A narrative review of the gut microbiota-testis axis: mechanisms and implications for male reproductive health

Introduction

Infertility, defined as the inability to conceive after at least 1 year of regular, unprotected sexual intercourse, affects approximately 8–12% of couples globally, with male factors accounting for nearly 50% of cases (1,2). Epidemiological data indicate that sperm counts in adult men have been declining by 1–2% annually over the past 20 years, with marked reductions in sperm morphology and motility (3). Since the 1970s, sperm counts in men from North America, Europe, and Oceania have decreased by 59.3% (4). Furthermore, time to pregnancy has significantly increased in couples of advanced maternal ages and those seeking multiple pregnancies in the United States (5). Thus, the decline in male reproductive health has become a pressing global challenge.

Microorganisms, the oldest forms of life on the earth, have existed for hundreds of millions of years, long before the emergence of humans. The human body, composed of approximately 10¹³ human cells and up to 10¹⁴ symbiotic microorganisms, functions as a “superorganism”. The composition and stability of the microbiota are influenced not only by the host’s genetic factors but also by environmental factors, including birth method, diet, lifestyle, and medication use (6). Through metabolic regulation, immune maturation, and endocrine homeostasis, the gut microbiota modulates a wide range of host physiological functions and has emerged as a critical bridge linking diet, environment, and human health (7).

In recent years, the association between the gut microbiota and male reproductive health has gained increasing attention, giving rise to the emerging concept of the “gut microbiota-testis axis”. Several reviews have summarized this field (8-10); however, most tend to present the gut microbiota and testicular function as two separate domains, lacking systematic integration of the direct mechanistic connections between them. Moreover, the strength of evidence varies considerably across different study designs [e.g., human observational studies, animal experiments, fecal microbiota transplantation (FMT), germ-free models, and antibiotic or probiotic interventions], yet few existing reviews have provided a stratified evaluation or critical comparison of evidence derived from these distinct approaches. This review aims to systematically summarize current progress in the interplay between the gut microbiota and testicular function, synthesize the available mechanistic evidence with particular attention to evidence strength derived from different study designs, and discuss methodological limitations and future directions in this field. We present this article in accordance with the Narrative Review reporting checklist (available at https://tau.amegroups.com/article/view/10.21037/tau-2026-0520/rc).

Methods

We conducted a comprehensive literature search in the PubMed/MEDLINE database up to May 2026. The search strategy combined medical subject headings (MeSH) terms and free-text keywords related to the gut microbiota [e.g., “Gastrointestinal Microbiome”(MeSH), “gut microbiota”(tiab), “gut microbiome”(tiab), “intestinal microbiota”(tiab), dysbiosis(tiab)) with terms pertaining to male reproductive health (e.g., “Semen Analysis”(MeSH), “Spermatozoa”(MeSH), “Infertility, Male”(MeSH), “semen quality”(tiab), “sperm quality”(tiab), “sperm motility”(tiab), “sperm concentration”(tiab), “sperm count”(tiab), “sperm morphology”(tiab), “sperm DNA fragmentation”(tiab), “male infertility”(tiab), asthenozoospermia(tiab), oligozoospermia(tiab), azoospermia(tiab)]. Separate search strategies were developed for human and animal studies (see Table 1 for the search strategy summary; a detailed evidence summary of all included studies is provided in Table S1).

The search covered all records from database inception to May 15, 2026. Studies were eligible for inclusion if they met the following criteria: (I) original research articles, including observational studies, animal experiments, FMT studies, germ-free models, and antibiotic or probiotic intervention studies; (II) investigations examining the association between gut microbiota and male reproductive function, encompassing semen quality, testicular function, and spermatogenesis; and (III) articles published in English. Exclusion criteria were: (I) reviews, case reports, conference abstracts, and editorials; (II) studies focusing exclusively on female reproductive health or non-mammalian models; (III) secondary analyses lacking original data; and (IV) non-English publications.

