-generation cephalosporins (cefotaxime and ceftriaxone), and the carbapenem ertapenem. Additionally, it shows resistance to chloramphenicol, trimethoprim, and tetracycline derivatives, including tigecycline (EUCAST, 2021). These intrinsic resistance mechanisms poses major therapeutic challenges and are primarily associated with low outer membrane permeability combined with the constitutive or inducible expression of multidrug efflux pumps (Meletis and Karastergiou, 2025).

Beyond intrinsic resistance, P. aeruginosa frequently acquires additional resistance through chromosomal mutations and horizontal gene transfer mediated by mobile genetic elements, particularly integrons, which facilitate the dissemination of resistance determinants among strains (Pang et al., 2019).

Several class D β-lactamases of the oxacilinases (OXA) family, including OXA-50, OXA-395, OXA-486, and OXA-488, are part of the P. aeruginosa chromosomal resistome (Bai et al., 2024; Mack et al., 2025) and detected across clinical isolates (Jangsangthong et al., 2024). Although these intrinsic OXA enzymes can contribute to reduced susceptibility to certain β-lactams, they exhibit only weak carbapenem-hydrolysing activity and, on their own, do not confer clinically significant carbapenem- resistance (Streling et al., 2022).

Carbapenem-resistant P. aeruginosa strains (CRPA) have become especially concerning because of its persistence in healthcare environments and its association with difficult-to-treat infections (World Health Organization, 2024). Several studies in companion animals have reported CRPA isolates with resistance rates ranging from 2.1% to 74.1% for meropenem and 6.7% to 77.6% for imipenem (Dégi et al., 2021; Hayashi et al., 2021; Jangsangthong et al., 2024; Płókarz et al., 2023). Carbapenem resistance in P. aeruginosa isolates from companion animals is most frequently associated with the production of metallo-β-lactamases (MBLs), particularly Verona Integron –encoded metallo-β-lactamases (VIM) and New Delhi metallo-β-lactamase (NDM) -type enzymes, with occasional reports of Imipenemase (IMP) -type carbapenemases (Badis et al., 2025; Jangsangthong et al., 2024; Nocera et al., 2025). Among these, the emergence of NDM-producing isolates in dogs and cats is of particular concern due to their association with MDR phenotypes and their relevance in a One Health context. Nonetheless, carbapenem resistance in P. aeruginosa is not solely mediated by carbapenemase production. In many cases, resistance is more commonly associated with non-enzymatic mechanisms, including loss or modification of the OprD porin and overexpression of efflux pumps (Quale et al., 2006), rather than direct carbapenemase activity. The coexistence of these mechanisms contributes to the complexity of resistance phenotypes and further limits therapeutic options in veterinary clinical practice (Lister et al., 2009), leading to treatment failure and contributing to increased healthcare costs, longer disease duration, and reduced quality of life in affected animals.

Furthermore, the circulation of internationally recognised high-risk clones of P. aeruginosa has become an important concern in companion animals. Among these, ST235 is currently the most clearly documented and clinically significant lineage (Elfadadny et al., 2023; Hayashi et al., 2021; Jangsangthong et al., 2024). This clone is widely recognised in human medicine as a successful epidemic lineage with a marked capacity to accumulate resistance determinants, including carbapenemase genes (Hayashi et al., 2021). The detection of ST235 in dogs and cats is therefore particularly important, as it suggests that companion animals may participate in the maintenance or dissemination of lineages that are already established in human healthcare settings.

Although ST235 is the best-established high-risk clone in companion animals, recent genomic studies from companion animals have also reported other internationally recognised high-risk lineages, including ST244 and ST111, indicating that pets can harbour clones commonly associated with human healthcare settings (Badis et al., 2025; Secker et al., 2025). Additional studies have also shown broad sequence-type diversity among canine and feline isolates, indicating that P. aeruginosa populations in companion animals are heterogeneous overall (Figure 1). Even so, the repeated detection of a limited number of globally disseminated lineages is epidemiologically significant because these clones are disproportionately associated with multidrug resistance, carbapenem resistance, persistence, and potential interspecies transmission. Taken together, the available evidence suggests that companion animals are not merely incidental hosts for P. aeruginosa, but may act as reservoirs of clinically important lineages that are relevant to both veterinary medicine and the wider One Health context (Jangsangthong et al., 2024; Nadăş et al., 2025; Secker et al., 2025).

