The human microbiome has become an area of interest in how the many microbial communities symbiotically inhabit the human body, and may affect human health. Current research aims to understand the many microbiomes’ possible roles in influencing host immune responses, inflammation, and metabolic pathways throughout the human body, and their contribution to various diseases. Studies have suggested that the gut microbiome can create conditions leading to cancer progression as well as immunological suppression (1). Recently, studies have suggested that alongside the gut microbiome, the microbiomes of the female reproductive system may play a similar role in gynecologic cancers (2). Gynecologic cancers, malignancies that affect the female reproductive tract, account for up to 30,000 deaths and 100,000 cancer diagnoses per year in the United States (3). The microbiomes of interest with regards to these cancers include the gut, vagina, and upper female reproductive tract, specifically the endometrium.
The vagino-uterine microbiome has only recently become an area of interest, and whether the female reproductive tract has distinct microbial communities aside from the vaginal microbiota has been a frequent debate. Various studies have identified the vaginal microbiota of multiple women and have observed changes that correlate alongside the menstrual cycle suggesting that bacterial composition may be regulated hormonally (4, 5, 6). These studies found that the Lactobacillus species was the majority species of most vaginal microbial communities among large groups of women (4, 5, 6). A large abundance of Lactobacillus is considered healthy due to the protective characteristics it provides including the production of lactic acid which maintains the low pH of the vagina protecting against pathogens (4, 7). As different Lactobacillus species dominate the mucosal layer of the vagina, five community state types of vaginal microbiotas have been identified in healthy women (4, 6, 7). Type I is dominated by Lactobacillus crispatus, Type II being L. gasseri, Type III being L. iners, Type IV being L. jensenii, and Type V being more diverse with higher anaerobic bacterial abundance (4, 6). However, healthy women with vaginal microbiotas containing a low abundance of Lactobacillus species have also been observed suggesting that more research must go into what indicates a “healthy” vaginal microbiota (6). A lack of Lactobacillus and an overgrowth of facultative anaerobes, resulting from microbial dysbiosis, can lead to many infections such as bacterial vaginosis and can even increase an individual’s risk for developing gynecological cancers through unknown mechanisms (4, 6, 7, 8). Vaginal microbial dysbiosis has been associated with changes to the mucosal environment, an increase in pro-inflammatory cytokines, immune cell activation, and epithelial barrier disruption (9, 10). These disruptions are linked to multiple adverse health outcomes and cancers including an increased risk of preterm birth, HPV infection, and cervical cancers (4).
Until recently, the vagina was the only part of the female reproductive system believed to contain its own microbiome. However, new studies have identified an upper female reproductive tract microbiome using 16S ribosomal RNA gene sequencing (7). This upper reproductive tract includes the uterine cavity and peritoneum, where the fallopian tubes and ovaries reside (5). The conclusions drawn suggested that the uterine cavity is not sterile, and that the endometrium, the inner epithelial and mucosal membrane layer that lines the uterus and changes depending on the menstrual cycle, contains its own microbiome (2, 11, 12). This endometrial microbiota differs from the vaginal microbiota as the biomass of bacterial cells decreases, Lactobacillus abundance decreases, and diversity increases (2, 11, 12). Fang and colleagues also compared the bacterial communities of the vagina and endometrium and identified Proteobacteria, Firmicutes, and Actinobacteria as the dominant species within the endometrial microbiome (13). However, other studies have found contrasting results with Bacteroides, Pseudomonas, and Vagococcus being the dominant species identified in the endometrial microbiome suggesting that the dominant bacterial species may be dependent on the individual, the menstrual cycle phase, or the menopausal phase (pre-, peri-, and post-) (2, 14). While studies are limited, it has been suggested that the endometrial microbial community composition also differs based on the individual’s fertility status, the presence of endometrial cancers, and those with endometriosis (12). In the study conducted by Walther-Antonio et al. patients with endometriosis had Shigella and Barnesiella as the most dominant genera among the endometria (15).
The gut microbiome has also been linked to affect the female reproductive system in a multitude of ways, including the development of endometriosis. Endometriosis is a common gynecological disease that consists of the proliferation and bleeding of lesions outside the uterus, primarily found on the ovaries and throughout the abdominal cavity, causing pain and potential infertility (16). The cause of this disease appears to be multifactorial, one of the factors being cyclic estrogen levels as studies have suggested lesion growth is estrogen-driven, and these lesions lead to chronic inflammation and activation of the body’s immune system (17). These specific factors have led researchers to view dysbiosis within the gut microbiota as a potential cause of endometriosis because it regulates multiple inflammatory cytokines as well as oestrogen metabolism (17). Based on the host’s condition and lifestyle, the gut microbiota can cause an increase in CD4+ T-lymphocyte activation which produces IL-17, a pro-inflammatory cytokine (18, 19, 20). Studies have suggested that high levels of IL-17 are important for the development of endometriosis due to its resulting effect in chronic inflammation and the hypervascularization of peritoneal surfaces (18, 19, 20). Leonardi and colleagues found that in patients with endometriosis, levels of Proteobacteria, Enterobacteriaceae, Streptococcus, and E. coli were elevated within the gut; However, their direct role in promoting the development of endometriosis was unknown (21).
