All Properties of Infertility Microbiome in a Review Article.

R.G. and A.D. initiated the idea of this study. A.D., Z.E., and R.G. contributed to data collection, interpretation, and final approval of data for the work. Z.E. and M.M. developed the first and final draft of the manuscript. S.M., S.S., and F.G. developed the second draft of the manuscript. All figures and tables were designed and checked by T.D. All authors reviewed and contributed to the revisions and finalized the drafts.

This study was approved by the Ethics Committee (Grant Number: IR.BHN.REC.1402.095) of Behbahan Faculty of Medical Sciences.

The health of the reproductive tract environment and the regular functioning of the reproductive endocrine system are both essential for women of reproductive age to achieve a successful pregnancy. Reproductive health, fertility maintenance, and gonadal development all rely on reproductive endocrine function. Simultaneously, maintaining pregnancy and promoting effective embryo implantation are facilitated by a healthy reproductive tract environment [ 172 ]. The vaginal microbiome of healthy, reproductive‐age women typically contains around 1 billion bacteria per gram of vaginal fluid and is characterized by low diversification, with Lactobacillus species accounting for as much as 95% of all bacteria [ 173 ]. Interestingly, the prevalence of Lactobacillus spp. seems to be age‐dependent and exclusively linked to reproductive age: in childhood, the majority consists of E . coli and anaerobes; during puberty, Lactobacillus spp. colonization begins, and after menopause, its abundance gradually decreases [ 102 ]. The vaginal microbiota undergoes constant changes, while sex hormone levels fluctuate as women age [ 174 ]. The vaginal microbiota is significantly influenced by the cyclical levels of menstrual cycle‐regulating hormones, particularly progesterone (P) and estradiol (E2), throughout a woman's reproductive years [ 175 ]. The components of the female genital tract's defensive barriers, such as the thickness of the epithelial barrier, the frequency of immune cells, the viscosity of mucus, and the vaginal microorganisms residing there, are all affected by the levels of sex hormones. Research indicates a connection between the health of mothers' children and their pre‐pregnancy body mass index (BMI), suggesting a potential relationship between gut microbiota and reproductive health even before conception [ 176 ]. Obesity is defined as having a body mass index (BMI) of 30 or higher. Studies have associated obesity with various obstetric and gynecological issues, such as pre‐eclampsia, macrosomia, and miscarriage, and have shown an adverse correlation with factors affecting the efficacy of assisted reproduction [ 177 ]. Obesity can impact reproductive function through several mechanisms, including altered microbial states, chronic inflammation, psychological effects, reduced immunological response, and subsequent effects on the reproductive hormonal environment [ 178 ]. In comparison to lean women, overweight and obese women have a greater prevalence of bacterial vaginosis (BV), according to a cross‐sectional study conducted in the United States with over 6000 participants [ 179 ]. Increasing BMI was linked to BV in a second large cross‐sectional investigation using self‐collected vaginal swabs, although statistical analysis did not reveal this to be an independent risk factor. Pregnancy‐related BMI was linked to a higher incidence of vaginal dysbiosis during pregnancy, as evidenced by prospective research. This was seen in terms of both a lower quantity of Lactobacilli and a higher incidence of BV [ 180 ]. Genetic variations in the absorption, distribution, metabolism, and excretion of dietary components may explain some of the observed effects in supplementation studies. However, there has been limited research on how genetic variation influences the relationship between fertility and diet. Ultimately, the development of individualized dietary recommendations to optimize fertility may stem from a deeper understanding of an individual's genotype [ 181 ] (Table  3 ). Overview of the potential impact of genetic variation in micronutrient and macronutrient metabolism on male fertility. Dietary patterns serve as vital and practical nutritional tools that offer insights into a person's eating habits, which play a role in numerous complex nutritional interactions impacting overall health and well‐being. Mediterranean and Western diets have been shown to have positive and negative effects on women's reproductive health, respectively [ 196 ]. Chavarro & Co. examined how dietary habits influenced female fertility. They suggested that a diet resembling the Mediterranean diet could be considered the optimal “fertility diet.” This dietary pattern was associated