Literature screening was conducted independently by two co-first authors (L.G. and Y.C.). Titles and abstracts were initially screened to exclude irrelevant studies, followed by full-text assessment of potentially eligible articles. Disagreements were resolved through discussion and consensus, with arbitration by the corresponding authors (Y.L. and L.J.) when necessary. Data extraction focused on study design, sample size, microbial profiling methods, key findings regarding microbial associations or mechanistic pathways, and evidence for causality (e.g., from FMT experiments). Additionally, we manually searched the reference lists of included articles to identify further relevant studies.

Gut microbiota: an overview

Early microbiological research relied on culture-based techniques, but many gut microorganisms cannot grow under laboratory conditions, limiting a comprehensive understanding of the gut microbiome (11). With the application of culture-independent molecular techniques, the composition and function of the gut microbiota have been more thoroughly revealed. To date, over 2,776 prokaryotic species have been isolated from human fecal samples (12).

Composition and development

In healthy individuals, the gastrointestinal tract harbors a distinct spatial gradient of microbial distribution. The stomach contains small numbers of acid-resistant bacteria derived from the oral cavity. Bacterial density is low in the proximal small intestine [10³–10⁴ colony-forming units (CFU)/mL] and increases substantially toward the terminal ileum (up to 10⁹ CFU/mL), with a transition from aerobic to anaerobic bacteria (13). The colon harbors the highest microbial density (10¹² CFU/mL), dominated by Firmicutes and Bacteroidetes, and is also the primary site of dietary fiber fermentation and short-chain fatty acid (SCFA) production. Once absorbed into systemic circulation, these metabolites can influence distant organs, including the testes (14). Beyond this spatial distribution, the gut microbiota also undergoes a characteristic temporal trajectory from infancy to adulthood. The gut microbiota begins to colonize early in life, influenced by mode of delivery and feeding practices (15). During weaning, the transition from a milk-based to a solid food diet triggers significant reorganization of the microbial community, which stabilizes around 3 years of age into an adult-like structure (16). External factors such as antibiotics and diet can temporarily disrupt this balance, though the microbiota can often recover to a baseline state over time (17). Maintenance of gut microbial homeostasis is critical for host health, and its disruption has been linked to various diseases. Given the temporal overlap between pubertal onset and gut microbiota maturation, whether microbiota-derived signals participate in regulating reproductive system maturation warrants further investigation.

Functions of the gut microbiota

These microbiota-derived signals arise from a broad array of metabolic activities carried out by the gut microbial community. Approximately 85% of carbohydrates, 66–95% of proteins, and nearly all fats are processed in the gastrointestinal tract, while dietary fibers (lignin, non-starch polysaccharides, resistant starch, oligosaccharides) resist host enzymes and undergo colonic microbial fermentation, producing gases (methane, hydrogen), SCFAs (acetate, propionate, butyrate), and organic acids (lactate, succinate) (18). Absorbed in the colon via passive diffusion or ion exchange, SCFAs supply ~10% of host energy and exert anti-inflammatory effects by reducing intestinal permeability, enhancing insulin sensitivity, lowering lipids, preventing hepatic steatosis, and alleviating metabolic endotoxemia (19). Undigested proteins are degraded by bacterial proteases/peptidases into neuroactive compounds, sulfur-containing and aromatic metabolites, polyamines, and ammonia, affecting nitrogen balance through the gut-brain axis (20). Bile acids are converted by microbial bile salt hydrolases (BSHs)-mainly from anaerobes (Bacteroides, Clostridium) and some aerobes (Actinobacteria, Proteobacteria)-into secondary forms (deoxycholic and lithocholic acids), partially reabsorbed via enterohepatic circulation, with excess excreted in feces (21). Other microbial metabolites, such as trimethylamine-N-oxide (TMAO) derived from choline and bioactive polyphenol derivatives, also influence host metabolism (22,23).

In addition to metabolic functions, the gut microbiota plays a critical role in maintaining the intestinal barrier. The intestinal barrier is a multilayered defense system comprising the outer mucus layer, the intermediate epithelial layer with intercellular tight junctions, and the underlying gut-associated lymphoid tissue (24,25). The mucus layer harbors commensal bacteria that help maintain microbial balance; intestinal epithelial cells regulate selective permeability through tight junctions (25); and the underlying lymphoid tissue provides immune protection (26). These layers work in concert to limit the translocation of luminal contents, particularly bacterial endotoxins such as lipopolysaccharides (LPS). When barrier integrity is compromised, LPS and other microbial products can activate Toll-like receptor 4 (TLR4) on immune cells, triggering immune and inflammatory responses (24), which may in turn affect distant organs, including the testes, via the circulatory system.