5.2 Disinfectant resistance

Effective cleaning and disinfection are fundamental components of infection control in veterinary and healthcare environments, playing a crucial role in preventing the spread of pathogenic microorganisms (Caveney, 2016). Chemical disinfectants are the most used biocides for this purpose due to their broad and often non-specific mechanisms of action, which target multiple cellular sites and processes, resulting in high antimicrobial efficacy. Unlike antibiotics, biocides exert multitarget effects that are largely independent of the bacterial metabolic state, which enables activity against both actively growing and dormant cells (Jones and Joshi, 2021).

The efficacy of biocides can be influenced by several factors, including intrinsic chemical properties, such as concentration, pH, and formulation, as well as external conditions including temperature, contact time, and the presence of organic material (Verdial et al., 2023). Despite their effectiveness, the overuse or inappropriate application of disinfectants may select for reduced of biocide susceptibility and, in some cases, contribute to cross-resistance or co-selection of antibiotic resistance mechanisms through processes such as efflux pump overexpression and decreased outer membrane permeability, raising concerns for antimicrobial stewardship (Maillard, 2018). These adaptative mechanisms, combined with the ability of P. aeruginosa to form strong biofilms on a wide range of biotic and abiotic surfaces, can hinder effective eradication of the organism from healthcare facilities and equipment. Consequently, inadequate disinfection procedures may fail to eliminate resistant strains, allowing their persistence and dissemination within veterinary and clinical environments (Lysitsas et al., 2025).

In P. aeruginosa, the mexAB-oprM, mexCD-oprJ, and mexEF-oprN operons represent some of the most extensively characterised efflux systems, playing a pivotal role in mediating tolerance and adaptive resistance to a wide range of biocidal compounds (Amsalu et al., 2020; Chuanchuen et al., 2001). The MexAB-OprM system has been associated with decreased susceptibility to multiple antibiotics and certain disinfectants, contributing to potencial cross-resistance (Haenni et al., 2017). Additionally, these efflux pump genes and their regulators (such as mexR, nalC, nalD, and mexS) are highly conserved and can be transmitted horizontally, including via bacteriophages-mediated mechanisms, raising concerns about their spread in both clinical and environmental settings (Wu et al., 2025) Companion animals can act as reservoirs for MDR P. aeruginosa strains carrying these efflux pump determinants, and potentially facilitating their dissemination beyond hospital environments (Lysitsas et al., 2025). Continuous monitoring and targeted interventions are recommended to control the spread of these resistance mechanisms in veterinary and public health contexts (Amsalu et al., 2020).

Chlorhexidine is one of the most widely used antiseptics in both human and veterinary medicine due to its broad-spectrum antimicrobial activity and residual effect on treated surfaces. However, recent studies have raised concerns regarding the adaptive potential of P. aeruginosa to this compound. Prolonged exposure to chlorhexidine can drive the evolution of P. aeruginosa populations with increased tolerance and reduced susceptibility to clinically relevant antibiotics, including β-lactams and aminoglycosides, due mainly to genetic and phenotypic changes affecting efflux systems and membrane permeability, which not only reduce chlorhexidine efficacy but also confer cross-resistance to antibiotics (Tag ElDein et al., 2021).

Complementary findings by Shepherd et al. (2018) demonstrated that P. aeruginosa can also develop enhanced tolerance to chlorhexidine following adaptation to other biocides, such as octenidine, under both laboratory and simulated clinical conditions. These adaptations involved broad-spectrum tolerance mechanisms, indicating that repeated or sublethal exposure to antiseptics may select for multidrug-tolerant phenotypes (Amsalu et al., 2020). Collectively, these findings highlight the importance of rational and controlled use of chlorhexidine in clinical and veterinary environments to prevent the emergence of cross-resistant P. aeruginosa strains.

Overall, the intrinsic and acquired resistance mechanisms of P. aeruginosa, its tolerance to disinfectants and its ability to form robust biofilms present great diagnostic and therapeutic challenges in veterinary and clinical settings. These traits facilitate the persistence and dissemination of multidrug-tolerant strains, emphasising the importance of rational use of antimicrobials and biocides, integrated infection control and continuous surveillance within a One Health approach.

5.3 Virulence factors

P. aeruginosa possesses a diverse array of virulence factors that contribute to its pathogenicity, colonisation, tissue invasion, and persistence within the host ( (Badis et al., 2025; de Sousa et al., 2023). The expression of these virulence determinants has been associated with increased disease severity and progression in companion animals (Hayashi et al., 2021). This extensive virulence repertoire contributes to the clinical epidemiologically importance of P. aeruginosa as an opportunistic pathogen in veterinary medicine (EFSA, 2022; Secker et al., 2023).