with a 27% decrease in the risk of infertility and a 66% reduction in ovulatory disorders. Additionally, it was linked to a higher ratio of monounsaturated fatty acids (MUFA) to trans saturated fats, consumption of full‐fat dairy products, a lower glycemic load, and intake of vitamins [ 197 ]. Research indicates that diet influences the composition of the microbiome, genome, and epigenome, in response to an individual's genetic background, thereby impacting female reproductive health and outcomes. Various dietary factors, including polyunsaturated fatty acids (PUFA), folate, fiber, starch, and adherence to the Mediterranean diet, appear to have a positive influence on female fertility and the efficacy of assisted reproductive technologies (ART). Given the impact of diet on reproductive health, understanding the correlation between dietary intake and female fertility is crucial. For instance, trans‐saturated fatty acids may have adverse effects on female reproductive health, contributing to ovulatory disorders, reduced fecundability, and endometriosis [ 198 ]. It is essential to grasp the link between food consumption and female fertility because diet directly influences the development of long‐term metabolic disorders that can affect reproductive health, such as obesity [ 196 ]. The vaginal microflora is crucial for successful reproduction as it plays a vital role in maintaining the host's normal physiological environment, a fact long recognized. Lactobacilli, predominant in the normal vaginal flora, especially in women of European descent, offer vaginal protection against urinary tract infections and sexually transmitted infections. Conversely, in women of African American descent, there is a different microbial composition. Alterations in the vaginal microbiota, such as reduced lactobacilli levels and increased populations of facultative and anaerobic organisms, can lead to bacterial vaginosis. This condition predisposes the host to a higher risk of bacterial infections and complications like low birth weight. During pregnancy, the vaginal microbiome changes, with a higher prevalence of Lactobacillus species and reduced microbial diversity. However, an altered vaginal microbiota with low levels of lactobacilli, especially during pregnancy, may lead to preterm labor and excessive inflammation [ 199 ]. Over the past two decades, it has become evident that the gut microbiota impacts numerous metabolic and immune pathways in hosts. Conversely, the intestinal ecosystem can undergo alterations, with the composition and metabolism of intestinal bacteria being influenced by environmental conditions, lifestyle choices, and host responses [ 200 ]. Recent research indicates that certain bacteria within the gut microbiota, particularly in humans, can trigger intestinal CD4+ TH17 responses, CD4+ TH1 responses [ 201 ], or CD8+ TH1 responses [ 202 ] in the mouse gut. Segmented filamentous bacteria (SFBs) can directly and indirectly activate epithelial innate defenses by stimulating Type 3 innate lymphoid cells to produce IL‐22 and T cells to produce IL‐17. Notably, IL‐22 enhances the expression of Reg3γ in epithelial cells. SFB induces the development of inducible gut‐associated lymphoid tissue and Peyer's patches simultaneously, where they elicit specific T‐cell and adaptive IgA responses. SFB markedly increases the population of T cells expressing ROR‐γt; in C57BL/6 mice, most of these cells differentiate into TH17 cells, while a smaller fraction transforms into FOXP3 regulatory T cells. Consequently, SFB stimulates TH17 responses against itself and other commensal bacteria, contributing to the maintenance of the intestinal barrier through the induction of a state of controlled inflammation. The presence of SFB enhances the barrier effect of the microbiota [ 203 ]. The interplay between signaling molecules known as cytokines and their receptors, expressed by immune cells and host tissues, leads to inflammation in response to invading foreign agents. Cytokine networks predominantly regulate inflammation and immune responses related to infections. The immune response can be compromised as invasive microorganisms often target these cytokine mediators. For instance, Pseudomonas aeruginosa ( P . aeruginosa ) secretes several proteases known to be significant virulence factors. Among these are aeruginolysin (an alkaline protease) and pseudolysin (an elastase), both substrates for key proinflammatory cytokines, IL‐6 and IL‐8. This interference with leukocyte recruitment may assist P . aeruginosa in establishing an infection. Additionally, the protease AprA, capable of directly degrading epithelial‐derived IFNλ and inhibiting IFN signaling, is secreted by P . aeruginosa strains isolated from individuals with cystic fibrosis, in a manner dependent on LasR [ 204 ].