Gut microbiota and semen quality: evidence from human studies

Human studies provide the most direct line of evidence linking the gut microbiota to male reproductive health. The relative accessibility of fecal and semen samples, combined with the maturity of high-throughput sequencing technologies and limited ethical constraints, has spurred increasing clinical investigations into this relationship. The field has progressed from descriptive association analyses to Mendelian randomization (MR)-based causal inference, and more recently to mechanistic explorations with human correlative data.

Observational studies: gut microbial signatures in men with impaired fertility

Multiple case-control studies have shown that men with impaired semen quality harbor gut microbial compositions distinct from those of fertile controls. Among the reported findings, two observations have demonstrated reasonable cross-population reproducibility. First, relative depletion of butyrate-producing taxa-particularly Faecalibacterium prausnitzii-has been associated with reduced sperm motility across several independent cohorts, while gut microbial α-diversity also tends to be lower in asthenozoospermia patients (27,28). Second, increased abundance of Bacteroides and Prevotella correlates positively with serum endotoxin levels and negatively with sperm motility, a pattern reported in both Chinese and Western populations (29,30). Beyond these, however, most differentially abundant taxa show limited consistency across studies. This heterogeneity likely reflects confounding factors such as dietary habits, geographic regions, or sequencing platforms, suggesting that a universally applicable “fertility-associated” gut microbial reference profile has not yet been established.

Notably, the relationship between gut microbiota and semen quality may be modulated by host metabolic status. In an analysis of 407 men stratified by enterotype, obesity was associated with asthenospermia risk only in the Prevotella-dominant enterotype P group, whereas no such association was observed in the Bacteroides-dominant enterotype B group (30). This finding suggests that metabolic disturbances may synergize with microbial dysbiosis to affect semen quality, although its generalizability requires further validation. In recent years, multi-omics integration has provided new analytical dimensions to this field. A combined metagenomic and metabolomic study of 19 patients with primary idiopathic male infertility identified coordinated correlations between eight microbial species and semen parameters, with a random forest model demonstrating moderate discriminative potential (31), offering preliminary proof-of-concept for non-invasive adjunctive diagnosis using fecal samples. However, the diagnostic performance of this model has not yet been validated in independent external cohorts, and its clinical utility remains remote.

Causal inference from MR studies

Observational studies are vulnerable to confounding and reverse causation. MR overcomes these limitations by using genetic variants as instrumental variables, enabling causal inference from observational data when core assumptions are met. Recent MR studies have identified several candidate microbial genera. A two-sample MR analysis using genome-wide association study (GWAS) data from the MiBioGen and FinnGen consortia identified Anaerotruncus as a potential risk factor for male infertility [odds ratio (OR) =1.96; 95% confidence interval (CI): 1.31–3.40] (32). Another MR study extended to 211 taxa and reported 16 causal associations between gut microbes and reproductive diseases, with Intestinibacter implicated in both male infertility and sexual dysfunction (33). On the protective side, a mediation MR analysis estimated that the protective effect of Bacteroides on male infertility (OR =0.57) may be partially mediated by hepatocyte growth factor (34). Another study reported candidate associations between five bacterial genera and sperm-related proteins, including SPACA3, SPAG11A, and ZPBP4 (35), providing molecular-level clues linking microbial taxa to reproductive physiology. Reverse MR analysis further suggested that reproductive pathologies may also influence gut microbial composition, indicating that the interaction is not unidirectional.

MR analyses are inherently limited by summary-level GWAS data with limited taxonomic resolution and cannot fully exclude horizontal pleiotropy. Although several independent studies have shown consistency in the causal direction of certain genera, these findings should currently be interpreted as clues for generating causal hypotheses rather than confirmatory conclusions.

