Gut microbiota-gonadal axis: the impact of gut microbiota on reproductive functions.

Increasing evidence shows that gut virome is essential in shaping the composition and function of gut microbiota ( 199 , 200 ). The gut viral community is dominated by prokaryotic viruses ( 201 ) such as bacteriophages that attack bacteria in a host-specific form ( 202 ). Through a phage-mediated gut microbiome modulation, gut virome alters the phenotype of the gut microbiota ( 203 , 204 ). The effect of gut viruses on gut microbiota determines their impacts on fertility. Rasmussen et al. ( 205 ) demonstrated that fecal virome transfer upregulated the proliferation of Akkermansia muciniphila , a commensal gut, and unexpectedly enhances fertility in a mice model. It is likely that these microorganisms influence gonadal metatranscriptomics profile; however, there is a dearth of data on the gut microorganisms, bacteria or viral, that may have a significant impact on gonadal metatranscriptomics profile.

The human body contains countless microorganisms, which makes the body a planet filled with ecosystems. Most of these reside in the gut, while others reside in the mouth ( 1 ), skin ( 2 ), vagina ( 3 ), and penis ( 4 ). The microbiome of individuals is unique to each person, just like the fingerprint and genome. From where do they originate? The human body serves as the largest reservoir of gut microflora. They transmit the microbes from another reservoir to reservoir. In addition, food, water, the environment, and animal also carry microorganisms that make up the human gut ( 5 ). The birth of a child is the very first form of acquisition and transmission of gut microflora. The source of the initial microorganism depends on the mode of delivery. As a child passes through the birth canal of a mother, it comes in contact with its primary microflora from the mother through the vaginal, via the faeco-oral or vaginal-oral route, while those born through caesarean section acquire theirs through the skin. It further encounters other bacteria and organisms through skin- to-skin contact, and breastfeeding. The exposure to these microorganisms is known as seeding. Thus, unlike the genome, microbiome composition originates from the biological mother. As the child is exposed to the world, the composition of its microbiome is influenced by factors such as the birth and growth environment, nature, nutrition, other members of the family, pets among others ( 6 ). Seeding is pertinent to the biological development of a child. The microbes colonize the gastrointestinal tract (GIT) and multiply rapidly, this ensures longevity of the microorganisms. Transmission of gut bacteria to the new born continues upon birth as the baby comes in contact with other humans, especially members of the family. Transmission also occurs from pets and the environment in which the child lives in its early life ( 5 ). The takeover by obligate anaerobes is determined by transmission ability among human population, that is, the ability to exit a host, enter and colonize another ( 7 ). At birth and through the first three years of life, the GIT is first dominated by facultative anaerobic bacteria, which are later replaced by obligate anaerobes as the child transitions to eating solid food ( 8 , 9 ). Some animals share similar microbiome with humans; Roseburia , Faecalibacterium , Bacteroides , Prevotella and Ruminoccocus are commensal bacteria found in humans, dogs, and cats ( 10 , 11 ), while intestinal infections caused by Salmonella enterica subsp. enterica serovar Enteritidis, enteropathogenic E. coli, Campylobacter jejuni , and C. difficile are transmitted between animals and humans ( 12 – 14 ). The interaction between humans and animals has also contributed to the incessant spread of antibiotic resistance. Therefore, there is a possibility that commensal bacteria are transmitted from animals to humans and vice versa. Interaction with pets and farm animals is thus a source of acquiring gut microbiota. Foods contain microorganisms that could make up the gut microflora ( 15 ). Breast milk supplies a baby about 8 million intestinal bacteria on a daily basis ( 5 ). It has been established that humans consume about 10 6 to 10 9 microorganisms daily from food. Although not all these survive the digestion process and those that do survive do not colonize the gut for a long term, gut microbiota acquired through food are obtained through horizontal gene transfer. Food serves as a source of external