The expression and relative importance of P. aeruginosa virulence factors vary according to the stage of infection. During the early phase, adhesion structures such as pili and flagella promote initial colonisation of epithelial surfaces (Curran et al., 2018; de Sousa et al., 2023; Jurado-Martín et al., 2021), particularly when host defences are compromised, as in inflamed ear canals (Secker et al., 2023) or damaged corneal tissue (Ledbetter et al., 2009).

As infection becomes established, extracellular enzymes and toxins, including exotoxin S, exotoxin A, elastases, and phospholipases, contribute to tissue damage, inflammation, and disruption of host barriers, thereby facilitating bacterial invasion, persistence, and progression of disease (de Sousa et al., 2023; Jurado-Martín et al., 2021; Płókarz et al., 2023). The clinical relevance of these factors is particularly evident in diseases such as canine OE and ulcerative keratitis, in which tissue injury can progress rapidly and become severe (Ledbetter et al., 2009; Secker et al., 2023). In chronic infections, biofilm-associated factors, including siderophore production and alginate-mediated matrix development, are thought to contribute to long-term persistence within host tissues and to a reduced therapeutic response (Płókarz et al., 2022; de Sousa et al., 2023).

Overall, the coordinated expression of virulence determinants enables P. aeruginosa to transition from an opportunistic coloniser to a highly pathogenic agent, contributing to both acute and chronic disease processes in companion animals.

5.3.1 Biofilms formation and regulation

Biofilm formation is one of the most important virulence factor of P. aeruginosa and is often responsible for failures of the antibiotic treatment. Antibiotic resistance is higher among biofilm-forming strains compared to strains without this property (Płókarz et al., 2022).

A biofilm is a complex, structured community of microorganisms embedded within a self-produced matrix composed of exopolysaccharides, proteins, and extracellular deoxyribonucleic acid (DNA), and other biomolecules (Flemming and Wingender, 2010). Robinson et al. (2019) demonstrated that Pseudomonas spp. isolates from canine otitis produced biofilms rich in extracellular polymeric substances, which constituted the predominant structural component and accounted for the majority of the total biofilm biomass.

P. aeruginosa is capable of forming biofilms on both abiotic surfaces, including medical devices such as urinary catheters and implants and biotic surfaces (Thi et al., 2020). Biofilms are primarily responsible for persistent and chronic infections (Robinson et al., 2019; Trøstrup et al., 2017), rather than the planktonic form that is more typical of acute infection. This transition is mediated by quorum-sensing mechanisms, which enable the bacteria to coordinate from a motile, planktonic lifestyle to a sessile, biofilm-associated state (Shrout et al., 2006) and support long-term persistence within the host (Yan and Wu, 2019).

Several genes regulate biofilm formation in P. aeruginosa, many of which are associated with virulence factor production. Table 2 provides a comprehensive overview of the main virulence-associated genes in P. aeruginosa, including biofilm-related genes.

Table 2. Virulence genes associated with biofilm formation and secretion system in Pseudomonas aeruginosa.

MDR, multidrug resistance.

The pel and psl gene clusters encode exopolysaccharides essential for biofilm architecture. pel gene is required for biofilm formation on solid surfaces (Colvin et al., 2013), while psl mediates surface adhesion and microcolony development (Overhage et al., 2005). The pslA and pelA genes contribute to the carbohydrate-rich matrix structure, and the ppyR gene, encoding a putative transmembrane protein, positively regulates psl expression, with its inactivation leading to reduced biofilm formation (Attila et al., 2008).

In addition to exopolysaccharide-related genes, flagellar components are important for pathogenicity and biofilm development (Ridha et al., 2023). The fliC gene encodes flagellin type B, the principal structural subunit of the bacterial flagellum and a key determinant of motility. Flagella facilitate initial surface attachment and dispersal, and their highly immunogenic nature contributes to host inflammatory responses (Ridha et al., 2023).

Another significant virulence factor is neuraminidase, an extracellular enzyme that enhances the colonisation by increasing bacterial adhesion through upregulation of potential host receptors and cleavage of terminal sialic acid residues from sialylated gangliosides (Xu et al., 2009).