Metabolite correlates and candidate pathways from human studies

Gut microbes may exert distal regulatory functions through circulating metabolites. TMAO correlated negatively with semen volume, total sperm count, and motile sperm count in 107 men, with choline-to-trimethylamine converting bacteria (Phocaeicola massiliensis, Veillonella spp., and Klebsiella pneumoniae) enriched in abnormal parameter groups (36). Butyrate is reduced in asthenozoospermia patients and correlates positively with sperm motility (27). Under high-altitude hypoxia, Clostridium symbiosum-derived succinic acid activates GPR91 in testicular macrophages, promoting CD68⁺CD163⁻ polarization and spermatogenic apoptosis (37). However, these candidate pathways require causal validation in humans. More broadly, the clinical translation of microbiota-based strategies faces major hurdles: substantial heterogeneity in microbial profiles across populations, unvalidated therapeutic targets, and uncertainty about reversing irreversible spermatogenic damage. While probiotic or synbiotic interventions have entered small-scale trials in idiopathic male infertility, most have used short-term sperm parameters as endpoints rather than pregnancy or live birth rates (38). Thus, despite accumulating evidence for a gut microbiota-testis axis, the path from mechanistic insight to evidence-based intervention remains to be defined.

Mechanisms of gut microbiota in regulating testicular function: evidence from animal studies

Spermatogenesis and testicular endocrine function depend on the coordinated actions of three key cell types: Leydig cells, which synthesize testosterone; Sertoli cells, which provide structural and metabolic support; and germ cells, which undergo meiosis to form mature spermatozoa (39). The functional integrity of these cells is tightly regulated by the hypothalamic-pituitary-gonadal axis and is also modulated by the gut microbiota and its metabolites.

Animal models and in vitro experiments provide an irreplaceable platform for dissecting this regulatory network. Compared with human studies, these models allow precise control of genetic background, environmental factors, and microbial composition, and enable direct testing of causality through interventions such as microbiota transplantation, antibiotic depletion, and metabolite administration. This section focuses on Leydig cells, Sertoli cells, and germ cells to systematically review the effects of the gut microbiota and its metabolites on each cell population, thereby elucidating the regulatory mechanisms of the gut microbiota-testis axis at the cellular level.

Effects on Leydig cells

Leydig cells, located in the testicular interstitium, are the principal site of testosterone synthesis. Testosterone biosynthesis relies on the continuous supply of cholesterol to mitochondria and the coordinated action of steroidogenic enzymes, including StAR, CYP11A1, CYP17A1, and 17β-HSD (39). Accumulating evidence from animal models indicates that the gut microbiota and its metabolites influence Leydig cell function through multiple interconnected mechanisms. This regulation is not unidirectional but rather constitutes a complex bidirectional network shaped by microbial composition, metabolite profiles, and systemic inflammatory status.

Cholesterol transport across the mitochondrial membrane is the rate-limiting step in testosterone synthesis, and this process is modulated by at least two types of microbial signals. The ornithine-LDLR axis exemplifies a clearly defined promoting pathway: when intestinal Muribaculaceae abundance is preserved, microbiota-derived ornithine enhances cholesterol uptake by upregulating low-density lipoprotein receptor (LDLR) expression in Leydig cells; conversely, berberine-induced depletion of this bacterial family reduces ornithine levels and compromises cholesterol uptake (40). The bile acid-FXR axis offers a distinct regulatory route: sea cucumber peptides suppress Alistipes abundance, elevate conjugated bile acid levels, and downregulate testicular farnesoid X receptor (FXR) expression, thereby relieving FXR-mediated negative regulation of steroidogenesis and promoting testosterone synthesis (41). On the inhibitory side, deoxynivalenol (DON) and doxycycline provide mechanistically distinct evidence. DON induces gut dysbiosis characterized by Desulfovibrio expansion, depleting the cholesterol precursor 5α-cholestanol while suppressing StAR and CYP17A1 expression (42); doxycycline directly impairs Leydig cell mitochondrial function and reduces Cyp11a1, Cyp17a1, and 17β-HSD transcript levels (43). Although both ultimately suppress testosterone synthesis, their distinct initial targets suggest that inhibition of steroidogenic enzymes represents a common downstream pathway shared by diverse microbiota-related insults. Notably, the same class of exogenous factors can produce opposing effects depending on the direction of microbial remodeling. PFOS, under specific exposure conditions, enriches Lactobacillus and drives arachidonic acid metabolite production, thereby activating the OXE-R-StAR pathway to stimulate testosterone synthesis (44). This bidirectional nature argues against a linear interpretation of the microbiota-Leydig cell relationship; a more plausible model is a regulatory network composed of multiple positive and negative feedback loops, in which the net effect depends on the integrated response of all nodes within the network.

