{"paper_id":"0da33905-2bc6-4d76-98b0-a37e498f6910","body_text":"The set of microorganisms that usually live inside the human body, also called human microbiota, generates the microflora. Its genomic constitution and its by-products are called the human microbiome [ 1 ]. This microbiome is unique for each individual and is never stable, due to environmental, nutritional and ecological changes. Furthermore, in everyone, due to environmental variations, each body site is home to a distinct microbial ecosystem [ 2 ]. Microorganisms can adapt to different survival conditions.  Bacteroidetes  and  Firmicutes , followed by  Proteobacteria ,  Fusobacteria ,  Tenericutes ,  Actinobacteria  and  Verrucomicrobia  were reported to be the most dominant, constituting about 90% of the total microbial population in humans [ 3 ]. A substantial amount of information is readily available regarding the human microbiome; much less is known regarding the human mycobiome, i.e., the type and number of fungal species that inhabit the human ecological niches. In fact, studies focusing on yeast infections are scarce compared with studies focusing on bacterial infections ( Figure 1 ).\nSeveral human ecological niches are colonized by millions of different microorganisms, of which the gut is by far the most studied. The gut is recognized as a virtual organ closely associated with the health and longevity of the host. The gut microbiome has both beneficial and adverse impacts on gut tissue homeostasis [ 4 ]. From a very early stage of life (after 3 years of age), the microbial composition of the intestinal microbiome varies over time, never managing to remain 100% stable [ 4 ]. The intestinal mycobiome is constituted in abundance by fungi, such as  Candida  spp.,  Aspergillus  spp.,  Fusarium  spp. and  Cryptococcus  spp., which can have a pathogenic effect on the host [ 5 ].\nAlthough most oral microbiome studies have been performed on bacteria, there has been a recent surge in studies on oral fungal communities [ 6 ]. Over the past few years, exploration of the composition of the mycobiome in oral samples has shown that they were colonized by  Candida  spp.,  Cladosporium  spp.,  Saccharomyces  spp.,  Penicillium  spp.,  Malassezia  spp.,  Aspergillus  spp.,  Cryptococcus  spp.,  Rhodotorula  spp. and  Trichosporon  spp., among others [ 6 ].\nCharacterization of fungal communities over time has revealed that mode of delivery influences colonization by  Candida  species, where the relative abundance of  Candida albicans  is highest on the skin of vaginally delivered infants [ 7 ]. On the other hand, characterization of the fungal communities in the skin has revealed abundant colonization by  Malassezia  spp.,  Candida  spp.,  Cladosporium  spp.,  Fusarium  spp. and  Cryptococcus  spp. [ 8 ]. In the vagina, although much less abundant than bacteria (values less than 1%), fungi, namely  Candida albicans , have been identified as the most predominant microorganism affecting vaginal health. One study demonstrated the presence of  Candida ,  Clavispora lusitanie ,  Malassezia ,  Rhodotorula ,  Aspergillus  and  Leptosphaerulina  as some of the most prevalent fungi in the vaginal mucosa [ 9 ].\nWith this review, we intend to obtain knowledge about the composition and role of commensal fungi in human health, with special focus on fungal microorganisms that can colonize the skin, vagina and mucous membranes.\n\nThis review was based on a literature search to obtain a blend of different types of systematic reviews, as well as to obtain information about yeasts in different niches (focusing mainly on the intestinal, vaginal and oral mucosae). The articles and dissertations mentioned were obtained through the following platforms: Scielo; PubMed and Web of Science. The titles of the subjects and the keywords chosen were mycobiome, intestinal microbiota, skin yeasts, fungi, vaginal microbiota, oral microbiota, vulvovaginal diseases and skin diseases. The inclusion criteria were language (Portuguese and English), text availability and preferred publication date (last 5 years).\n\nThe different mycobiomes in the human body not only influence a particular niche, but rather, a range of niches in the human body, due to the interrelationships between the different microbial communities.\nIn recent years, there has been an increase in human invasive fungal infections, mostly caused by pathogens such as  Candida  spp.; however, some less common ones, such as  Rhodotorula  spp., have been recognized as emerging pathogens [ 10 ]. Transmission of mycobioma of the skin and gastrointestinal tract is thought to be via the vaginal route during delivery, composed mostly of Candida albicans [ 11 ].