The group of bacteria that reside both within and outside of the human body is known as the human microbiome [ 135 ]. This complex collection of microorganisms is essential to human health as it has co‐evolved with us and is involved in many physiological functions. Everybody has a different microbiota, with different species inhabiting different body parts [ 136 ]. Previously thought to be mostly sterile, the male reproductive system is now understood to be a complex mosaic of microbial communities. Different microbial communities are present in the male genital canal, which includes areas like the urethra and the coronal sulcus. In the male reproductive system, Corynebacterium , Streptococcus , and Staphylococcus are the predominant bacterial genera [ 137 ]. Individual differences may be seen in the microbial makeup of this tract, which can be impacted by things like sanitary habits, sexual behavior, and the existence of sexually transmitted infections (STIs) [ 138 ]. Further investigation reveals that the testes, which were previously thought to be completely germ‐free, actually have a modest quantity of microbiota. Particular bacterial communities are also seen in the epididymis, a duct where sperm develop; the importance of these communities is currently being studied. The microbiota of the prostate and seminal vesicles, two glands essential for the formation of seminal fluid, might affect the fluid's composition and overall health. The urethra, which connects to the outside world, has a varied microbiota that affects its well‐being. In addition to sperm, semen also carries microbial communities that can impact the fertility and health of sperm. Last but not least, the microbiota of the penile skin, which includes the glans and shaft, is altered by things like circumcision [ 136 ]. Optimal reproductive health requires a varied and well‐balanced microbiota in the male genital system [ 137 ]. The male reproductive system may be harmed and chronic prostatitis may be caused by bacteria such as E . coli and Ureaplasma urealyticum ( U . urealyticum ), which could cause inflammation [ 139 ]. The microbial makeup of the male urinary system can also affect a person's vulnerability to STIs and the likelihood of the infection spreading to a partner [ 140 ]. Any change in the composition and activity of microbial communities within the human microbiome is referred to as microbial dysbiosis. It is characterized by a departure from the normal or ideal microbial composition, which causes modifications to the metabolic processes and component distributions within the microbiota [ 141 ]. A contributing cause to male infertility is thought to be dysbiosis in the microbiota of the male reproductive system. Precision medicine depends on the identification of specific microbial imbalances, which allows for customized therapies to address the underlying reasons of male infertility [ 142 ]. Male reproductive tract dysbiosis is complicated and varies between illnesses and people. It frequently results in a rise in facultative anaerobic species and a decrease in microbial diversity [ 141 ]. The quality and functioning of sperm can be negatively impacted by such dysbiosis, which can have a negative influence on the male reproductive system [ 136 ]. The development of male infertility is substantially influenced by oxidative stress. The risk of infertility is increased by elevated oxidative stress levels or DNA‐damaged sperm [ 143 ]. Infertility, urethritis, and prostatitis have all been linked to dysbiosis of the male genital tract microbiota. Certain bacterial species, such as U . urealyticum and E . coli , have been connected to inflammation in the male reproductive system and chronic prostatitis [ 31 ]. Through various processes such as inflammation, oxidative stress, decreased sperm function and motility, and disrupted testis function, microbial dysbiosis can have a deleterious impact on male fertility (Figure  2 ). Effect of microbial dysbiosis on male infertility. Maintaining redox equilibrium is crucial for sperm functioning in several critical areas. However, an imbalance in the production and removal of reactive oxygen species (ROS) due to oxidative damage can detrimentally affect sperm quality [ 144 ]. Semen samples from infertile patients often exhibit significantly elevated ROS levels, found in 25%–40% of cases [ 145 ]. Depending on the amounts and characteristics of reactive molecules, the duration of exposure, the effectiveness of antioxidants, ambient temperature and oxygen tension, and other factors, the degree of oxidative damage to spermatozoa can vary greatly among infertile men. Prolonged exposure to high concentrations of ROS can damage various essential cellular macromolecules, such as proteins, lipids, and nucleic acids, ultimately impairing several cellular processes [ 146 ]. Dysregulated gut microbiota stimulates dendritic cells and macrophages in the testis, leading to increased release of pro‐inflammatory substances. Men experiencing infertility often exhibit chronic inflammation in the male reproductive