Bridging soil health and human health: Mycobacterium vaccae as a keystone link

1 Introduction

In recent decades, the understanding of the relationship between environmental microorganisms and human health has undergone a significant transformation. Among these microorganisms, Mycobacterium vaccae has emerged as a subject of growing scientific interest due to its potential positive impact on several aspects of human health. This saprophytic, non-pathogenic bacterium, naturally present in soil, represents a proposed link between the health of terrestrial ecosystems and human well-being, illustrating the interconnectedness between components of the biosphere (1, 2). Preclinical research indicates that exposure to M. vaccae can foster regulatory immune responses and, in early studies, favorable behavioral outcomes (3).

M. vaccae belongs to the family Mycobacteriaceae, a diverse group comprising both pathogenic and non-pathogenic species. It is a Gram-positive, aerobic, non-spore-forming bacterium characterized by a lipid-rich cell wall containing mycolic acids, a defining feature of mycobacteria (4). Unlike pathogenic species such as M. tuberculosis and M. leprae, M. vaccae is non-pathogenic and appears to exert predominantly immunoregulatory rather than pro-inflammatory effects (5). Following the 2018 phylogeny-based taxonomic revision of the genus Mycobacterium, M. vaccae was reassigned to the genus Mycolicibacterium and is now formally designated Mycolicibacterium vaccae (6). Nevertheless, because the term M. vaccae remains the standard nomenclature throughout the immunological, neurobiological, and clinical literature, as well as in clinical trial registries and regulatory documents, it is used consistently throughout this review.

M. vaccae is a resident member of soil microbial communities, particularly in organic matter-rich soils of vegetated ecosystems (7). Interest in its potential health benefits emerged from epidemiological observations linking reduced environmental microbial exposure with increased allergic and immune disorders, leading to the formulation of the hygiene hypothesis and its subsequent refinement as the “old friends hypothesis,” which proposes that diminished contact with co-evolved environmental microorganisms such as M. vaccae contributes to immune dysregulation in industrialized societies (8). The hygiene hypothesis and the old friends hypothesis are related but conceptually distinct: the former, in its original formulation, emphasized reduced childhood infections, whereas the latter emphasizes loss of contact with organisms with which humans co-evolved and that shape immune regulation throughout life; the two terms are not used interchangeably in this review (see Supplementary Text S1). Subsequent studies, particularly those by Dorothy Matthews, Susan Jenks, and Christopher Lowry, have provided experimental evidence, primarily from rodent models, supporting the immunomodulatory and neurobehavioral effects of this bacterium (9, 10).

Research on M. vaccae represents an emerging, multidisciplinary field spanning soil microbiology, immunology, neuroscience, and public health. Initial studies in animal models revealed immunomodulatory effects, including the capacity to balance Th1/Th2 immune responses (11). These findings expanded scientific understanding of how environmental microorganisms may influence aspects of human physiology, from immune regulation to mood-related behavior in animal models, although the translational relevance of these preclinical findings to human health remains to be established.

The relevance of M. vaccae to contemporary public health is discussed against a backdrop of increasing rates of chronic inflammatory disease, allergic disorders, and mental health problems in many industrialized societies, alongside a global trend toward urbanized lifestyles and reduced direct contact with natural environments. Whether reduced exposure to soil microorganisms such as M. vaccae specifically contributes causally to this disease burden has not been established and remains an association drawn primarily from correlational evidence.