bacteria species and as genes for commensal gut microbes to acquire. Probiotics, prebiotics, and synbiotics also influence the gut microbiota. Probiotics are viable bacteria and yeasts (predominantly Bifidobacterium and Lactobacillus, Lactococus , Streptococcus , Enterococcus species) ( 16 ) that confer health benefits when consumed in the right quantity, usually as food supplements or with some foods ( 17 ). Prebiotics are fibre-rich foods that support the growth of human microflora ( 18 ). When both are taken together, this becomes symbiotic ( Figure 1 ). Of the numerous benefits of probiotics, they mainly are involved in the development of normal flora of the gut in order to ensure a balance between invaders and bacteria responsible for normal functioning of the organism ( 19 , 20 ). Probiotics restore the natural microbiome of the gut after drug therapy ( 21 , 22 ). Studies have shown that prebiotics (artichokes, asparagus, bananas, berries, chicory, garlic, green vegetables, legumes, onions, tomatoes, as well as barley, cereals, linseed, oats, vegetables, and wheat Fruit) modify the growth of gut bacteria. They selectively foster the growth of microorganisms in hosts gut and modify the gut environment such that normal flora can effortlessly grow and reproduce, but unconducive for pathogens of the gut ( 23 , 24 ). The association between probiotics, prebiotics, and symbiotics. Probiotics are viable bacteria and yeasts that confer health benefits when consumed in the appropriate proportions, usually as food supplements or with some foods, while prebiotics are fibre-rich foods that support the growth of human microflora. When both are taken together, this becomes symbiotic. The environment is another reservoir of microorganisms; indoor airborne microbes circulate through ventilation systems, while outdoor organisms could be carried by humans to become inborne. Bacteria reside on surfaces within and outside the home environment, many of which are skin-resident. However, intestinal bacteria belonging to the families of Bacteroidaceae , Prevotellaceae , Ruminococcaceae , and Lachnospiraceae have been isolated from bathroom and toilet surfaces ( 25 , 26 ), and could be transmitted into human gut via poor hygiene. Water harbours a lot of intestinal pathogens, which are linked to gastrointestinal diseases. When improperly treated water is consumed, there is a risk of consuming bacteria pathogens such as Shigella sonnei , Shigella flexneri , and V. cholerae. The means of transmission is not fully understood, but Blautia spp and E. coli have been isolated from water and linked to be of human origin ( 27 ). The microbial composition gets perturbed by very many factors, which can alter or destroy the function and makeup of the microbiome. The gut microbiome is in a constant state of change through life; its role in both health and disease are been studied. Studies have established the link between gut microflora and human metabolism, nutrition, physiology, and immune function. The exact contribution to disease progression is not clear, however, a disruption of these commensal microbes is an environmental factor that impacts on hosts metabolism and plays a role in diseases such as diabetes, obesity, and atopy- and gut-related Irritable bowel syndrome IBS, and Inflammatory Bowel Disease, IBD ( 28 ). The microbes in the child reach a steady state around age 2 or 3; these ecosystems are however altered by external factors to form the composition which dominates through the entire life of the child ( 29 , 30 ). If the microbiota would return to its previous state after a disruption is determined by the extent of disruption, exposure to other microbes, and the composition of microbiota. Food is one of the factors that influences the abundance and diversity of the gut microorganisms. Certain foods have been linked with the general state of health by affecting the microflora of the intestinal tract. According to a study by McIntosh et al. ( 31 ), low fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAP) diet such as dairy, fruits, vegetables, proteins, nuts and seeds, grains increased Actinobacteria in the gut, while high FODMAP diet reduced bacteria that in turn produced gas. Uchida et al. ( 32 ) demonstrated that cheese increased the abundance of Bifidobacterium and Foligné et al. ( 33 ) showed that cheese also decreased Bacteroides and Clostridia ; some of these strains of bacteria are culprits in gut infections. Food additives, high intensity