Nuclear receptors serve as key transducers of microbial metabolic signals to Leydig cells. The bile acid-FXR axis has already established that nuclear receptors can directly mediate microbial metabolite regulation of steroidogenic enzyme transcription. The tryptophan-aryl hydrocarbon receptor (AHR) axis further supports this paradigm: gut microbiota-derived tryptophan metabolites act as endogenous ligands for the AHR; in cholestasis models, AHR ligand depletion correlates with suppressed androgen synthesis, and exogenous AHR agonist supplementation partially restores testosterone production (45). The common feature of such pathways is that microbial metabolites do not act directly on Leydig cells, but rather modulate Leydig cell transcriptional responses to luteinizing hormone signaling through the nuclear receptor system-essentially a regulation of hormonal sensitivity.

In addition, gut dysbiosis can indirectly impair Leydig cell function through systemic inflammation. EBCN (46) and 6:2 Cl-PFESA (47) induce gut dysbiosis while simultaneously activating Leydig cell necroptosis or suppressing steroidogenic enzyme expression. In Ggt1-deficient mice, elevated levels of the microbial metabolite phenylacetylglycine (PAGly) activate the Leydig cell β2AR-STAT3-SOCS3-STAT5B-Klk1b signaling cascade. The reproductive toxicity of DON can be transmitted to healthy recipients via FMT (42), indicating that gut dysbiosis alone is sufficient to drive Leydig cell dysfunction. Thus, inflammatory pathways are interwoven with metabolic pathways-dysbiosis simultaneously alters metabolite profiles and compromises intestinal barrier function; the former acts directly on Leydig cells, while the latter triggers systemic inflammation through LPS and other endotoxins, further exacerbating local testicular damage.

Effects on Sertoli cells

Sertoli cells are the only somatic cells within the seminiferous epithelium, and their functional integrity is essential for spermatogenesis. At the structural level, adjacent Sertoli cells form the blood-testis barrier (BTB) through tight junctions composed of occludin, ZO-2, and E-cadherin, along with gap junctions, desmosomes, and basal ectoplasmic specializations, which collectively sequester post-meiotic germ cells from the systemic immune system to maintain the immune-privileged microenvironment (48). At the metabolic level, Sertoli cells convert glucose to lactate via glycolysis, providing the primary energy substrate for developing germ cells (39).

The gut microbiota exerts bidirectional regulation on BTB integrity. Positive regulation is primarily mediated by SCFAs, particularly butyrate. Germ-free mice exhibit increased BTB permeability, accompanied by significantly reduced expression of occludin, ZO-2, and E-cadherin; notably, colonization with butyrate-producing Clostridium tyrobutyricum restores BTB integrity (49). In addition, butyrate-secreting Faecalibacterium prausnitzii has been reported to restore adherens junction protein levels and mitigate BTB damage (10). The BTB-protective effect of melatonin in heat-stress models has been shown to depend on gut microbiota remodeling (50). Negative regulation is observed in multiple models of gut dysbiosis. Antibiotic treatment (51), DON exposure (42), and bisphenol S (BPS) exposure (52) each induce gut microbial dysbiosis, leading to downregulation of tight junction proteins or activation of p38/ERK MAPK inflammatory signaling, ultimately compromising BTB function. FMT experiments across multiple models consistently indicate that altered gut microbial composition can induce BTB permeability changes (42,51,53), thereby elevating the “microbiota-BTB” axis from a correlative observation to a mechanistically validated pathway with a clear causal direction. Mechanistically, compromised gut barrier function allows microbial products such as LPS to enter the circulation, activating the LPS/TLR4 pathway in the testis and subsequently triggering NF-κB and MAPK inflammatory cascades, which reduce tight junction protein expression and disrupt BTB integrity (54).