\nAlthough the skin is the niche where  Malassezia  is most commonly found, in recent years it has been identified in other unrelated niches, including the human gut, breast milk and internal organs, including those of the central nervous system [ 10 ]. Transmission of  Malassezia  spp. by this way is suggested, although there is controversy as to whether transmission is initiated from the placenta [ 12 ]. Therefore, the fungal diversity of the skin and gut of newborn babies is derived from the maternal mycobioma [ 13 ].\nCryptococcus neoformans  is a known human pathogen, particularly in the immunocompromised host. However, other species of this genus can be also found colonizing several human niches [ 14 ]. In  Figure 2 , we summarize the fungal communities that inhabit the four different niches under analysis. We were able to conclude that  Candida  spp. is the common fungus present in the microbiota of the skin, vagina, intestine and oral cavity; on the other hand, the three other yeasts analyzed in this review, that correspond to the ones that are frequent colonizers of the human body but also are human pathogens, do not inhabit a niche only. Therefore, our research in this subject has revealed that these four genera possibly comprise opportunistic yeast species.\nThis genus is one of the most commonly found in the normal human microbiota, managing to colonize niches such as oral mucous membranes, skin, gastrointestinal, genital and urinary tracts without causing infections. However, in immunocompromised people or people with chronic illnesses, this yeast can become pathogenic, thus causing infections called candidiasis [ 15 ]. Within this genus, there are species that are one of the most frequent causes of opportunistic infections,  Candida albicans , the drug resistant,  C. glabrata , the new global threat to public health,  C. auris  and other emerging species such as  C. tropicalis ,  C. parapsilosis  and  C. krusei  [ 16 , 17 ].  C. albicans  is a yeast that can reside in the human body while also living in certain environmental reservoirs [ 18 ]. Disturbances induced either by antibiotics, immune system anomalies, alterations in the microbiome and/or alterations in the integrity of the mucocutaneous barrier allow  Candida  spp. to become an opportunistic pathogen in the context of a series of virulence factors [ 19 , 20 ].\nC. albicans  colonization has been found in the vaginal niche by culture-dependent methods in approximately 20% to 30% of asymptomatic women and by molecular methods in about 65% [ 21 ]. When the infection is symptomatic, it is defined as vulvovaginal candidosis (or candidosis), 80% to 92% of which are caused by  C. albicans  [ 22 ].\nThe success in the occurrence of infections comes from specific characteristics of this genus. One is the change from a yeast form to a branched hyphal form. As a rule, the yeast form is associated with commensalism, dissemination, or transmission of the infection [ 23 ]. The balance between the two forms is maintained by the responses of the innate immune system [ 24 ].\nThe genus  Cryptococcus  is a group of capsulated opportunistic fungi. Of the 37 species discovered,  C. neoformans  and  C. gattii  are the species that most cause pathogenesis in man [ 14 ].  Cryptococcus  spp. is an environmental yeast and is therefore very recurrently found in nature (soil, animal feces, among others).  C. neoformans  essentially affects immunocompromised individuals (elderly people, patients with HIV/AIDS and recently transplanted organs) in addition to causing infections of the central nervous system [ 25 ]. It is estimated that annually, 223,000 people with HIV/AIDS develop cryptococcal meningitis, of which 181,000 eventually die [ 26 ].\nC. gatti  manages to infect both immunocompetent and immunosuppressed individuals, obtaining a mortality rate of almost 33% [ 27 ]. That said, this genus has become a lethal pathogen that poses a serious threat to public health [ 27 ].\nThis yeast is included in the genus Basidiomycota, class  Malasseziomycetes  and family  Malasseziaceae  [ 28 ].  Malassezia , a commensal yeast that is lipophilic and lipid dependent for its growth, except  M. pachydermatis , is the main component of the fungal skin microbiota of many mammals, corresponding to more than 90% of the total fungal population in the skin niche [ 29 ].