Płókarz et al. (2022) investigated the prevalence of selected virulence-associated genes (fliC, nan1, pelA, pslA, and ppyR) in P. aeruginosa isolates from dogs and cats. Their study revealed that fliC in canine strains and nan1 in feline strains were significantly linked to biofilm-forming ability (Table 2). The ppyR gene was detected in nearly all isolates, fliC and pslA were present in slightly more than half, and nan1 and pelA occurred in less than half of the examined strains. Importantly, the presence of fliC in dogs and the absence of nan1 in cats were both significantly associated with enhanced biofilm formation, suggesting host-specific genetic influences on biofilm development in companion animals.

The algD gene encodes GDP-mannose dehydrogenase, a key enzyme in alginate biosynthesis, and is therefore closely linked to the production of the alginate-rich extracellular matrix that supports biofilm maturation and persistence (Hulen, 2023). In veterinary medicine, algD studies remain limited (Nocera et al., 2025), its detection is related with bacterial survival rather than with acute tissue damage alone.

Biofilm formation and virulence are closely regulated by quorum sensing systems that coordinate gene expression according to bacterial population density. Among these, the transcriptional regulators LasR and RhlR are particularly important, as they control the expression of multiple factors involved in biofilm maturation, exoenzyme and toxin production, tissue damage, and persistence (Jurado-Martín et al., 2021). Although direct veterinary studies specifically addressing lasR and rhlR in companion-animal isolates remain limited (Badis et al., 2025), quorum sensing is increasingly recognised as a major contributor to chronic, biofilm-associated, and treatment-refractory infections such as canine OE (Secker et al., 2023). Consequently, LasR and RhlR can be regarded as key regulators linking bacterial communication with chronicity, virulence, and reduced treatment responsiveness (Barbosa et al., 2025).

Biofilm formation is a key virulence determinant in P. aeruginosa infections and plays an important role in the development of chronic and recurrent disease in companion animals (Płókarz et al., 2022; Secker et al., 2023). Within biofilms, bacterial cells are embedded in a structured extracellular polymeric matrix that enhances adhesion to host tissues and limits the penetration of antimicrobial agents while also contributing to reduced susceptibility to host immune defenses (Tuon et al., 2022).

Clinically, biofilms are strongly associated with treatment-refractory infections, particularly in conditions such as chronic OE, non-healing wounds, and catheter-associated UTI. In these settings, biofilm-associated bacteria can survive antimicrobial therapy and act as a reservoir for reinfection, leading to repeated clinical episodes (de Sousa et al., 2023; Pye, 2018). This explains, in part, why infections caused by P. aeruginosa are often difficult to eradicate despite appropriate antimicrobial selection.

In addition to contributing to antimicrobial tolerance, biofilms also facilitate horizontal gene transfer and the accumulation of resistance determinants, further complicating treatment (Michaelis and Grohmann, 2023). The interplay between biofilm formation, virulence factor expression, and antimicrobial resistance creates a highly resilient pathogenic system that underpins the persistence of P. aeruginosa in clinical settings (Secker et al., 2023; Tuon et al., 2022). Consequently, effective management of these infections requires not only antimicrobial therapy but also strategies aimed at disrupting biofilms and addressing underlying predisposing factors (Bajwa, 2019), as we discuss in the next section.

5.3.2 Secretion systems

P. aeruginosa employs several secretion systems to deliver toxins and hydrolytic enzymes that facilitate host invasion and immune evasion, with the type III secretion system (T3SS) being predominantly important for pathogenicity (Hayashi et al., 2021). In addition to the type III secretion system (T3SS), P. aeruginosa also uses type I, II, IV, and VI secretion systems to deliver a variety of virulence factors and enzymes that contribute to host invasion, immune evasion, and environmental persistence (Jurado-Martín et al., 2021; Sainz-Mejías et al., 2020).

In companion animals, the relevance of T3SS-associated virulotypes has been documented in canine and feline isolates (Hayashi et al., 2021; Płókarz et al., 2023), while broader mechanistic studies have shown that these toxins and enzymes collectively promote cytotoxicity, tissue destruction, and persistence (Jurado-Martín et al., 2021; Qin et al., 2022). These factors contribute to host cell injury through different but complementary mechanisms. ExoS is primarily associated with the invasive phenotype and interferes with host cell signaling and cytoskeletal organisation, thereby promoting bacterial dissemination and intracellular survival (Hayashi et al., 2021; Jurado-Martín et al., 2021). ExoU, in contrast, is a potent phospholipase linked to the cytotoxic phenotype and is associated with rapid cell lysis, extensive epithelial damage, and more severe disease (Foulkes et al., 2019). Cytotoxic strains expressing ExoU have been associated with more aggressive infections, characterised by extensive epithelial damage and poorer clinical outcomes (Borkar et al., 2014), whereas ExoS-positive strains are more typically linked to an invasive phenotype that favours dissemination and chronic colonisation (Sawa et al., 2014). In companion animals, studies confirm the presence of both virulence profiles (Hayashi et al., 2021; Płókarz et al., 2023), although direct correlations with disease severity and treatment response remain limited.