This narrative, non-systematic review (Supplementary Text S1) synthesizes current knowledge on Mycobacterium vaccae, focusing on its soil ecology, interactions with the human immune system, preclinical and clinical evidence, and implications for public health and environmental management. A narrative rather than a systematic format was adopted because the underlying evidence base spans multiple disciplines; soil microbiology, immunology, neuroscience, and clinical medicine, with heterogeneous study designs, outcome measures, and M. vaccae strains/preparations that are not amenable to pooled quantitative synthesis around a single clinical endpoint; a systematic review or meta-analysis would be more appropriate for a future, narrower question (e.g., efficacy of a specific formulation for a specific indication), and we flag this as a direction for subsequent work rather than a limitation unique to the present synthesis. Relevant literature was identified through PubMed, Web of Science, and Scopus using terms including “Mycobacterium vaccae,” “Mycolicibacterium vaccae,” “old friends hypothesis,” “soil microbiome,” “immunoregulation,” “gut-brain axis,” and “biodiversity hypothesis,” complemented by manual screening of reference lists. Both preclinical and clinical studies were included, with distinctions between these evidence levels highlighted where appropriate.

2 Soil ecology of Mycobacterium vaccae

The best-documented ecological function of M. vaccae is its catabolic versatility toward hydrocarbons and related xenobiotic compounds. Strain JOB-5, the most extensively studied isolate, expresses an inducible propane monooxygenase that allows it to co-metabolize a broad range of recalcitrant pollutants, including trichloroethylene (TCE), other chlorinated aliphatic solvents, 2,4,6-trinitrotoluene (TNT), and the gasoline additive methyl tert-butyl ether (MTBE) (7, 12, 13). This catabolic capacity has made M. vaccae one of the more extensively characterized soil bacteria for potential bioremediation of groundwater and soil contaminated with chlorinated solvents and related industrial pollutants, and represents its most robustly demonstrated contribution to soil biogeochemistry.

3 Pathways and mechanisms of human exposure to Mycobacterium vaccae

Humans interact with M. vaccae through multiple routes, with inhalation being among the most studied mechanisms. During soil-disturbing activities such as gardening, agriculture, or walking through natural environments, microscopic soil particles containing M. vaccae can be aerosolized and inhaled, bringing the bacteria into contact with the respiratory mucosa and its associated immune cells (14). Direct dermal contact represents another pathway, particularly through unprotected soil contact during gardening or outdoor play (15). Ingestion constitutes a third mechanism, occurring both through inadvertent soil ingestion during outdoor activities, particularly relevant in children, and through consumption of vegetables grown in soils containing the bacterium (16).

As illustrated in Figure 1, reported exposure to M. vaccae differs between rural and urban environments, reflecting differences in soil microbial composition. In rural contexts, particularly traditional agricultural areas, the presence of M. vaccae in soil is often more abundant and diverse, facilitating more regular exposure (15). In contrast, urban environments show reduced abundance and diversity of M. vaccae in available soils, attributable to factors including extensive paving, chemical soil contamination, and altered urban soil management (17). Comparative studies have documented more sporadic and limited patterns of exposure among urban residents, concentrated mainly in parks and green spaces, with considerable variability depending on urban design and the availability of natural areas (18). It should be noted that most of the cited comparative studies characterize overall soil or environmental microbial diversity rather than M. vaccae abundance specifically; quantitative, species-level data confirming this rural–urban gradient for M. vaccae itself remain scarce, and the pattern described here is therefore best regarded as inferred from broader microbiome data rather than directly demonstrated for this species.

Urbanization and modern lifestyles have substantially altered human exposure to M. vaccae and other environmental microorganisms, potentially contributing to a “microbial deficit” through reduced contact with microbiologically diverse soils (14). Intensive agricultural practices may further modify soil microbial communities, potentially decreasing M. vaccae abundance in some rural environments. Epidemiological studies suggest associations between reduced exposure to microbial “old friends” and increased prevalence of inflammatory diseases, allergies, and neuropsychiatric disorders in urban populations, although these relationships remain correlational and influenced by multiple confounding factors (18). These observations have encouraged strategies to restore beneficial microbial exposures through approaches such as biodiverse urban green spaces and community gardening programs.