\nThe mechanisms by which these yeasts can trigger diseases are poorly studied and, therefore, are not yet clearly identified. However, the proposed hypothesis is that the diseases can be induced by direct invasion of fungal filament tissue or indirectly by immunological and metabolic mechanisms [ 30 , 31 , 32 , 33 ].\nStudies already carried out on the composition of the fungal microbiota of breast milk revealed two new pieces of information [ 32 , 34 ]: in its composition, the proportion of fungi is much greater than that of bacteria (contrary to what is found in many other niches) and among the fungi found, 40% of the total of genera identified by pyrosequencing are represented by  Malassezia .\nDifferent species of  Malassezia  spp. can lead to different clinical manifestations, which can range from hypopigmentation without visible inflammation, to eczema with scaling and inflammation. Of all the skin diseases, although still controversial, malassezia is a pathogenic agent of pityriasis versicolor and malassezia folliculitis. The discovery of this yeast in seborrheic areas still needs further study since its commensal state is still difficult to distinguish from its pathogenic phase [ 35 ]. Scientific evidence for the role of  Malassezia  in psoriasis and atopic dermatitis is less robust than for other diseases [ 36 ].\nThe pathogenic role and its relevance in the intestinal microbiota have not been studied nor seen as important by researchers, although they are frequently found in human stool samples [ 37 , 38 ]. However, human fungal infections by  Rhodotorula  spp. have been increasing in recent years [ 39 ]. Until 2018, clinical cases have been reported, where of the 40 species,  R. mucilaginosa  is the most common cause of human infection (72%), followed by  R. glutinis  (21%) and  R. minuta  (7%) [ 40 , 41 , 42 ].\nThis yeast can be found as contaminants of the skin, nails, lungs, urine, feces, central nervous system and blood. The infection can be installed endogenously or exogenously, as well as by the hands of health professionals, contaminated materials and inanimate environmental sources [ 43 ]. In addition to these transmission routes,  Rhodotorula  spp., being an environmental yeast, can be found in nature by isolating it from environmental sources such as air, soil and plants [ 44 ].\nIt is estimated that there are about three million species of fungi, of which about 300 cause infections in humans ( Table 1 ) [ 45 ]. In studies previously performed, the authors identified about 66 different genres of fungi present, the most prevalent being  Candida ,  Cladosporium  and  Saccharomyces  [ 46 ].\nThe ecological relationships between bacteria, archaea, viruses and fungi maintain the host’s equilibrium and are part of several vital processes, such as nutrition and protection against pathogens. Being one of the largest eukaryotic kingdoms, fungi have a variety of life cycles with adaptations in metabolism and morphogenesis that allow them to adapt to environmental changes and give them survival throughout the human body, such as the intestine, skin, oral cavity, urogenital tract and digestive system [ 47 ]. There is a correlation between the occurrence of changes in the fungal community and the diseases caused in humans.\nIllnesses that are caused by colonizing fungi. The upward and downward arrows represent the increase and decrease, respectively, of fungi colonisation in the various diseases.\nThe most studied fungal phyla present in the human intestine have been  Ascomycota  as the most predominant phylum in the intestine, followed by  Zygomycota  and  Basidiomycota  [ 69 , 70 , 71 ]. A newborn’s gut microbiome is highly dependent on its diet. Breastfed babies have higher levels of bacteria belonging to the Actinobacteria class [ 72 , 73 ] and the genera belonging to  Lactibacillus  and  bifidobacterium  [ 72 , 73 , 74 , 75 ]. High amounts of oligosaccharides and various fatty acids that make up breast milk positively influence the intestinal microbiome and their metabolites may help to act against hypersensitivity (allergy) and asthma counter reactions [ 76 ].\nSaccharomyces  is a non-pathogenic yeast, thermotolerant and resistant to the action of gastric, enteric and pancreatic juices.  