sweeteners, polyphenols from tea, coffee, berries, and some vegetables have also been proven to influence the gut microbial diversity ( 34 – 40 ). Also, drugs are important modulators of the gut microbial composition. Many researchers have studied how commonly used drugs alter the composition, functions, and abundance gut microbiota ( 41 , 42 ). Weersma and others ( 43 ) reported on how 19 groups of commonly administered drugs modulate different gut microflora among Belgium Flemish people. ACE inhibitors, beta-blockers, laxatives, lipid-lowering statins, metformin, proton pump inhibitors (PPI), and selective serotonin reuptake inhibitor antidepressant have been reported to modulate gut microbiota ( 43 – 45 ). A study in the Netherlands reported a decrease in the diversity of gut microbes with the use of PPIs ( 41 ), which agrees with the reports by Imhann et al. ( 46 ) who opined that PPIs altered the bacterial population among some populations; in the report, some population increased while others decreased. A similar study reported a decrease in microbial diversity of gut microflora from faecal samples obtained from the cohorts ( 47 ). In general, PPIs alter commensal organisms of the intestine ( Enterobacteriaceae , Enterococcaceae , and Lactobacillaceae ) decrease Ruminococcaceae and Bifidobacteriaceae , but increase bacteria resident in the oral cavity ( Rothia dentocariosa and Rothia mucilaginosa , the genus Actinomyces and the family Micrococcaceae ) ( 46 ). Another drug with a wide range of application, which has a modulatory effect on gut microbiome is Metformin. It is used to control blood glucose levels and prevent complications such as renal injury, blindness, and sexual/erectile dysfunction in diabetic patients. Although its mode of action is not fully understood, it has been reported to cause a change in gut bacteria. It was reported that the use of metformin among a group of people resulted in a change in over 80 species of bacteria when compared to the control group. The use of metformin caused a significant increase in Escherichia coli and reduced Intestinibacter. In addition, the study reports one- third of the total population to which metformin was administered suffered gastrointestinal disorders such as like diarrhoea, bloating and nausea, which were caused by an increase in Escherichia coli population ( 48 , 49 ). In addition to metformin and PPIs, other commonly used drugs such as laxatives, statins, antidepressants and opioids have been reported to influence gut microbiome ( 41 , 44 , 46 ). An increase in Bacteroides species has been reported in patients on laxatives, which is similar to the findings in mice that were administered polyethylene glycol (PEG) ( 50 – 52 ). Similarly in a study, the authors administered broad spectrum antibiotics consisting of neomycin, vancomycin, and metronidazole to 11 human cohort suffering bacteria gastrointestinal infection for 5 days, with the aim to measure the effect of these antimicrobials on gut microbiota ( 53 ). The study showed a non-negligible change in the composition and diversity of the microbiome, with the highest alteration occurring one month after antibiotic intervention. Specifically, Enterobacteriaceae remained dominant till the 7th day post antibiotic therapy. By the 30th day, Lachnospiraceae , Enterobacteriaceae , and Ruminococcaceae were greatly reduced but finally returned to their previous state by day 90 post-antibiotics ( 53 ). The entry of an invading microorganism, which successfully colonizes the gut and competes with the normal flora for space and nutrient may cause a depletion of the resident flora and outnumber same. Since there are several factors that disrupt the human microbiota, it is therefore almost impossible to define a healthy microbiota. This large variability could cause commensal and/or mutualistic microorganisms to turn pathogenic. The opportunistic invasion could result in infection and inflammation. A healthy microbiota is one that returns to its previous state after recovering from a disruption ( 54 ). The immune system maintains a constant symbiotic relationship with microorganisms to maintain a state of balance. These microbial populations control the host’s physiological and metabolic functions, they are involved in the maturation of intestinal immune cells ( 55 , 56 ) and maintaining homeostasis, as well as exert strong immunomodulatory effects in response to invasions ( 57 ). Although the exact mechanisms have not been fully