The relationship between gut and endometrial microbiome dysbiosis and inflammation and estrogen regulation has been investigated for its role in endometrial cancer. There are two types of endometrial cancer with Type I being estrogen-dependent and Type II being estrogen independent, and Type I endometrial cancer affects more women than Type II (22, 23). Studies have observed a difference in the microbial composition of the endometrium between healthy patients and those with hyperplasia, the initial stage of cancer development, which suggested the role this microbiota may have in precancerous cell transformation of the endometrium. Walther-Antonio et al. sequenced the microbial communities from endometrial cancer patients and found that the taxa Firmicutes, Spirochaetes, Actinobacteria, Bacteroidetes, and Proteobacteria were enriched when compared to health patients. However, the closest correlation was found between the species Atopobium vaginae and Porphyromonas species and endometrial cancer. Porphyromonas gingivalis has already been considered a biomarker of increased risk of death from aerodigestive cancer, while Atopobium vaginae has been associated with bacterial vaginosis due to its role it producing pro-inflammatory cytokines leading to inflammation (15). Walther-Antonio et al. suggested that the resulting chronic inflammation leads to dysregulation of the host immune system which can facilitate infection by Porphyromonas species. The gut microbiome’s possible role in the development of endometrial cancers is due to its involvement in estrogen level regulation through bacteria whose products can metabolize estrogens, specifically bacteria with β-glucuronidase activity (24, 25). β-glucuronidase activity has a significant role in generating carcinogenic metabolites within the intestine, promoting the reabsorption of estrogens into the bloodstream, and facilitating the binding of estrogen receptors (24, 25). Through dysbiosis of the gut microbiome, an increase in bacteria with β-glucuronidase activity is possible which causes greater reabsorption and receptor binding of estrogens which greatly contributes to the development hyperplasia of the endometrial layer and endometrial cancer (23, 24, 25, 26).
The human microbiomes have key roles in regulating human physiological processes, and recent studies have begun looking at the relationships between the microbiomes of the female reproductive system and the gut and gynecological diseases such as endometriosis and endometrial cancer. These microbiomes are involved in estrogen regulation and can initiate inflammatory conditions leading to immunological changes all of which ultimately lead to the development of cancer. Dysbiosis of these microbiomes cause a shift in the microbial composition, and bacteria that produce pro-inflammatory cytokines can become dominant leading to chronic inflammation which is involved in the development of endometrial lesions and tumors. More research is required to further understand the microbial compositions of the female reproductive system microbiomes and identify bacteria that may have a direct role in causing these gynecological diseases as well as the mechanisms at which the microbiota affects the endometrium during its diseased state.
References
Vétizou M., Pitt J.M., Daillère R., Lepage P., Waldschmitt N., Flament C., Rusakiewicz S., Routy B., Roberti M.P., Duong C.P.M., et al. Anticancer immunotherapy by CTLA-4 blockade relies on the gut microbiota. Science. 2015;350:1079–1084.
Chen C., Song X., Chunwei Z., Zhong H., Dai J., Lan Z., Li F., Yu X., Feng Q., Wang Z., et al. The microbiota continuum along the female reproductive tract and its relation to uterine-related diseases. Nat. Commun. 2017;8:1–11.
Cancer Stat Facts: Ovarian Cancer. [(accessed on 12 February 2021)]; Available online: https://seer.cancer.gov/statfacts/html/ovary.html
Song, S. D., Acharya, K. D., Zhu, J. E., Deveney, C. M., Walther-Antonio, M. R., Tetel, M. J., & Chia, N. (2020). Daily vaginal microbiota fluctuations associated with natural hormonal cycle, contraceptives, diet, and exercise. MSphere, 5(4), e00593-20.
Van De Wijgert, J. H., Borgdorff, H., Verhelst, R., Crucitti, T., Francis, S., Verstraelen, H., & Jespers, V. (2014). The vaginal microbiota: what have we learned after a decade of molecular characterization?. PloS one, 9(8), e105998.
Gajer, P., Brotman, R. M., Bai, G., Sakamoto, J., Schütte, U. M., Zhong, X., ... & Ravel, J. (2012). Temporal dynamics of the human vaginal microbiota. Science translational medicine, 4(132), 132ra52-132ra52.
Rizzo AE, Gordon JC, Berard AR, Burgener AD, Avril S. The Female Reproductive Tract Microbiome-Implications for Gynecologic Cancers and Personalized Medicine. J Pers Med. 2021 Jun 11;11(6):546.
Lehtoranta, L., Ala-Jaakkola, R., Laitila, A., & Maukonen, J. (2022). Healthy Vaginal Microbiota and Influence of Probiotics Across the Female Life Span. Frontiers in Microbiology, 787.