system, exacerbating fertility issues, as evidenced by epidemiological studies [ 147 ]. Experimental models of autoimmune orchitis further elucidate this association, demonstrating significant macrophage infiltration and the release of numerous inflammatory markers. These findings underscore the complex interaction between inflammation and male reproductive health [ 148 ]. The gut microbiota influences male reproductive activity, subsequently impacting the host's physiology and fitness through the microbiota–gut–testis axis. Exploring novel treatments for testicular dysfunction by modulating the gut microbiota is particularly intriguing. The gut microbiota can be altered by androgens or other communication molecules secreted by the testis. Conversely, endotoxins, induced ROS, metabolites (such as SCFA), regulation of gut metabolism and nutrient intake, or the hypothalamic–pituitary–gonadal (HPG) axis are mechanisms through which the gut microbiota influences testicular function. Probiotics, along with other microbial regulators, medications, toxins, nutrition, and lifestyle choices, can all influence the gut microbiota [ 149 ]. Sperm concentration and motility were clearly reduced in mice fed a high‐fat diet, according to Ding et al. [ 150 ]. Furthermore, the gut microbiota of these animals exhibited an increase in Firmicutes and Proteobacteria and a decrease in Bacteroidetes and Verrucomicrobia. After transplanting the fecal microbiota from mice treated with alginate oligosaccharide to mice treated with busulfan, Zhang et al. [ 151 ] observed a noteworthy increase in sperm concentration and motility. Specifically, Bacteroidales and Bifidobacteriales populations of “beneficial” bacteria increased in response to this modification. Alginate oligosaccharides have been shown by Zhao et al. [ 152 ] to have the ability to mitigate the inhibition of spermatogenesis in mice brought on by busulfan. An increase in Lactobacillaceae and Bacteroidales, two types of good bacteria, and a reduction in Desulfovibrionaceae , a kind of harmful bacteria, were associated with this impact. Artificial diets have been shown to cause microbiota dysbiosis in mice and to negatively impact their reproductive systems. Spermatogenesis defects have also been linked to elevated endotoxins, dysregulated testicular gene expression, and localized epididymal inflammation. Sex hormones have been shown to play a role in the communication between microorganisms and hosts and to influence host reproduction [ 153 ]. Dysbiosis of the gut microbiota is linked to the regulation of host hormone levels through close communication between the gut and testicular tissues, so preventing reproduction in wild animals [ 154 ]. Staphylococcal species, which live in harmony with humans, have the ability to directly damage reproductive tissues or to do so through hematogenous pathways. They may also orchestrate the inflammatory response in the reproductive system by way of an innate immune route that is activated by TLR2 [ 155 ]. S . aureus is the species of Staphylococcus that is most closely associated with male infertility. Because S . aureus increases aberrant morphology and decreases sperm concentration, it may be a significant negative factor contributing to the decline of male reproductive function [ 156 ]. Furthermore, it has been found that aberrant sperm are linked to elevated S . aureus concentrations in seminal vesicles. In the treatment of male infertility, colonization of S . aureus in the male reproductive system should not be disregarded as it can also immobilize and agglutinate spermatozoa [ 157 ]. One of the most prevalent sexually transmitted infections of the male reproductive system is C . trachomatis [ 158 ]. The detection rate of C . trachomatis in infertile men was found to be several times higher than in healthy fertile men [ 159 ]. Numerous studies support the detrimental effects of C . trachomatis infection on sperm count, motility, normal morphology, production of reactive oxygen species, total antioxidant capacity, and the ability of sperm from infertile men to penetrate eggs. Additionally, C . trachomatis infection may increase the incidence of male infertility by causing the synthesis of anti‐sperm antibodies [ 157 ]. One of the most well‐known sexually transmitted infections is the HPV, which can cause cancers linked to HPV in both men and women. Harmful effects of HPV infection on male infertility have also been reported, including decreases in sperm count, motility, volume, and normal sperm morphology, which suggests that HPV infection is a risk factor for male infertility. However, other viruses, including adeno‐associated virus, cytomegalovirus, and herpesviruses, have also been linked to male infertility; however, the evidence currently available does not clearly link these factors to male fertility [ 160 ]. A study conducted by Tian et al. [ 161 ] revealed that Candida albicans ( C . albicans ) inhibits human sperm motility and disrupts the ultrastructure of human spermatozoa in vitro, potentially linking it to male