Saccharomyces  has been used commercially in the production of probiotic foods. In recent decades,  S. cerevisiae  and  S. boulardii  have shown great promise as probiotic treatments [ 77 ]. Several studies have shown  S. cerevisiae  and  S. boulardii  to be associated with an increase in the proportion of  Bacteroidetes  in the gut microbiota composition and a decreased relative abundance of  Firmicutes  and  Proteobacteria . Furthermore, this yeast can prevent inflammation by promoting pro-inflammatory immune function and increasing the production of short-chain fatty acids [ 77 , 78 , 79 , 80 ].\nOther studies have also reported that  Malassezia ,  Candida  and  Saccharomyces  constitute the intestinal microbiota, with  S. cerevisiae ,  M. restricted  and  C. albicans  identified in 96.8%, 88.3% and 60.8% respectively [ 81 ]. Similar to other microbial communities, fungi also have a high capacity to produce metabolites that can be applied for medicinal or therapeutic purposes. In addition to this functionality, they can also influence host homeostasis, causing biological effects on them as part of fungus–host interactions [ 82 ].\nChanges in the gut microbiome are associated with increased disease and dysbiosis of fungal communities which may contribute to susceptibility or increased disease severity. It is important to note that these changes, when caused by external influences, will facilitate invasion and that gastrointestinal (GI) infections affect the composition of the microbiota.\nIn CD (Crohn’s disease), there is an increase in the fecal fungal burden as well as an increase in the  Basidiomycota — Ascomycota  ratio. Regarding the opportunistic yeasts that cause this disease, an increase in the abundance of  C. albicans ,  C. tropicalis ,  C. glabrata ,  Aspergillus clavatus ,  Cryptococcus neoformans ,  Cyberlindnera jadinii ,  Clavispora lusitaniae ,  Debaryomyces hansenii  and  Kluyveromyces marxianus  and a decrease in  Saccharomyces cerevisiae  have been found [ 48 , 49 , 50 , 51 ].\nAlthough not considered a critical illness, it affects about 10–15% of individuals, reducing their quality of life [ 30 ]. Studies have found an increase in the prevalence of  Candida  spp. (notably  C. albicans ),  Malassezia  spp.,  Cladosporium  spp. and  Saccharomyces cerevisiae . In contrast, yeasts such as  Mycosphaerella  spp.,  Aspergillus  spp.,  Sporidiobolus  spp. and  Pandora  spp. suffer a decrease in their abundance [ 52 , 53 , 83 , 84 ].\nThe intestinal microbiome and its changes have been associated with the pathogenesis of diseases such as colorectal adenoma, which induces colorectal cancer (CRC), esophageal squamous cell carcinoma (ESCC), gastric cancer, hepatocellular carcinoma (HCC), cholangiocarcinoma and pancreatic ductal adenocarcinoma (PDAC) [ 85 ].\nPatients with these pathologies and with polyps have been observed with fungal dysbiosis, having a very high  Ascomycota/Basidiomycota  ratio leading to the expansion of opportunistic fungi [ 55 ].\nSpecies such as  Trichosporon  and  Malassezia  were considered two of the populations capable of facilitating the progression and growth of colorectal cancer [ 54 ]. Other studies have found that although no single type has been identified as the single leading cause, there is evidence to show an association between  Fusobacterium  species ( F. mortiferum ,  F. nucleatum , and  F. necrophorum ) with colorectal cancer [ 37 ]. Lev et al. also demonstrated that people with this pathology contain higher percentages of  Klebsiella ,  E. coli ,  Streptococcus  and  Enterococcus  as opposed to  Rothia  which is in low amounts [ 86 ].\nThere are many causes that can lead to obesity, such as genetic, lifestyle and environmental factors. However, in addition to these causes, the intestinal microbiota plays a key role in the presence and development of obesity. The microbiota of overweight individuals has a greater capacity for fermentative processes and for capturing energy from the diet. It also has a high proportion of  Firmicutes  to  bacteroides / Prevotella  leading to an increase in the microbiota gene involved in polysaccharide degradation and an increase in SCFAs [ 87 ].\nCandida ,  Nakaseomyces ,  Penicillium ,  Porphyromonas ,  Campylobacter ,  Bacteroides ,  Staphylococcus ,  Parabacteroides ,  Dialister  and  Ruminococcus  have been the most identified genera in overweight individuals while Mucor racemosus and M. fuscus have been found the most in non-obese patients [ 55 , 56 ]. It is now obvious that obesity is somehow associated with gut dysbiosis, low-grade inflammation and a host of metabolic disorders.