4 Immunomodulatory effects of Mycobacterium vaccae

Mycobacterium vaccae exerts modulatory effects on the immune system in experimental models, acting as an immune regulator that promotes balanced responses and can reduce pathological inflammation. Dendritic cell priming by M. vaccae has been shown to inhibit Th2-skewed responses, in contrast to priming by specific Toll-like receptor 2 agonists, through selective activation of the CREB signaling pathway (19). Separately, M. vaccae (strain NCTC 11659) has been shown to modulate the proinflammatory effects of lipopolysaccharide (LPS) in macrophages, consistent with an anti-inflammatory, regulatory action on innate immune cells (20). This cascade of cellular interactions is schematically illustrated in Figure 2.

M. vaccae is one of the best-studied experimental models supporting the “old friends hypothesis,” originally derived from the hygiene hypothesis, which proposes that reduced exposure to co-evolved environmental microorganisms contributes to the increasing prevalence of allergic, autoimmune, and inflammatory diseases in industrialized societies (8). This concept is supported by epidemiological evidence showing lower rates of atopy and inflammatory disorders in populations with greater exposure to microbially diverse environments, particularly traditional farming communities (14), and by animal studies demonstrating that M. vaccae attenuates allergic sensitization through suppression of Th2-skewed immune responses (19, 21). At the cellular level, M. vaccae interacts with immune cells via recognition of microbial molecular patterns that activate predominantly regulatory and anti-inflammatory signaling pathways, distinguishing it from the pro-inflammatory responses induced by pathogenic mycobacteria (19, 20).

5 Influence of Mycobacterium vaccae on mental health outcomes

Research on M. vaccae, conducted predominantly in rodent models, has reported effects relevant to the gut–brain axis, a bidirectional communication pathway connecting the central nervous system to the enteric nervous system and gut microbiome. Studies have shown that M. vaccae administration modifies gut microbiome composition in the context of allergic airway disease models, with reported shifts toward a more favorable microbial profile (22); this effect appears strain- and context-dependent, since other preparations of M. vaccae have shown no measurable effect on overall gut microbiome diversity or composition despite exerting downstream immunometabolic effects.

M. vaccae has been shown to influence serotonergic signaling relevant to mood and cognition. Studies in murine models have reported that immunization with M. vaccae selectively increases tryptophan hydroxylase 2 (tph2) mRNA expression in specific subregions of the dorsal raphe nucleus, the rate-limiting enzyme in serotonin biosynthesis (23). Foundational work from the same research group identified activation of a specific population of serotonergic neurons in the dorsal raphe nucleus following M. vaccae exposure, and follow-up studies reported increased serotonergic, rather than dopaminergic, activity in the medial prefrontal cortex, with antidepressant-like behavioral effects (10).

Preclinical evidence indicates that M. vaccae exerts antidepressant, anxiolytic, and stress-resilience effects in animal models. Heat-inactivated M. vaccae induces antidepressant-like behavior through activation of serotonergic neurons in the dorsal raphe nucleus (24), while oral administration of live bacteria reduces anxiety-like behavior and improves learning performance (9). In addition, immunization with heat-killed M. vaccae enhances resilience to chronic psychosocial stress, reducing anxiety-related behaviors, colitis severity (23), microglial activation, neuroinflammation, and pro-inflammatory cytokine production (25). Direct human evidence for neuropsychiatric or mood-related outcomes is limited and does not by itself establish an antidepressant or anxiolytic effect of M. vaccae in people. Human evidence remains limited; however, a phase III clinical trial reported that heat-killed M. vaccae (SRL172), used as an adjunct to chemotherapy, improved quality of life, including emotional and cognitive well-being, in patients with advanced non-small-cell lung cancer, although without improving survival and without directly demonstrating anxiolytic efficacy (26). Quality-of-life improvements in that trial were a secondary endpoint measured with general instruments not specifically designed to assess anxiety or depression, and should not be equated with a validated psychiatric treatment effect.