infertility. Similarly, a study by Burrello et al. (2004) demonstrated that the presence of C . albicans increased sperm DNA debris and hindered oocyte fertilization. The relationship between fungal communities, particularly C . albicans , and male infertility is an area of research that is still being explored and understood [ 162 ]. Male fertility can be impacted by a wide range of variables, such as microbial dysbiosis, immunological interactions, metabolic processes, and STIs [ 137 ]. Male microbiome‐produced metabolites may have an immediate or long‐term impact on the reproductive system, which may impair fertility. Aside from sperm quality, other parameters might also be affected by such microbial imbalances [ 163 ]. Couple fertility may be impacted by the “seminovaginal microbiota,” which is transferred between partners during intercourse. Both couples' microbiota should be taken into account in thorough fertility evaluations [ 164 ]. The Prevotella genus has been associated with poor‐quality semen in analyses of human semen samples, indicating that some Prevotella species may be to blame for impaired spermatogenesis and male infertility. Given that Prevotella in semen comes straight from the testis, the finding that its quantity in semen was negatively linked with semen concentration [ 25 ] is particularly noteworthy. Moreover, there is a clear correlation between Prevotella abundance and BMI, indicating that it might be a factor in obesity‐related infertility [ 165 ]. Furthermore, Pseudomonas spp. are expressed in greater abundance in the semen of infertile patients. Pseudomonas abundance has a direct correlation with the total number of motile sperm [ 37 ]. Additionally, it was recently shown that males with aberrant sperm concentrations have greater Pseudomona s and Pseudomonas fluorescens abundances in their semen [ 166 ]. There is a shortage of research on the testicular microbiota. Changes in the testicular microbiota have been linked to male infertility, according to preliminary research by Alfano et al. Infertile men have lower levels of Proteobacteria and Bacteroidetes in their testicles. The microsurgical testicular sperm extraction tests produced no sperm, but the Firmicutes and Clostridium abundances changed, Peptoniphilus asaccharolyticus was completely absent, and Actinobacteria was elevated [ 167 ]. Following a vasectomy, Lundy et al. [ 168 ] found that the amount of Collinsella (phylum Actinobacteria) and Staphylococcus (phylum Firmicutes) in semen was reduced. This finding suggests a possible connection between male infertility and the microbiota of the testicles. There are several microbial populations in the male genital canal, which includes areas like the urethra and the coronal sulcus. In the male reproductive system, Corynebacterium , Streptococcus , and Staphylococcus are the predominant bacterial genera [ 169 ]. Research on male infertility has primarily focused on semen samples, consistently revealing differences in the microbiota composition of infertile male semen. Notably, there is an increased abundance of Prevotella and Staphylococcus , and a decreased abundance of Lactobacillus and Pseudomonas . Studies by Baud et al. [ 170 ] and Farahani et al. [ 171 ] have highlighted a negative correlation between Prevotella prevalence and sperm motility, while a reduced Lactobacillus abundance was directly linked to abnormal sperm morphology. Comparisons of rectal samples from infertile men with those from fertile individuals have shown variations in the abundance of Anaerococcus and an elevated presence of Lachnospiracea . Anaerococcus , conversely, was found in higher concentrations in urine samples from infertile males. Additionally, Collinsella was less prevalent in semen samples from infertile men, while Aerococcus was more prevalent. Further investigations revealed a statistically significant inverse correlation between leukocytospermia and semen viscosity and the quantity of Aerococcus . Furthermore, a statistically significant inverse relationship between the concentration of semen and Prevotella abundance was found. On the other hand, Pseudomonas abundance and sperm count were found to have a statistically significant positive connection, but with inverse proportionality to semen pH. Further substantial longitudinal research across different institutions is necessary to validate the results of these investigations [ 168 ]. Current methods generally focus on the investigation of microbiota composition and diversity utilizing techniques including culture‐based techniques, PCR, quantitative PCR (qPCR), and next‐generation sequencing (NGS) to determine microbial dysbiosis in infertility. In culture‐based approaches, bacteria are isolated and cultivated from clinical samples, and then they are identified using DNA sequencing or biochemical testing. Nonetheless, these techniques frequently exhibit a bias toward culturable bacteria and cannot fully capture the diversity of microbial populations (Figure  3 ). Molecular techniques to analyze the composition and diversity of the microbiota.