\nAutoimmune diseases, unlike IBD, are directly linked to an abnormal development of the intestinal microbiota throughout life. A study on the quantitative changes in  Candida  species in patients with real-time PCR (qPCR) DM1 and DM2 was carried out, where  C. albicans  was the most common strain found in the stool of these patients [ 55 ]. However, no significant changes were observed between DM1 and DM2 patients in terms of  C. albicans  colonization [ 55 ].\nRegarding genetics, nutrition and lifestyle choices, these also influence the prevalence of DM1, particularly if we are to assess the incidence rate in various countries around the world. Other studies have reported, in addition to an increase in  C. albicans , that in type 1 diabetes there is also an increase in the genus  Saccharomyces  [ 57 ]. In type 2 diabetes, in addition to an increase in  C. albicans , there an increase in  Cladosporium  spp.,  Meyerozyma  spp.,  Mortierella  spp. and  Aspergillus  spp. has also been found [ 58 ].\nIn addition, it has been speculated that the high prevalence of  Saccharomyces  may be due to the consumption of foods containing yeast (beer and bread), while the high percentage of  Candida  is related to the consumption of carbohydrates [ 46 ].\nDespite the abundance of the mycobiome, the candida genus remains one of the few that unquestionably contributes to the emergence of the most common infections in the oral mucosa [ 6 ]. However, although fungi that are present in a smaller percentage do not have a significant influence on metabolic activities, they can play a modulator role in immune responses or an opportunistic pathogenic role under surveillance conditions, harming the host [ 6 ]. In some rare cases, involving severe immunosuppression,  Cryptococcus  spp. and  Aspergillus  spp. were described as causing lesions in the oral mucosa [ 58 ].\nOPC can be classified into three main conditions: acute, chronic and chronic mucocutaneous candidosis [ 63 ]. Some of the risk factors include nutritional deficiencies, local dysbiosis, salivary hypofunctions, smoking, use of dentures and a dysfunction in T-cell immunity [ 63 ].  C. albicans  is the fungus responsible for causing this disease, and life-threatening systemic infections can develop when this fungus enters the bloodstream [ 63 ].\nThe role of the oral mycobiome in caries has been a recent focus. A study by Baraniya in 2020 found that advanced caries were associated with an abnormal increase in the prevalence and abundance of  C. albicans  in adults and of  C. dubliniensis  in children [ 64 , 65 ].\nFurthermore, one study reported a regressive trend in mycobiome diversity as caries severity increased [ 66 ]. Interestingly, in another study,  C. albicans  was associated with severe disease, while  C. dubliniensis  was shown to have a gradual and steady increase as the disease set in and grew [ 66 ]. On the other hand, in caries-free children, one of the most common fungi was found to be  Malassezia globosa  [ 64 ].\nIn summary, it is thought that  C. albicans  will be involved in more advanced lesions while  C. dubliniensis  plays a pivotal role earlier in the disease process. Some investigations suggest that some fungi are found in caries-free children, namely  Debaromyces  spp.,  Rhodotorula  spp. or  malassezia  spp. [ 64 , 66 ].\nAlthough fungal communities have already been detected in the subgingival plaque, their role is still unclear. Research has reported an increase in yeast detection, namely  Candida  spp., in subjects with periodontitis [ 67 ]. However, this study was carried out through cultures, because when tests were carried out at the molecular level, this increase was not verified [ 68 ]. This study saw that this pathogenesis was associated with a decrease in microbiome diversity and changes in the relative richness of two genera (decrease in  S. cerevisiae  and an increase in  Filobasidiales  species) when compared to individuals with or without mild disease [ 68 ].\nMost studies on the microbiome focus on understanding the bacterial composition, but the microorganisms present on the skin are not limited to bacteria, but also include viruses, fungi and arthropods. Notably, the skin being the largest organ in the human body, it serves as a defensive obstruction against possible injury and microbial attack [ 36 ].