6 Therapeutic potential of Mycobacterium vaccae

The therapeutic potential of M. vaccae for inflammatory and allergic disorders has been the subject of growing preclinical and, to a lesser extent, clinical investigation. In animal models of allergic asthma, preventive or therapeutic administration of M. vaccae preparations has been shown to reduce bronchial hyperresponsiveness, pulmonary eosinophilic infiltration, and allergen-specific IgE levels, effects attributed primarily to induction of regulatory T cells and modulation of the Th1/Th2 balance (22, 27). A clinical study in children with atopic dermatitis reported improvements following administration of heat-inactivated M. vaccae, including reductions in the SCORAD (Scoring Atopic Dermatitis) index and cytokine changes consistent with a shift away from a pro-allergic profile (28). Therapeutic mechanisms documented across these applications include induction of tolerogenic dendritic cells, expansion of regulatory T cells, and modulation of macrophage activity toward M2 anti-inflammatory phenotypes (20).

M. vaccae-based interventions for neuropsychiatric outcomes represent an emerging research area supported by preclinical evidence of neuromodulatory effects, and should currently be regarded as hypothesis-generating rather than clinically validated. Antidepressant-like responses in rodent models appear with a shorter onset than conventional antidepressants in similar assays, suggesting distinct and potentially complementary mechanisms of action (24). In models of anxiety and post-traumatic stress, preventive administration of heat-killed M. vaccae attenuates behavioral and physiological responses to chronic psychosocial stress, potentially involving hypothalamic-pituitary-adrenal axis modulation (23). Current therapeutic strategies explore oral administration through the gut-brain pathway and parenteral delivery for systemic immunomodulation, with ongoing efforts to optimize formulations and dosing regimens (29).

The development of vaccines and immunotherapies based on M. vaccae extends beyond classic inflammatory disorders. Its use as a vaccine adjuvant is among the most studied strategies, exploiting its capacity to polarize immune responses toward Th1 profiles that enhance antigen immunogenicity. This approach has shown promising results in experimental tuberculosis vaccines, where M. vaccae preparations combined with BCG enhanced protective immunity compared with BCG alone in a phase III clinical trial (30). In oncology, M. vaccae-based immunotherapy (SRL172) has been evaluated as an adjuvant treatment in non-small-cell lung cancer, with the principal randomized trial reporting improved quality of life but no survival benefit (26). For atopic conditions, clinical trials evaluating M. vaccae formulations, including atopic dermatitis, indicate modest and inconsistent efficacy rather than the robust, meta-analytically confirmed effects sometimes claimed in secondary literature (28). These effects vary considerably across M. vaccae strains and preparations, which are not necessarily interchangeable and should not be assumed to produce equivalent outcomes. Live preparations (e.g., strain ATCC 15483) have been used mainly in oral/ingestion protocols in rodent behavioral studies, whereas heat-inactivated preparations, including strain NCTC 11659 and the clinical-grade formulation SRL172, have been used predominantly in immunological, stress-resilience, and oncology/allergy trials. Differences in bacterial viability, antigen presentation, and dosing between live and heat-killed preparations plausibly contribute to the heterogeneity of reported outcomes across studies, and findings obtained with one strain or preparation should not be generalized to another without direct comparative evidence (29).

Although M. vaccae is considered non-pathogenic and has shown a favorable safety profile as an adjuvant in tuberculosis and cancer immunotherapy trials, reported adverse events have generally been limited to mild, transient injection-site reactions and low-grade fever (31). Nevertheless, important safety uncertainties remain, including the absence of data in immunocompromised populations, differences between live and heat-killed preparations, the potential for cross-reactivity with other mycobacteria affecting tuberculosis diagnostics, and the lack of standardized dosing, administration routes, and manufacturing protocols. Because M. vaccae shares antigenic determinants with other mycobacteria, prior exposure or immunization could in principle interfere with tuberculin skin testing or interferon-gamma release assays used for tuberculosis diagnosis; this possibility has biological plausibility but, to our knowledge, has not been systematically evaluated in the literature reviewed here and should be studied directly before clinical or public-health application. Data in immunocompromised individuals, including people living with HIV, transplant recipients, and those receiving immunosuppressive or biologic therapy, are essentially absent from the reviewed literature, which is a significant gap given that the proposed mechanism of action is immunomodulatory and could plausibly behave differently in a dysregulated or suppressed immune system. Addressing these limitations through dedicated safety and toxicology studies, particularly in vulnerable populations, is essential before broader clinical translation.