The authors have nothing to report.

Numerous studies have examined the effects of probiotics on infertile couples, but the results have been inconsistent (Table  4 ). The variations may be due to differences in probiotic species/strains, formulations, doses, intervention length, and patient circumstances. Probiotic supplements may consist of a single strain of bacteria or a combination of multiple strains or species. Research indicates that multi‐strain and/or multi‐species probiotics may, in certain instances, be more efficacious than single‐strain probiotics, as the various strains or species may synergistically enhance each other's benefits [ 218 ]. This study analyzed clinical trials that used either a single species of probiotics or a combination of several species, showing better outcomes. Lactobacillus acidophilus and Lactobacillus rhamnosus were two of the most commonly used species in treating infertility in the studies reviewed. These two bacterial species are among the best‐known probiotics globally for addressing a wide range of medical issues [ 219 , 220 ]. Many researchers have studied its genetic, biological, and physiological properties. Clinical trial probiotic strains must be evaluated for safety, efficacy, and characteristics. As probiotics continue to be a valuable complementary intervention tool for modulating dysbiosis of the microbiota—which has been linked to a number of metabolic disorders and diseases—the dosage of probiotics is another critical factor to think about when studying their effects on the physiological functions in humans and other animals [ 221 , 222 ]. In order to restore eubiosis, a healthy microbiota, it may be necessary to administer certain probiotic strains at certain concentrations, on the other hand, the misuse of probiotics may present risks and safety issues for individuals with compromised immune systems [ 223 , 224 ]. The optimal dosage of probiotics remains unclear; however, a dose exceeding 10 6  CFU/g (CFU/mL) is widely regarded as capable of producing highly effective outcomes. In the trials reviewed in this study, infertile patients were administered daily probiotic doses ranging from 10 8 to 10 11  CFU before conception. The treatment results among infertile patients indicate that the optimal suggested dosage is an average of ≥ 10 8  CFU per g, demonstrating efficacy in inducing remission and reducing relapse and rate of complications. The outcomes align with the results of the meta‐analysis conducted by López‐Moreno and Aguilera [ 225 ]. Investigation of the dose and mechanism of probiotics' effect on infertility treatment in several clinical trials and ongoing studies. 58 C ( n  = 14): 34.6 (33.5–35.8) RA ( n  = 21): 39.4 (38.5–40.4) INF ( n  = 23): 39.4 (38.5–40.4) D 6 months 340 S ( n  = 158): 35.10 ± 3.38 C ( n  = 158): 35.51 ± 3.25 D 6 days 56 S ( n  = 28): 28.42 ± 6.10 C ( n  = 28): 32.75 ± 15.99 B . coagulans (GBI‐30) Lrhamnosus , L . helveticus 10 11 10 10 10 10 D 12 weeks 80 18–40 years L . crispatus LBV88 L . rhamnosus LBV96 L . gasseri LBV150N L . jensenii LBV116 D 4 weeks 41 S ( n  = 20): 37 (32–42) C ( n  = 21): 36 (30–43) D 6 months 160 S ( n  = 80): 37.53  ±  5.12 C ( n  = 80): 37.56  ±  4.63 D 90 days 74 S ( n  = 38): 30.6 (4.0) C ( n  = 36): 31.5 (4.5) Lactobacillus gasseri