\nPityriasis versicolor is a chronic skin infection characterized by discrete or confluent, scaly, dark or depigmented patches, mainly on the upper torso, but which can extend to the neck, abdomen and other locations. The  Malassezia  species identified mainly in pityriasis versicolor lesions are  M. globosa  and  M. sympodialis  and  M. furfur . In addition to the findings of the action of this yeast in pityriasis versicolor, there are two facts that further emphasize its role in this disease: (i) a positive culture is found more in samples cultured from skin lesions than from healthy skin [ 59 ], and (ii) in its hyphal form, it is usually found in samples obtained from pityriasis versicolor lesions, regardless of the species of malassezia present [ 88 ].\nThe relationship between malassezia colonization and seborrheic dermatitis was first described by Louis-Charles Malassez in 1874. Seborrheic dermatitis is an inflammatory dermatosis with a predilection for anatomical areas with a high concentration of sebaceous glands, such as the middle third of the face, chest, back and scalp. The prevalence of seborrheic dermatitis also increases with age (2.0% in <35 years; 3.6% in 35–64 years; 4.4% ≥65 years) and there is an association with other fungal diseases such as tinea pedis, onychomycosis and pityriasis versicolor. The age dependence of seborrheic dermatitis is probably responsible for the increased prevalence (14.3%) reported in the Rotterdam study [ 89 ].\nFungi such as  Malassezia  is found in the sebaceous gland where lipids are the main source of energy. As this yeast is usually commensal, the mechanism that triggers this disease has been increasingly investigated [ 60 ]. Its DNA has been detected in about 90% of skin lesions; the skin lesions of atopic dermatitis and colonization increase with the severity of the disease [ 90 ].\nPsoriasis is an inflammatory skin disease that affects approximately 2% of the world’s population. Little is known about the role of the skin microbiome in psoriasis. However, some studies have already been carried out on this disease. In one study, an increase in  Brevibacterium ,  Kocuria palustris  and  Gordonia  was found to be associated with back and elbow injuries [ 61 ]. In that same study, it was also found that there was a high abundance of the  Malassezia  fungus. Strains such as  M. biota  were detected on the coast, while  M. sympodialis  dominated elbow mycosis [ 61 ].\nIn psoriatic elbow skin, there was found to be a significant correlation between the occurrence of  Kocuria ,  Lactobacillus  and  Streptococcus  with  Saccharomyces , which was not observed in healthy skin [ 62 ].\nThe composition of the female microbiota varies according to age, hormone production, menstrual cycle, drug use and sexual activity. Evidence has shown that the composition of the microbiota is important for reproductive and genital tract health.\nMucosal infections, characterized by fungal colonization associated with an inflammatory host response, are extremely common and cause a negative impact on the quality of life of patients.\nVVC is a multifactorial condition with an abnormal immune system response. Undeniably, colonization is carried out by both bacteria and fungi, with communication between them through physical interactions, production of metabolites and chemical interactions, among others. Most in vivo studies have revealed an inhibitory capacity on the part of  Lactobacillus  on the growth, dimorphic transition, virulence factors and biofilm formation of  C. albicans  [ 8 ]. For example, about 75% of women of childbearing age have at least one episode of VVC and up to 9% of them suffer from recurrent VVC (more than four episodes per year) [ 63 , 91 , 92 ]. Some of the risk factors associated with this pathogenesis include a very high level of estrogen, the use of oral contraceptives and uncontrolled diabetes [ 63 ].\nMost clinicians accept that Candiduria is defined as colonization or contamination, but it may be the only sign of invasive candidosis. The consensus is that Candiduria is quite common in hospitalized patients [ 93 , 94 ]. The spread of the disease usually happens through contamination. It is usually diagnosed in elderly patients, with  Candida  being the most frequently isolated microorganism in nosocomial urinary tract infections (UTIs). Overall,  C. albicans  accounts for 50 to 70% of cases, followed by  C. glabrata  and  C. tropicalis , which is the third most common species. There are studies that claim that the use of antibiotics is associated with an increase in their incidence; however, these estimated values can be disregarded, as the standard urine culture is not very sensitive [ 95 ].