7 Integrating M. vaccae ecology into urban planning, agriculture, and public health

Conventional agriculture with intensive agrochemical use has been shown to reduce soil microbial diversity. This has renewed interest in sustainable agricultural practices that preserve and enrich the soil microbiome. Techniques such as crop rotation, organic fertilization, minimum tillage, and cover cropping have been associated with increased populations of beneficial soil microorganisms (32, 33). A meta-analysis comparing conventional and organic farms found that organic systems maintain higher levels of overall soil microbial abundance and activity; this analysis did not isolate M. vaccae specifically, and no species-specific comparison for M. vaccae was identified (34). Natural environments has been associated with indicators of wellbeing in observational studies (35). Direct soil contact, such as gardening, has been suggested as one plausible means of increasing exposure to soil-derived microorganisms including M. vaccae, though this specific link has not been directly quantified (36).

8 M. vaccae as a paradigm of ecosystem and human health

The emerging “Planetary Health” paradigm treats Mycobacterium vaccae as an illustrative case study of the interdependence between terrestrial ecosystems and human health (37). This conceptual framework, associated with the Rockefeller Foundation–Lancet Commission on Planetary Health, proposes that human health is critically dependent on the integrity and biodiversity of natural systems. M. vaccae is often discussed as a tangible example of this interconnection, given reported associations between soil-derived microbial exposure and immunological outcomes (38). Broader initiatives in the planetary health field, such as the Planetary Health Alliance, advocate for systematic monitoring of environmental microbial diversity as an indicator of ecosystem integrity (39). It should be emphasized that the evidence linking soil biodiversity loss specifically to human disease burden remains largely correlational, indirect, and susceptible to confounding by socioeconomic and lifestyle factors; none of the studies identified in this review establishes a direct, quantitative, causal relationship between M. vaccae exposure (or loss thereof) and disease outcomes at the population level. Accordingly, the “keystone link” framing introduced in Section 1, and the planetary-health narrative more broadly, should be understood as a heuristic and a hypothesis warranting further, dedicated investigation, rather than an established causal pathway. This holistic approach suggests that ecosystem conservation may be relevant to public health strategy, not solely to environmental policy (40).

Anthropogenic environmental changes are altering the distribution and abundance of soil microorganisms more broadly, with potentially relevant implications for human health. Deforestation and conversion of natural ecosystems to agricultural monocultures reduce soil microbial diversity (41). Soil degradation resulting from erosion and loss of organic matter poses a risk to microbial populations more broadly (42). According to the Food and Agriculture Organization, approximately one-third of the planet’s soils are degraded, with fertility loss occurring faster than natural recovery (43).

9 Technical barriers and methodological approaches to studying M. vaccae

The study of Mycobacterium vaccae in natural environments is constrained by methodological challenges that have limited understanding of its ecology and distribution. Its slow growth and specialized culture requirements hinder isolation using conventional microbiological methods, although environmental DNA (eDNA) sequencing, 16S rRNA metabarcoding, and shotgun metagenomics have substantially improved detection and functional characterization without cultivation. Nevertheless, efficient DNA extraction from mycobacterial cells and discrimination of closely related species remain challenging, often requiring multi-gene or whole-genome sequencing. Quantifying human exposure is equally difficult, as current approaches, including quantitative PCR of environmental samples, serological markers, behavioral questionnaires, and emerging wearable air-sampling technologies, each have important limitations. Integrating multiple exposure assessment methods with geospatial models of soil, land use, and climatic conditions currently provides the most comprehensive strategy.