EB01 DSM14869 Lactobacillus rhamnosus PB01 DSM14870 > 10 8 > 10 8 D 10 days 40 S ( n  = 20): 27.50 (7.25) C ( n  = 20): 27.50 (6.75) L . acidophilus B . bifidus L . rutri L . fermentum D 2 months 2 × 10 9 2 × 10 9 2 × 10 9 2 × 10 9 39 S ( n  = 20): 30.8 ± 0.9 C ( n  = 19): 29.1 ± 1.3 2 strains of B . lactis (W51 and W52) L . acidophilus (W22) L . paracasei (W20) L . plantarum (W21) L . salivarius (W24) L . lactis (W19) D (4 capsules) 3 months S ( n  = 20): 30.8 ± 0.9 C ( n  = 19): 26.0 ± 5.3 L . acidophilus L . reuteri L . fermentum B . bifidum D 12 weeks 70 18–55 years L . acidophilus L . plantarum L . fermentum L . gasseri 5 × 10 10 10 5  × 10 10 7 × 10 9 2 × 10 10 D 3 months 50 S ( n  = 25): 32.23 ± 4.11 C ( n  = 25): 33.01 ± 3.91 L . casei L . rhamnosus L . bulgaricus L . acidophilus B . breve B . longum S . thermophiles D 10 weeks 94 S ( n  = 49): 33.59 ± 0.7 C ( n  = 45): 33.59 ± 0.7 D 2 weeks Abbreviations: ART, assisted reproductive technology; C, control group; CFU, colony‐forming units; D, daily; Flortec, a probiotic associated with a prebiotic; INF group, had a history of infertility (inability to conceive) despite being the recipients of ART for at least three times, including two cycles, at least, of in vitro fertilization (IVF); iOAT, male idiopathic oligoasthenoteratospermia; PCOS, polycystic ovary syndrome; RA group, had a history of recurrent miscarriage with three or more pregnancy losses during the first 12 weeks of pregnancy. Potential therapeutic approaches for altering the vaginal microbiota in the context of infertility in couples include targeted antimicrobial therapies, probiotics, prebiotics, fecal microbiota transplantation (FMT), vaginal microbiota transplant trials (VMT), and engineered probiotics. Oral probiotics, such as Lactobacillus and Bifidobacterium species, have the capacity to colonize the gut and subsequently influence reproductive health [ 226 , 227 , 228 ]. Alternatively, they may be directly administered to the vaginal tract to restore a healthy microbial balance. These probiotics adhere to the vaginal epithelium, produce lactic acid to sustain an acidic pH, and inhibit the proliferation of pathogenic bacteria [ 229 ]. Prebiotics, which are non‐digestible dietary fibers, are essential for promoting the growth of beneficial bacteria and restoring microbial balance. They may improve sperm quality, decrease oxidative stress, and regulate the immune response in the context of male infertility [ 230 , 231 ]. FMT may influence infertility by modifying the vaginal and intestinal microbiota, reducing inflammation, and improving overall reproductive health [ 232 , 233 ]. Furthermore, VMTs hold potential for both clinical treatment and research regarding disease etiology, positioning them as an intriguing strategy for engineering the female sexual microbiome [ 234 ]. Engineered probiotics use genetically modified bacteria to perform specific functions, serving as an alternative to conventional pharmaceutical therapies. Treatment involving genetically modified organisms should be classified as drug therapy rather than, as with traditional probiotics, a dietary supplement [ 234 ]. Studies assessing individual bacterial strains or consortia have shown some success in treating vaginal and reproductive conditions [ 235 ].

