The vaginal microbiota, host defence and reproductive physiology

The interaction between the human host and the vaginal microbiota is highly dynamic. Major changes in the vaginal physiology and microbiota over a woman's lifetime are largely shaped by transitional periods such as puberty, menopause and pregnancy, while daily fluctuations in microbial composition observed through culture‐independent studies are more likely to be the results of daily life activities and behaviours. The vaginal microbiota of reproductive‐aged women is largely made up of at least five different community state types. Four of these community state types are dominated by lactic‐acid producing Lactobacillus spp. while the fifth is commonly composed of anaerobes and strict anaerobes and is sometimes associated with vaginal symptoms. The production of lactic acid has been associated with contributing to the overall health of the vagina due to its direct and indirect effects on pathogens and host defence. Some species associated with non‐Lactobacillus vaginal microbiota may trigger immune responses as well as degrade the host mucosa, processes that ultimately increase susceptibility to infections and contribute to negative reproductive outcomes such as infertility and preterm birth. Further studies are needed to better understand the functional underpinnings of how the vaginal microbiota affect host physiology but also how host physiology affects the vaginal microbiota. Understanding this fine‐tuned interaction is key to maintaining women's reproductive health. Interaction of the human vaginal microbiota with physiology, host defence and reproduction. The host supplies glycogen and glycogen breakdown products via stimulation by oestrogen production as a carbon source to the vaginal microbiota, therefore favouring Lactobacillus growth. In turn, lactic acid and bacteriocins produced mainly by Lactobacillus spp. contribute to the host defence while species associated with community state type (CST)‐IV, aerobic vaginitis or bacterial vaginosis (such as Gardnerella vaginalis or Lactobacillus iners) contribute to mucosa degradation and susceptibility to other infections. Specific host defences including the innate immune response/inflammation, cell recruitment (such as NK cells or macrophages, pink and yellow circles, respectively), cytokines (pink ‘Y’ shapes) and antimicrobial peptides (AMPs; blue circles) are dependent on host genetic polymorphisms, CST and pathogen presence (such as Chlamydia trachomatis and HIV) and may also have a negative effect o