Clinical research on M. vaccae also faces significant methodological limitations. Natural exposure is typically chronic, low-dose, and multimodal, whereas clinical trials generally use discrete, higher-dose interventions with heterogeneous formulations and protocols, limiting comparability across studies. Additional challenges include selecting validated biomarkers that capture both immunological and neuropsychiatric responses, designing appropriate placebo controls, and accounting for interindividual variability related to immune status and genetic background. Future progress will depend on more rigorous study designs, including crossover and pragmatic trials, to better evaluate the health effects of M. vaccae under real-world conditions.

10 Future research directions

Future research on Mycobacterium vaccae is increasingly exploring how environmental microorganisms influence immune, neurological, and ecosystem health. Promising directions include microbial epigenetics, investigating whether M. vaccae induces long-term epigenetic changes in immune and neuronal cells, and environmental neuroimmunology, examining its potential modulation of the gut–brain axis through immune, neuronal, and endocrine pathways. The concept of microbial ecotherapy has also emerged, proposing controlled exposure to M. vaccae-rich soils as a complementary strategy for mood and stress-related disorders, although this remains unvalidated in humans. From an evolutionary perspective, the “old friends” hypothesis continues to explore the role of M. vaccae in immune calibration and the consequences of reduced exposure to environmental microorganisms in modern societies. Advances in CRISPR-Cas9 gene editing, organ-on-chip and microfluidic platforms, high-resolution metabolomics, encapsulation technologies, and computational multi-omics are expected to improve mechanistic understanding, formulation development, and, potentially, personalized applications.

Despite these advances, major knowledge gaps remain. The biological specificity of M. vaccae relative to closely related environmental bacteria, the molecular pathways linking immune recognition to neurobehavioral outcomes, and the optimal dose, route, and duration of exposure have yet to be established. In addition, interindividual variability associated with genetic, epigenetic, microbiome, and environmental factors remains poorly understood. Addressing these questions will require transdisciplinary research integrating soil microbiology, immunology, neuroscience, ecology, public health, and the social sciences, ideally incorporating standardized, strain-specific reporting and formal risk-of-bias assessment.

11 Conclusions

Research on Mycobacterium vaccae reflects growing interest in the links between environmental microbiomes and human health. Preclinical evidence over the past two decades indicates that this saprophytic soil bacterium exerts regulatory effects on immune function and, in animal models, neurobehavioral physiology, through mechanisms including suppression of inflammatory cascades, expansion of regulatory T cells, and TLR2-mediated signaling implicated in gut-brain-immune communication. These mechanistic findings align with, though do not prove, epidemiological associations between biodiverse soil contact and reduced chronic inflammatory and allergic disease, while evidence for neuropsychiatric outcomes remains largely preclinical and should not, at this stage, be interpreted as evidence of clinical efficacy in humans. Clinical translation is still early: inactivated M. vaccae formulations have been tested in oncological and allergic indications, but psychiatric applications remain supported mainly by preclinical rationale. Clinical translation is further constrained by the absence of standardized exposure metrics, dosing regimens, and manufacturing protocols across strains and preparations, and by the limited number of adequately powered, independently replicated human trials with prespecified, validated outcome measures; consequently, the findings summarized in this review should be regarded as hypothesis-generating and mechanistically informative rather than practice-changing. Whether ongoing soil biodiversity loss meaningfully contributes to human disease burden remains to be established through dedicated research.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fsoil.2026.1957461/full#supplementary-material

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Footnotes

Edited by: Hamisi J. Tindwa, Sokoine University of Agriculture College of Agriculture, Tanzania

Reviewed by: Sarah E. Little-Letsinger, University of texas at Austin, United States

Sneha Gautam, Karunya Institute of Technology and Sciences, India