ExoT disrupts cytoskeletal dynamics and impairs phagocytic cell function, while ExoY alters host cell architecture and vascular permeability (Jurado-Martín et al., 2021; Płókarz et al., 2023).

P. aeruginosa populations are genetically diverse and can be divided into phylogroups based on genome-wide phylogenetic relationships rather than solely by the presence or absence of individual type III secretion system effectors. Nevertheless, the distribution of ExoS and ExoU is strongly associated with the major phylogenetic lineages and has often been used as a practical marker of invasive and cytotoxic backgrounds, respectively (Freschi et al., 2018; Płókarz et al., 2023). In general, phylogroup A is predominantly associated with the ExoS-positive invasive lineage, whereas phylogroup B is strongly associated with the ExoU-positive cytotoxic lineage (Ozer et al., 2019). Phylogroup C represents a distinct and less common genomic lineage and cannot be defined simply as exoS/exoU double positive or double negative. Therefore, although ExoS and ExoU are useful indicators of major virulence backgrounds, definitive phylogroup assignment requires broader genomic analysis (Botelho et al., 2023).

In companion animals, most canine isolates exhibit ExoS-positive, with smaller proportions with ExoU-positive (Badis et al., 2025; Hayashi et al., 2021; Secker et al., 2025), reflecting the evolutionary and pathogenic diversity of the species in veterinary settings. Also, strains carrying ExoU are often associated with multidrug resistance, more severe clinical outcomes, and outbreaks of carbapenem-resistant lineages, including epidemic clones such as ST235 (Hayashi et al., 2021; Badis et al., 2025). Veterinary studies confirm these patterns, with most isolates clustering with PAO1 (ExoS-positive) and fewer aligning with PA14 (ExoU-positive) (Płókarz et al., 2023; Newstead et al., 2025; Secker et al., 2025).

In addition to the T3SS, P. aeruginosa also uses type I, II, IV, and VI secretion systems to export a wide range of virulence factors that contribute to tissue damage, immune modulation, interbacterial competition, and environmental persistence (Hayashi et al., 2021; Jurado-Martín et al., 2021).

The type I secretion system (T1SS) represents one of the simplest secretion pathways, where the cell exports selected proteins directly from the cytoplasm to the extracellular environment (Jurado-Martín et al., 2021). The Apr system is the best-characterised T1SS. It releases AprA alkaline protease and related substrates such as AprX (Filloux, 2011). T1SS, primarily contributes to virulence through extracellular protease activity, which can cause tissue damage, interfere the host’s immune system, and enable the bacteria to persist (Jurado-Martín et al., 2021). Direct studies of T1SS in veterinary medicine remain limited, but the profiling of canine virulence has identified aprA (de Sousa et al., 2023; Badis et al., 2025), which supports the importance of this secretion pathway in infections of companion animals.

The type II secretion system (T2SS) of P. aeruginosa is responsible for releasing several extracellular toxins and enzymes, such as exotoxin A, LasB elastase, LasA protease and phospholipases (Durand et al., 2003). T2SS plays an important role in pathogenesis, since these factors promote tissue damage, immune evasion, and bacterial persistence (Jurado-Martín et al., 2021; Qin et al., 2022). Although there is limited veterinary data addressing the T2SS specifically, these secreted factors have been detected in isolates from companion animals (Płókarz et al., 2023; Badis et al., 2025).

The type IV secretion system (T4SS) in P. aeruginosa is primarily associated with horizontal gene transfer and genomic plasticity. It plays a key role in spreading resistance and virulence determinants, thereby contributing to bacterial adaptation and persistence (Juhas et al., 2015; Jurado-Martín et al., 2021).

The type VI secretion system (T6SS) is a contractile injection apparatus that delivers effector proteins into neighbouring bacterial cells and, eukaryotic host cells. This system is especially important for bacterial competition, niche adaptation, and persistence, although it can also contribute to host interaction and virulence (Jurado-Martín et al., 2021). P. aeruginosa encodes three T6SS clusters, designated H1-, H2-, and H3-T6SS, and the system secretes hallmark structural and effector-associated proteins such as Hcp and VgrG (Hachani et al., 2011; Robinson et al., 2023). Within the clinical context, the T6SS acts as a multifunctional system that supports survival in complex microbial environments while simultaneously enhancing pathogenic fitness. Direct veterinary studies remain scarce, but its relevance is likely important in chronic polymicrobial infections and biofilm-associated disease.