Beyond BTB regulation, the metabolic support function of Sertoli cells is also modulated by the gut microbiota. Nicotinamide mononucleotide (NMN) enhances lactate production in Sertoli cells by reducing lysine acetylation of lactate dehydrogenase C (LDHC), an effect closely associated with gut microbiota-metabolite interactions (55). Conversely, a high-concentrate diet induces gut dysbiosis and depletes circulating L-citrulline, leading to Sertoli cell DNA damage and BTB disruption phenotypes that are effectively reversed by L-citrulline supplementation (53). Together, these findings suggest that gut dysbiosis may indirectly compromise Sertoli cell metabolic support by disrupting the systemic availability of key metabolites, thereby impairing their capacity to sustain spermatogenesis.

Effects on germ cells

Germ cells reside within the seminiferous epithelium, where they undergo mitotic proliferation, meiosis, and morphological differentiation to form mature spermatozoa. This process is tightly regulated by hormonal signals from Sertoli cells and metabolic cues from the testicular microenvironment (39). Animal studies indicate that the gut microbiota and its metabolites influence germ cell development through both direct metabolic signaling and indirect inflammatory pathways, impacting spermatogonial differentiation, meiotic progression, and post-meiotic germ cell survival.

Meiosis depends on retinoic acid (RA) signaling, whose initiation and progression are regulated by systemic vitamin A metabolism. The gut microbiota influences this pathway by modulating bile acid metabolism and subsequent vitamin A absorption. In a metabolic syndrome sheep model, excessive diet-induced gut dysbiosis reduced bile acid levels and impaired vitamin A absorption, leading to disrupted RA signaling and spermatogonial arrest; FMT from healthy donors confirmed that gut microbiota composition affects spermatogonial differentiation through the vitamin A-RA axis (56). PFOS exposure impairs hepatic CDCA synthesis and reduces Ligilactobacillus murinus abundance, affecting aspartic acid metabolism and causing spermatogonial arrest, a pathway validated by FMT (57). In addition, dysbiosis alters the microRNA cargo of circulating exosomes: upregulation of miR-211-5p in gut-derived exosomes targets testicular Meioc, interfering with meiosis (58); antibiotic-induced microbiota depletion similarly perturbs exosomal function, leading to abnormal RA metabolism and meiotic disruption (59). In chronic stress models, glucocorticoid-induced dysbiosis depletes Lactobacillus and disrupts vitamins A and E metabolism, upregulating Sting1 and downregulating Slc9c2, thereby impairing sperm motility (60).

The gut microbiota and its metabolites also regulate the balance between germ cell survival and programmed cell death. FMT from young to aged mice alleviates aging-associated spermatogenic dysfunction via 3-hydroxyphenylacetic acid (3-HPAA), which upregulates GPX4 and suppresses ferroptosis (61). Conversely, dysbiosis promotes germ cell death: heat stress induces intestinal inflammation and increases permeability, allowing endotoxins to enter the circulation and trigger necroptosis, an effect mitigated by probiotic intervention (62). Inflammatory signals further compromise germ cell survival: glyphosate exposure increases Prevotella_1 and Bacteroides abundance, driving IL-17A production and testicular oxidative damage (63); polystyrene microplastics induce dysbiosis and elevate IL-17A through Th17 cell migration, leading to testicular inflammation and impaired spermatogenesis, a pathway recapitulated by FMT (64).

Recent evidence has identified additional microbial metabolites that directly promote germ cell development. Lactiplantibacillus plantarum SNI3 increases testicular γ-glutamyl-glutamate (γ-GluGlu) levels and enhances sperm count independently of testosterone, suggesting a non-hormonal mechanism (65). NCG supplementation enriches Paraprevotella, elevates vitamin B6 and testicular glutathione content, and reduces lipid peroxidation, thereby protecting germ cells from oxidative damage (66). Portulaca oleracea polysaccharides protect against acrylonitrile-induced reproductive damage by remodeling the gut microbiota and restoring the metabolite 2-aminopropenoic acid (67).