Shannon B., Gajer P., Yi T.J., Ma B., Humphrys M.S., Thomas-Pavanel J., Chieza L., Janakiram P., Saunders M., Tharao W., et al. Distinct Effects of the Cervicovaginal Microbiota and Herpes Simplex Type 2 Infection on Female Genital Tract Immunology. J. Infect. Dis. 2017;215:1366–1375.
Anahtar M.N., Byrne E.H., Doherty K.E., Bowman B.A., Yamamoto H.S., Soumillon M., Padavattan N., Ismail N., Moodley A., Sabatini M.E., et al. Cervicovaginal Bacteria Are a Major Modulator of Host Inflammatory Responses in the Female Genital Tract. Immunity. 2015;42:965–976.
Moreno I., Codoñer F.M., Vilella F., Valbuena D., Martinez-Blanch J.F., Jimenez-Almazán J., Alonso R., Alamá P., Remohí J., Pellicer A., et al. Evidence that the endometrial microbiota has an effect on implantation success or failure. Am. J. Obstet. Gynecol. 2016;215:684–703.
Khan K.N., Fujishita A., Masumoto H., Muto H., Kitajima M., Masuzaki H., Kitawaki J. Molecular detection of intrauterine microbial colonization in women with endometriosis. Eur. J. Obstet. Gynecol. Reprod. Biol. 2016;199:69–75.
Fang R.-L., Chen L.-X., Shu W.-S., Yao S.-Z., Wang S.-W., Chen Y.-Q. Barcoded sequencing reveals diverse intrauterine microbiomes in patients suffering with endometrial polyps. Am. J. Transl. Res. 2016;8:1581–1592.
Verstraelen H., Vilchez-Vargas R., Desimpel F., Jauregui R., Vankeirsbilck N., Weyers S., Verhelst R., De Sutter P., Pieper D.H., Van De Wiele T. Characterisation of the human uterine microbiome in non-pregnant women through deep sequencing of the V1-2 region of the 16S rRNA gene. PeerJ. 2016;4:e1602.
Walther-António M.R.S., Chen J., Multinu F., Hokenstad A., Distad T.J., Cheek E.H., Keeney G.L., Creedon D.J., Nelson H., Mariani A., et al. Potential contribution of the uterine microbiome in the development of endometrial cancer. Genome Med. 2016;8:1–15.
Krog, M. C., Madsen, M. E., Bliddal, S., Bashir, Z., Vexø, L. E., Hartwell, D., ... & Nielsen, H. S. (2022). The microbiome in reproductive health: protocol for a systems biology approach using a prospective, observational study design. Human Reproduction Open, 2022(2), hoac015.
Burney, R. O., & Giudice, L. C. (2012). Pathogenesis and pathophysiology of endometriosis. Fertility and sterility, 98(3), 511-519.
Costello, M. E., Robinson, P. C., Benham, H., & Brown, M. A. (2015). The intestinal microbiome in human disease and how it relates to arthritis and spondyloarthritis. Best Practice & Research Clinical Rheumatology, 29(2), 202-212.
Ivanov, I. I., Atarashi, K., Manel, N., Brodie, E. L., Shima, T., Karaoz, U., ... & Littman, D. R. (2009). Induction of intestinal Th17 cells by segmented filamentous bacteria. Cell, 139(3), 485-498.
Laschke, M. W., & Menger, M. D. (2016). The gut microbiota: a puppet master in the pathogenesis of endometriosis?. American journal of obstetrics and gynecology, 215(1), 68-e1.
Leonardi, M., Hicks, C., El‐Assaad, F., El‐Omar, E., & Condous, G. (2020). Endometriosis and the microbiome: a systematic review. BJOG: An International Journal of Obstetrics & Gynaecology, 127(2), 239-249.
Setiawan, V. W., Yang, H. P., Pike, M. C., McCann, S. E., Yu, H., Xiang, Y. B., ... & Australian National Endometrial Cancer Study Group. (2013). Type I and II endometrial cancers: have they different risk factors?. Journal of Clinical Oncology, 31(20), 2607.
Boutriq, S., González-González, A., Plaza-Andrades, I., Laborda-Illanes, A., Sánchez-Alcoholado, L., Peralta-Linero, J., ... & Queipo-Ortuño, M. I. (2021). Gut and Endometrial Microbiome Dysbiosis: A New Emergent Risk Factor for Endometrial Cancer. Journal of Personalized Medicine, 11(7), 659.
Flores R., Shi J., Fuhrman B., Xu X., Veenstra T.D., Gail M.H., Gajer P., Ravel J., Goedert J.J. Fecal microbial determinants of fecal and systemic estrogens and estrogen metabolites: A cross-sectional study. J. Transl. Med. 2012;10:253.
Beaud D., Tailliez P., Anba-Mondoloni J. Genetic characterization of the β-glucuronidase enzyme from a human intestinal bacterium, Ruminococcus gnavus. Microbiology. 2005;151:2323–2330.
Baker J.M., Al-Nakkash L., Herbst-Kralovetz M.M. Estrogen–Gut microbiome axis: Physiological and clinical implications. Maturitas. 2017;103:45–53.