Lastly, other exoenzymes such as phospholipases PlcN and PlcH further enhance membrane disruption, facilitate tissue invasion, and may support persistence within damaged host environments (de Sousa et al., 2023; Qin et al., 2022). Together, these virulence factors create a highly coordinated system of cytotoxicity, tissue injury, and immune evasion that enables P. aeruginosa to establish both acute and chronic infections (Jurado-Martín et al., 2021).

6 Integrated therapeutic strategies

6.1 Pseudomonas aeruginosa in otitis externa

Management of OE caused by P. aeruginosa requires an integrated therapeutic approach that combines effective antimicrobial treatment with control of inflammation, restoration of the ear canal microenvironment, and identification of the underlying predisposing factors. Successful clinical resolution depends not only on reducing the bacterial burden, but also on addressing primary and perpetuating causes such as allergic skin disease, endocrinopathies, foreign bodies, or chronic structural changes within the ear canal. In recurrent or chronic cases, failure to correct these underlying factors is a major contribuitor to treatment failure and disease relapse (Nuttall, 2023; Pye, 2018).

6.1.1 Inflammation control

Control of inflammation is a central component of treatment. Chronic otitis is often accompanied by oedema, pain, hyperplasia of the ceruminous glands and epithelium, and progressive canal narrowing, all of which impair drainage and reduce the penetration of topical medication. For this reason, glucocorticoids (prednisolone), administered topically and in selected cases systemically, are commonly used to reduce swelling, improve patient comfort, and facilitate ear cleaning and drug delivery. This approach is supported by current reviews on canine otitis management (Bajwa, 2019; Nuttall, 2016).

6.1.2 Topical therapy

Topical therapy remains the cornerstone of treatment because it allows higher antimicrobial concentrations at the site of infection compared with those obtained systemically (Secker et al., 2023).

The choice of topical antimicrobial should be guided whenever possible by cytology, culture, and susceptibility testing, especially in recurrent or previously treated cases (KuKanich et al., 2022). Fluoroquinolones, aminoglycosides, and polymyxin B are commonly used against P. aeruginosa, although resistance patterns vary by region and over time (Pye, 2018; Secker et al., 2023) (Table 1). Aminoglycosides require caution when tympanic membrane rupture is suspected because ototoxicity remains a concern, even though the clinical risk may vary depending on the product and treatment context (Nuttall, 2016; Pye, 2018).

Successful topical therapy depends on adequate cleaning and the use of appropriate topical agents. Removal of exudate, cerumen, and surface debris is important because these materials can act as a physical barrier, contribute to microbial persistence, and reduce the activity of some antimicrobials (Palmeiro et al., 2004; Secker et al., 2023).

Preparations containing Tris-EDTA are a useful adjunct in Pseudomonas spp. otitis. By chelating divalent cations that stabilise the Gram-negative outer membrane, EDTA increases membrane permeability and can enhance the activity of several topical antimicrobials (Pye, 2018; Savaliya et al., 2025). Veterinary studies and reviews have shown that Tris-EDTA can enhance the in vitro activity of marbofloxacin and gentamicin against MDR P. aeruginosa, including isolates growing in biofilm-associated conditions (Buckley et al., 2013). A practical approach is to instil the Tris-EDTA solution before the antimicrobial preparation and allow a short contact period before the antibiotic is applied (Burrows et al., 2013).

Silver sulfadiazine has shown encouraging in vitro activity as an alternative topical option, particularly in resistant infections and may be considered in selected refractory cases, particularly when guided by susceptibility data and local clinical experience (Burrows et al., 2013; von Silva-Tarouca et al., 2019).