The American Society for Reproductive Medicine (ASRM, 2023) has described infertility as a medical condition that involves the inability to achieve a successful pregnancy. This can be due to various factors such as medical history, sexual history, reproductive history, age, physical examinations, diagnostic tests, or a combination of these elements. Infertility may require medical intervention, which could include the utilization of donor gametes or embryos to facilitate a successful pregnancy, whether the individual is trying to conceive alone or with a partner. In cases where there is no apparent cause of infertility despite regular unprotected intercourse, evaluation should commence after 12 months for female partners under 35 years of age and after 6 months for female partners aged 35 years or older [ 1 ]. Infertility affects almost one in six people worldwide at some point in their lives [ 2 ]. The prevalence of infertility among couples of reproductive ages is between 12.6% and 17.5% worldwide [ 3 ]. The term “microbiome” describes the assortment of bacteria, fungi, and viruses that inhabit and are present in and on the human body [ 4 ]. The microbiome is crucial for many physiological processes, including immunity, metabolism, and even reproduction [ 5 ]. An imbalance in the microbiome, referred to as dysbiosis, could be a factor in the inability to conceive for both males and females [ 6 ]. Several reproductive disorders, such as pelvic inflammatory disease and endometriosis, have been associated with an imbalance in the vaginal microbiome of women [ 7 ]. Also, studies showed that lower quality of sperm and infertility have been linked to imbalances in the gut microbiome in men [ 8 ]. So, various factors such as race, age, habits, and sexual activity can play a role in the diversity of microbiota in the genital region [ 9 ]. The connection between microbial origins of pelvic inflammatory disease (PID) leading to infertility is associated with sexually transmitted microorganisms, such as Chlamydia trachomatis ( C . trachomatis ), Neisseria gonorrhoeae , Mycoplasma genitalium ( M . genitalium ), and microorganisms linked to bacterial vaginosis, mostly anaerobic [ 10 ]. M . genitalium is a type of bacteria that can cause infertility and is transmitted through sexual contact. Research indicates that women who have M . genitalium infections have a higher likelihood of experiencing infertility due to damage or dysfunction of the fallopian tubes [ 11 , 12 ]. Furthermore, Gardnerella vaginalis ( G . vaginalis ), a bacterium frequently linked with bacterial vaginosis, has been connected to female infertility as it can disturb the balance of the vaginal microbiome and trigger inflammation [ 13 ]. Prostatitis and epididymitis, which may result in poor sperm quality and function and ultimately infertility, have been linked to Escherichia coli ( E . coli ) and other uropathogenic bacteria in males [ 14 ]. M . genitalium is another bacterium that has been linked to male infertility, and it can cause urethritis, prostatitis, and epididymitis. Additionally, this bacterium can result in reduced sperm motility and changes in sperm morphology [ 15 ]. Lactobacillus has been suggested as a potential probiotic for maintaining semen quality and could also be beneficial in mitigating the harmful effects of Prevotella and Pseudomonas [ 8 ]. Adjusting the microbiome equilibrium using alterations in diet, probiotics, or transferring fecal microbiota may enhance fertility results for males and females [ 16 ]. The purpose of this review is to collect information about the microbiome of infertility in women and men and investigate how the type of microbiome affects future fertility.

The authors declare no conflicts of interest.

Data were from the four international information databases Medline, Scopus, Embase, and Google Scholar. The search strategy was based on the combination of the following terms: “microbiota,” “microbiome,” “microfilm,” “microflora,” “fertility,” or “infertility.” Each keyword combination was linked using the Boolean operators OR and AND. The search strategy was adapted to the specifications of each database. We evaluated the selected studies by examining their titles, abstracts, and full texts. To filter out irrelevant studies, a set of exclusion criteria was applied, such as modeling studies, commentaries, duplicate articles, editorials, guidelines, news articles, and studies lacking adequate data. Finally, duplicate articles were identified and removed using EndNote X9 software (Thomson Reuters, San Francisco, CA).