\nFrom all the studies carried out in patients with Candiduria, a set of risk factors that are associated with an increase in its incidence was gathered. In diabetes mellitus, frequent or prolonged use of antimicrobials, broad-spectrum antibiotics (which suppress the gastrointestinal and genital flora), can lead to ICU stay and use of indwelling catheters, among others.\nStudies indicate that alterations in intestinal microbial diversity (dysbiosis) can lead to an increase in host susceptibility and an interruption of mucosal immune tolerance, which will influence skin health in the future [ 96 ]. Other studies have associated a bidirectional interaction between gastrointestinal health and skin homeostasis through the metabolic activity and immune impact of the gut microbiome [ 97 , 98 ]. For example, commensal intestinal microorganisms can promote skin allostasis by controlling T-cell differentiation [ 97 ].\nAlthough a healthy gut microbiota is critical to host health, overgrowth of the host and changes in diversity can also result in disease processes, for example, patients with Crohn’s disease are also found primarily with psoriasis as a comorbidity [ 99 , 100 ].\nStudies carried out deepening the knowledge of the possible relationship between the intestinal and oral microbiome reported that patients with intestinal diseases exhibited a considerable abnormal increase in oral microorganisms in the lumen and in the tissues of the intestinal mucosa [ 101 , 102 ]. Thus, it is plausible to believe that the cavity serves as a reservoir; however, we have still not obtained comprehensive information on which microorganisms act as pathogens.\nSome researchers have already found evidence that suggests the existence of a gut–vagina axis. There are already some data that the intestinal and vaginal microbiota can be completely linked, for example, in the onset and progression of endometriosis [ 103 ]. Yet, another study about this connection between endometriosis and the intestine showed that after 42 days of persistence of endometrial injury, a distinct intestinal microbiota develops [ 104 ], that is, not only does the intestinal microbiota change the vaginal one, but vice versa.\nThese hypotheses open many doors to new preventive, diagnostic and therapeutic possibilities, and are therefore an emerging area for investigation.\n\nWith this review, we intended to synthesize the existing literature about the microbiota and mycobiota. In addition, we highlighted the communities that live in the different niches of the human body, namely the oral, vaginal, intestinal and skin cavities, as well as the diseases that occur when there are changes in the composition of the microbiota.\nIt should be noted that the importance and significant role that fungal communities play in human health are becoming increasingly more evident. However, more studies are needed to understand the effect of geography on the human mycobiome. Other aspects such as dietary habits or age should also be studied in detail to provide a deeper knowledge on the dynamics of the human mycobiome.\nFurthermore, future research involving the study of the mycobiome should have the evolution and improvement in the health of humans, animals and also plants as an objective, as well as the improvement in the ecosystem as a whole. The increase in the amount of data available on mycobiomes allows for a greater knowledge to be attained on their use in increasing productivity and possible therapies. One of the major problems for health is the acquisition of resistance of microorganisms to antifungal drugs.\nTo contribute to possible alternatives for solving this problem, systems based on the mycobiome for monitoring and sanitization are being tested and considered good solutions, especially in the hospital environment. When the imbalance caused at a more specific level is known, it is possible to know in which human niches there are greater chances of proliferation of these pathogens. In addition, sanitizing products containing probiotics are already being developed to modulate these mycobiomes, making it difficult for pathogens to appear in these environments.\nTherefore, for future work, it is crucial to start including these yeasts in studies so that we begin to have a more realistic perception of the action of fungal communities in the homeostasis of the microbiota and, consequently, of human health, and to further deepen the knowledge of clinical mycology.","source_license":"CC-BY-4.0","license_restricted":false}