6.1.3 Systemic antimicrobial therapy

Systemic antimicrobial therapy should generally be reserved for selected situations rather than used routinely for otitis externa alone. Reasonable indications include confirmed otitis media, severe canal obstruction or proliferative changes preventing effective topical drug penetration, extension beyond the external ear canal (e.g., middle or inner ear involvement), or failure of an appropriately administered topical treatment protocol (KuKanich et al., 2022). When systemic therapy is required, culture and susceptibility testing are essential to guide antimicrobial selection. Fluoroquinolones are among the oral agents most considered for P. aeruginosa infections in veterinary dermatology; however, their interpretation has become more complex following updates to CLSI veterinary antimicrobial breakpoints and the introduction of the susceptible dose-dependent (SDD) category (Papich et al., 2023). This revision indicates that some isolates may require higher yet still approved dosing regimens to achieve an adequate probability of therapeutic success, depending on MIC values and pharmacodynamic targets. Ultimately, management of P. aeruginosa otitis should be viewed as restoration of the ear canal environment rather than simple bacterial eradication. Cleaning, anti-inflammatory therapy, optimisation of topical drug penetration, and correction of primary and perpetuating factors all contribute to reducing bacterial persistence and preventing recurrence. In this context, antimicrobials are one component of therapy, rather than the sole determinant of success.

6.1.4 Multidrug-resistant strains and biofilm matrix challenges

The emergence of MDR P. aeruginosa strains represents a major therapeutic challenge in companion animal practice, particularly in chronic canine OE. In these cases, successful management rarely depends on a single antimicrobial agent and instead requires intensification of the multimodal approach, including careful cleaning, optimisation of topical therapy, control of inflammation, and treatment of the underlying cause. Culture and antimicrobial susceptibility testing are essential, particularly when empirical treatment has failed or when resistance to first-line topical agents is suspected (KuKanich et al., 2022; Secker et al., 2023).

In MDR infections, reduction of microbial burden and disruption of the local environment that supports persistence become especially important. Repeated ear cleaning and removal of exudate and debris can improve drug penetration and reduce the protective effect of surface material and biofilm. Tris-EDTA remains a valuable adjunct because it increases outer membrane permeability in Gram-negative bacteria and can enhance the activity of some topical antimicrobials (Pye, 2018; Savaliya et al., 2025). In selected refractory cases, compounded topical preparations such as ticarcillin-clavulanate (Bateman et al., 2012) or amikacin (Klinczar et al., 2022) may be considered, but they should be used cautiously and preferably in specialist settings, taking into account issues of formulation stability, safety, and stewardship. Silver sulfadiazine may also be considered in refractory infections because of its in vitro activity against canine P. aeruginosa otitis isolates (von Silva-Tarouca et al., 2019).

Recognition of the importance of biofilm in chronic P. aeruginosa infections has stimulated interest in adjuvant antibiofilm approaches. N-acetylcysteine has shown antibacterial and antibiofilm activity (Zhao and Liu, 2010) against common canine otitis pathogens, including P. aeruginosa, in vitro, and therefore represents a potentially useful adjunct in chronic and biofilm-associated infections (May et al., 2016; Savaliya et al., 2025). However, NAC is best presented as a promising investigational adjunct rather than an established standard of care due to mixed results in studies in vitro with conventional antimicrobials, with many interactions being indifferent and some antagonistic rather than clearly synergistic (May et al., 2019; Son and Bae, 2021).

7 One Health perspective

From a One Health perspective, the importance of P. aeruginosa, in companion animals extends beyond its role as a difficult-to-treat opportunistic pathogen in individual patients. Increasing molecular evidence shows that dogs and cats can harbour lineages of clear relevance to human medicine, including internationally disseminated high-risk clones and carbapenem-resistant strains: a notable example is ST235, which has been identified in companion-animal isolates in Japan and widely recognised in human medicine as an epidemic clone with a strong capacity to acquire and disseminate resistance determinants, including carbapenemase genes (Hayashi et al., 2021). More recent studies of canine otitis from the United Kingdom have also reported ST111 and ST244, two high-risk clones are well known in human healthcare-associated infections (Secker et al., 2025). In Algeria, a cluster of carbapenem-resistant ST244 cases was reported in dogs and cats, highlighting the capacity of high-risk clones to circulate in veterinary settings (Badis et al., 2025). In Greece, an XDR feline isolate carrying bla_NDM-1 was characterised, demonstrating that companion animals may also harbour carbapenemase-producing strains of major public-health concern (Lysitsas et al., 2025).