elucidated, studies have demonstrated that the interaction between gut microbiota and the host immune system undeniably impacts inflammation and glucose tolerance. Gut microbiota plays an important role in the maturation of CD4 + TH cells, which is crucial for host defense and the development of autoimmune disease by producing pro-inflammatory cytokines ( 58 ). Certain commensal bacteria of the guts are responsible for the induction of Treg cells ( 58 ). In addition, immunoglobulins and innate lymphoid cells (ILCs) are also dependent on this microbial community for development ( 58 ). Gut microbiota shapes the transcriptional landscape of the hepatic endothelium, thus modulating hepatic endothelial sphingosine metabolism and the sphingosine-1-phosphate pathway ( 59 ). A study by Zhao et al. ( 60 ) showed that Akkermansia muciniphila supplementation repressed metabolic inflammation in mice fed a chow diet. This study demonstrated that A. muciniphila , a gut bacterium, regulates host immune response by inhibiting inflammatory pathways, ER stress, and lipogenesis in insulin-responsive tissues, leading to improved insulin action and glucose tolerance ( 60 ). A. muciniphila protects the gut from invasion and infections ( 61 ). The study in addition to this reported an increase in α-tocopherol, β-sitosterol ( 60 ). Another study with a gnotobiotic mouse model carried out by Desai et al. ( 62 ) that aimed at studying the relationship between dietary fiber deprivation on gut microbiota and the mucus defense effect, showed that a malfunction of gut microbiota results in inflammation and increased susceptibility to invasion, which arises from the degradation of the colonic mucus barrier. The mucus barrier is made up of antimicrobial peptides and immunoglobulins, which a potential microorganism must successfully bypass to cause an infection ( 63 ). Sonnenburg et al. ( 64 ) opined that there is a connection between diet and the mucus barrier. Authors have as well reported depletion of the colonic mucus barrier as a response to reduced dietary fiber ( 65 – 67 ). Other studies suggested these diets support the growth of normal flora of the gut. A disruption in the population and physiology of gut microflora (gut dysbiosis) is implicated in the pathogenesis of diseases, including host susceptibility to pathogens, inflammatory bowel disease (IBD), and colon cancer ( 68 , 69 ). Successful treatment of gut dysbiosis negatively modulates inflammasomes and represses unsolicited immune system activation ( 54 ). Gut microbiota ensures a balance in mucus secretion and production. As reported, an imbalance of mucus production leads to inflammation of the intestine ( 70 ) and supports the entry and invasion of commensal bacteria in the inner mucus layer in murine models of colitis and ulcerative colitis patients ( 71 ). In a balanced system, phagocytes are sequestered within the lamina propria. This is necessary to ensure that the immune system maintains a state of unresponsiveness to commensal bacteria. The phagocytes are not activated as long as the epithelial barrier is not compromised. However, the immune system becomes activated through a cascade of processes once an invader/pathogen is detected. S. Typhimurium and Pseudomonas aeruginosa promote caspase1 /Interleukin-1 converting enzyme (ICE) by inducing pro-inflammatory IL-1β ( 58 , 72 ). When active, caspase 1 cleaves inactive inflammatory cytokines IL‐1β and ‐18 and converts them to their active forms. The cytokines thus activate other immune cells to attack and ward off the invading pathogens ( 58 , 72 ). Growing evidence has shown that gut microbiota regulates T lymphocytes ( 73 ). Some studies have suggested as well that the development of B-cells takes place in the intestinal mucus, and it is controlled by signals from commensal microorganisms, resident in the gut ( 74 ). Kamada et al. ( 58 ) also posited that the gut microorganisms positively modulate innate immunity by stimulating ILCs to produce IL-22. This is in agreement with other authors that documented that the production of IL-22 likely depends on commensal gut bacteria or their metabolites, as germ-free mice lacked the ability to produce IL-22 ( 75 ). Mice lacking the IL-22 production cells were more susceptible to C. rodentium infection than their counterparts. This suggests that IL-22 production, which is gut microbiota-dependent, is crucial for protection against pathogen invasion. Summarily, gut microorganisms might modulate host defense