Male reproductive tract microbiota: a local microbiome independent of the gut microbiota

When considering the association between the gut microbiota and testicular function, the local microbial communities residing within the male reproductive tract itself represent a noteworthy dimension. Although the resident microbiota in semen, prostate, and testis differ from the gut microbiota in terms of ecological niche and regulatory mode, they are not mutually exclusive-the reproductive tract microbiota may serve as an intermediary link between the gut microbiota and the testis: the gut microbiota alters the reproductive tract microenvironment through systemic immune and metabolic signals, thereby influencing the composition and function of local microbial communities. Additionally, the local microbiota may also exert independent effects on testicular function directly through inflammatory or immune responses. Therefore, examining the reproductive tract microbiota within the framework of the “gut microbiota-testis axis” may contribute to a more comprehensive understanding of the potential pathways through which the gut microbiota influences male reproductive health.

With respect to the seminal microbiota, Lactobacillus is typically the dominant genus, with its abundance positively correlated with sperm motility, whereas enrichment of Proteobacteria, Anaerococcus, and Bacteroides ureolyticus is associated with diminished sperm quality (68-70). Studies have reported that infertile men exhibit elevated seminal α-diversity, with Prevotella abundance negatively correlated with sperm concentration and Pseudomonas abundance positively correlated with total motile sperm count (71). Another study identified 45 differentially abundant taxa and 147 differential metabolites, four of which demonstrated diagnostic potential [area under the curve (AUC) >0.97] (72). Beyond the seminal compartment, the prostate also harbors a resident microbiota, with detectable levels of Escherichia coli, Propionibacterium, Acinetobacter, and Pseudomonas, among which Escherichia coli is recognized as a major pathogen in bacterial prostatitis (73). The testis, long considered a sterile organ, is now known to contain small numbers of bacteria, predominantly Actinobacteria, Bacteroidetes, Firmicutes, and Proteobacteria, in normozoospermic men. In contrast, men with idiopathic non-obstructive azoospermia (iNOA) exhibit increased bacterial DNA load, reduced taxonomic richness, and loss of Bacteroidetes and Proteobacteria (74). Nevertheless, the precise mechanisms by which the male reproductive tract microbiota influences reproductive health, as well as its interplay with the gut microbiota, remain to be further elucidated.

Conclusions

In summary, the gut microbiota-testis axis has emerged as a critical physiological pathway linking the microbiome to male reproductive function. The evidence reviewed here supports two core conclusions. First, although human studies remain preliminary, several genera-including Anaerotruncus, Bacteroides, and Faecalibacterium prausnitzii-have shown reproducible associations with semen quality, with microbial metabolites potentially serving as important mediators. Second, the underlying mechanisms can be attributed to microbiota-derived metabolic and inflammatory signals that act on Leydig cells, Sertoli cells, and germ cells, ultimately affecting testosterone synthesis and spermatogenesis (Figure 1). Animal experiments, particularly FMT studies, have provided direct evidence for these causal links, whereas most human studies remain correlational.

Figure 1 Schematic illustration of the proposed gut microbiota-testis axis in male reproductive regulation. BTB, blood-testis barrier; LPS, lipopolysaccharides; SCFA, short-chain fatty acid.

Looking forward, several critical gaps must be addressed. Methodologically, large-scale longitudinal cohort studies are urgently needed to determine whether early-life gut microbial signatures predict subsequent declines in sperm quality. Mechanistically, the pathways through which microbial metabolites influence epigenetic modifications in germ cells and their potential transgenerational effects require further elucidation; the interplay between the gut microbiota and local microbial communities of the reproductive tract, and their relative contributions, also remain unclear. Therapeutically, microbiota-based intervention strategies must be validated in randomized controlled trials with fertility outcomes as primary endpoints. Clinically, the substantial inter-individual variability in microbial composition calls for personalized rather than one-size-fits-all approaches. Fundamentally, a consensus definition of a “healthy” gut microbiota has yet to be established. Future research should integrate multi-omics data to construct robust and functionally relevant biomarker systems, thereby advancing the field from descriptive correlations toward mechanism-driven clinical translation.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://tau.amegroups.com/article/view/10.21037/tau-2026-0520/rc

Peer Review File: Available at https://tau.amegroups.com/article/view/10.21037/tau-2026-0520/prf

Funding: This work was supported by the National Natural Science Foundation of China (No. 88250206).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tau.amegroups.com/article/view/10.21037/tau-2026-0520/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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