These findings, demonstrate P. aeruginosa should be regarded not only as an opportunistic pathogen of individual companion animals, but also as a bacterium circulating across interconnected human, animal, clinical, and environmental niches (Newstead et al., 2025; Toyting-Hiraishi et al., 2025). Its capacity to persist in humid environments, form biofilms, tolerate disinfection pressure, and collect resistance determinants favours long-term survival and dissemination across these settings (Newstead et al., 2025). This concern is further strengthened by the close contact between pets and their owners, repeated exposure to veterinary healthcare environments, and the frequent use of antimicrobials in chronic or recurrent infections (Price et al., 2024). Together, these factors support the view that companion animals may act as reservoirs and transmission nodes within a broader epidemiological network, contributing to the maintenance and circulation of clinically important P. aeruginosa lineages across species and shared environments (Secker et al., 2025). Figure 3 illustrates the interconnectedness of human, animal, and environmental health. The overlap between these domains highlights how different factors influence health outcomes across sectors. These interactions collectively facilitate the development and dissemination of antimicrobial resistance, emphasising the need for an integrated, multidisciplinary approach to health management.

A particularly illustrative example of this interconnectedness was reported in the United States, where two canine cases of carbapenemase-producing P. aeruginosa in a veterinary hospital were linked to a contemporaneous human outbreak strain associated with contaminated over-the-counter artificial tears (Price et al., 2024). This report highlights how veterinary and human healthcare systems can become connected through shared resistant strains, and why surveillance in veterinary medicine cannot be separated from broader infection prevention and control efforts. Taken together, these findings support the view that management of P. aeruginosa in companion animals should be framed within a One Health model that integrates antimicrobial stewardship, molecular surveillance, infection control, and genomic epidemiology across veterinary and human settings (Newstead et al., 2025).

8 Future directions and knowledge gaps

Despite growing characterisation of virulence determinants and antimicrobial resistance in P. aeruginosa from companion animals, several important knowledge gaps remain (Table 3). Current veterinary studies mainly document the presence of virulence genes, resistance profiles, and biofilm-forming capacity (Secker et al., 2023), but there is still limited evidence linking specific genotypes to clinical presentation, disease severity, progression, or treatment outcome (Wood et al., 2023). As a result, the relationship between isolate characteristics, treatment choices, and meaningful clinical endpoints remains poorly defined. For example, studies show broad virulence gene repertoires and strong biofilm formation in canine and feline isolates, yet clear associations between these profiles and particular disease syndromes have not been established (Robinson et al., 2019; Secker et al., 2025). Future research should therefore prioritise prospective cohort studies that integrate microbiological and genomic data with treatment response and outcomes such as clinical cure, recurrence, chronicity, and functional sequelae including hearing or vision loss.

Table 3. Therapeutic research gaps for Pseudomonas aeruginosa.

In addition, little is known about how the canine and feline host immune response interacts with P. aeruginosa during acute and chronic infection, particularly in otitis, skin disease, and urinary tract infection, or how these host-pathogen interactions influence biofilm formation and long-term persistence (Płókarz et al., 2022). Addressing this gap would improve prognostic assessment and support more targeted therapeutic decision-making in companion animals.

Important limitations also remain at the epidemiological and clinical levels. Most resistance data come from in vitro susceptibility surveys, whereas robust clinical trials comparing treatment protocols, routes of administration, and duration of therapy for otitis, pyoderma, keratitis, and urinary tract infection are scarce (Badis et al., 2025; de Sousa et al., 2023). In addition, current One Health evidence is still based largely on case reports, small genomic studies, and localised investigations, making it difficult to define the true role of companion animals in the maintenance and transmission of antimicrobial-resistant P. aeruginosa (Pereira et al., 2025). Overall, future work should integrate genomic surveillance, clinical data, environmental studies, and experimental models to better define pathogenesis, transmission, and treatment outcomes, ultimately improving both animal and human health management.

9 Conclusions

P. aeruginosa is a highly adaptable opportunistic pathogen with notable genetic diversity and multiple virulence mechanisms, enablinhg colonisation of human, animal, and environmental niches. Its pathogenicity is driven by biofilm formation, quorum sensing and secretion systems, such as T3SS. In companion animals, ExoS⁺ isolates are predominant, whereas ExoU⁺ strains, although less common, are associated with increased cytotoxicity, multidrug resistance, and high-risk lineages such as ST235. These shared human- animal traits highlight its One Health relevance. Combined antimicrobial resistance, disinfectant tolerance, and biofilm formation contribute to persistent and difficult to treat infections, underscoring the need for surveillance, antimicrobial stewardship, and improved therapeutic strategies.

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Footnotes

Edited by: George Cosmin Nadăș, University of Agricultural Sciences and Veterinary Medicine of Cluj-Napoca, Romania

Reviewed by: Alex Odoom, University of Ghana, Ghana

Kamal Alsaad, Iraq University College, Iraq