by activating the production of IL-22 through ILC stimulation. Gut microbiota has also been reported to suppress neutrophil extracellular traps (NET)ing neutrophil hyperactivity in mesenteric ischaemia/reperfusion injury, while ensuring immunovigilance by enhancing neutrophil accumulation ( 76 ). As previously mentioned, some intestinal microbiota regulates the production of T lymphocytes, which play important roles in the pathogenesis of some diseases ( 58 , 77 – 82 ). TH17 cell differentiation is induced by the colonization by segmented filamentous bacteria (SFB), which confers protection against C. rodentium invasion ( 78 ). There is growing evidence that TH17 cells are essential in regulating immune responses in the intestine and that they protect against some pathogens. SFB are commensal organisms that colonize the epithelia of the host ileum; they are attached to the surface of the absorptive gut epithelium but do not induce inflammatory responses ( 83 ). Although the presence of SFB in humans is still debatable, some studies have reported the isolation of representative members such as Eubacterium, Prevotella , Roseburia, Escherichia , and Klebsiella Clostridia spp from human intestinal mucosa ( 2 , 84 , 85 ). Furthermore, the hyper reaction of immune cells to invading pathogens could result in damage to the host intestinal mucosa. Treg cells regulate the intensity of immune responses in order to prevent host damage ( 81 ). As previously stated, the production of Treg cells is gut microbiota-induced. Thus, gut microbiota regulates the host’s immune protection. Studies demonstrate that B. fragilis plays a crucial role in promoting IL-10-producing Treg cells, which fight against invasion of the host by Helicobacter hepaticus ( 81 ), Bifidobacterium infantis ( 86 ), and reduce the severity of S. Typhimurium infection ( 87 ). Gut bacteria have as well been reported to play a role in the production of IgA and CD4+ T cells. These immune cells target specific antigens ( 88 – 90 ). The exact role and mechanisms by which gut microflora regulate adaptive immune responses is still under investigation, but based on evidence from different studies, commensal organisms of the intestinal mucosa play important roles in activating various immune cells that serve as barriers for invaders and prevent epithelial invasion and disruption; they as well contribute to clearing off pathogens via opsonization. It is at least safe to say gut microbiota release microbial molecules that enhance host defense responses ( 58 , 91 ). Putting together, commensal microbes protect the host from pathogen invasion, prevent infections, limit the severity of infection, and are involved in pathogen clearance upon infection of the gut. In addition, they play important roles in the upregulation and downregulation of immune cells and are crucial to maintaining homeostasis.

There are existing pieces of compelling evidences, however little, which prove beyond reasonable doubts the link between the gut microbiota and reproduction. Most studies agree that gut microbiota influences gonadal functions by modulation steroid sex hormones, insulin sensitivity, immune system, and gonadal microbiota. Also, ingestion of probiotics and prebiotics also modifies gonadal functions by modulating the gut and gonadal microbiota. Although the mechanisms involved in gut microbiota-gonadal cross talk are complex and yet to be fully explored, the roles of gut microbiota, as well as probiotics and prebiotics that promote gut microbiota, should not be downplayed. Human studies validating the findings in animal models are important to curtail the reported global decline in fertility, especially for couples seeking conception. Also, it is important to investigate the gut microorganisms that may have a significant impact on gonadal metatranscriptomics profile. In addition, the role of gut virome and epididymal microbiota in reproduction should be explored.

VA: Data curation, Investigation, Project administration, Resources, Writing – original draft, Writing – review & editing. BA: Data curation, Investigation, Project administration, Resources, Writing – original draft, Writing – review & editing. PA: Data curation, Investigation, Project administration, Resources, Writing – original draft, Writing – review & editing. TA: Data curation, Investigation, Project administration, Resources, Writing – original draft, Writing – review & editing. RA: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing.