In recent years, the understanding of how the vaginal microbiome impacts health has expanded. The bacterial inhabitants of the vagina not only influence the direct local environment protecting it from infection, ranging from candida, bacterial vaginosis (BV) and sexually transmitted infection acquisition, it also impacts fertility, birth outcomes and possibly the success of IVF. Understanding of the vaginal microbiome environment offers insights into manipulation that have the propensity to change the clinical approach to female health.
As a microbiome site, the vaginal micro-environment is different to other human microbiome sites in the body, with high bacterial diversity considered to be a sign of dysbiosis. An imbalanced vaginal environment is usually characterised by a reduction in hydrogen peroxide (H2O2) and lactic acid-producing Lactobacilli spp. and an increase in polymicrobial facultative anaerobic organisms such as Gardnerella vaginalis, Mycoplasma hominis and Prevotella species. This shift of microbes is aligned with a subsequent shift in vaginal pH to a more alkaline nature. The optimal vaginal pH is considered to be less than 4.5. 
With higher lactobacilli counts considered to be aligned with eubiosis and strong first-line immune defence, a microbiome without a dominance in these bacteria is more prone to immunological issues. The species L. crispatus, L. iners, L. gasseri and L. jensenii, are specific to the human vagina, with a lactobacilli dominant vaginal environment considered to be unique amongst mammals.
The composition of the vaginal microbiome has been subtyped into communities based on their dominant microbe type. The work of Ravel (2011) identified five different community subtypes and each was aligned with their own benefits, risk and pH attributes. Four communities were dominated by Lactobacillus: L. iners, L. crispatus, L. gasseri or L. jensenii, whereas the fifth had lower proportions of lactic acid bacteria and higher proportions of strictly anaerobic organisms (prevotella, dialister, atopobium, gardnerella, megasphaera, peptoniphilus, sneathia, eggerthella, aerococcus, finegoldia and mobiluncus). The community subtypes and their presentations had statistical correlations with ethnicity. The research found that women in the study of Hispanic or Black ethnicity had a higher percentage of vaginal microbiome type CST IV not dominated by Lactobacilli spp. The associated low lactobacilli is believed to be governed by genetically determined differences between hosts and does not necessarily represent a disease state in itself.
Influences on the composition and balance within the environment are governed on a local as well as a systemic level. Hormonal fluctuations are closely implicated in the dominance of the lactobacilli within the vagina. Oestrogen is identified as having the biggest impact by upregulating epithelial cell density and the expression of glycogen, which is involved in the fermentative processes of the lactobacilli species and central to their microbiome survival. Conversely, low oestrogen states (such as those seen in menopause) are associated with an increasing vaginal microbiome diversity, increased risk of infection and vaginal atrophy.
Exogenous influences, like the use of sanitary items, lubricant quality and type of sexual and intimate contact, can also influence vaginal microbiome stability and composition. With most influences being attributed to pH changes, as well as microbe and biofilm introduction predisposing to infection and imbalance.[5,6,7]
Sexual interaction may have a considerable impact on the microbiome stability, with many clients reporting that symptom presentation is associated with sexual contact. Studying the interaction between microbiomes of sexual partners and their homogenisation has revealed that pathobiont microbe communities and clades are passed back and forth by unprotected sexual intercourse of an oral, digital and penetrative nature.
Vodstrcil (2017) performed comparative genome analysis on the vaginal microbiomes of women engaging in unprotected sex and found that the microbiome was more likely to be dominated by Gardnerella vaginalis and L. iners, which are both implicated in BV occurrence and dysbiosis. In addition, the analysis revealed four distinct G. vaginalis clades with acquisition of multiple clade types associated with unprotected penile-vaginal sex.
Understanding of commensal clade transmission suggests that antibiotic resistant and pathogenic biofilm G. vaginalis species could be transmitted through the act of intercourse resulting in BV. Research that has focussed on the microbiome of male anatomy and seminal discharge also confirms that non STI pathogenic microbes exist on the penile coronary sulcus and meatus and can be transmitted through sexual interaction.
As with many microbiome sites with external interaction, the influences on the vaginal microbiome are many. Understanding the basic eubiotic composition of the vagina is important to identify influences that may disturb and cause imbalance. Supporting lactobacilli dominance and vaginal pH are key components of care in the vaginal health.
- Onderdonk AB, Delaney ML, Fichorova RN. The human microbiome during bacterial vaginosis. Clin Microbiol Rev 2016;29:223-238. [Abstract]
- Deng ZL, Gottschick C, Bhuju S, et al. Metatranscriptome analysis of the vaginal microbiota reveals potential mechanisms for protection against metronidazole in bacterial vaginosis. mSphere 2018;3(3):e00262-18.[Abstract]
- Ravel J, Gajer P, Abdo Z, et al. Vaginal microbiome of reproductive-age women. Proceedings of the National Academy of Sciences of the United States of America 2011;108 (Suppl. 1):4680-4687.[Abstract]
- Mirmonsef P, Hotton A L, Gilbert D, et al. Free glycogen in vaginal fluids is associated with lactobacillus colonization and low vaginal pH. PLoS ONE 2014;9(7):e102467.[Abstract]
- World Health Organization. Use and procurement of additional lubricants for male and female condoms: WHO/UNFPA/FHI360 advisory note, 2012.[Source]
- Verstraelen H, Verhelst R, Vaneechoutte M, et al. The epidemiology of bacterial vaginosis in relation to sexual behaviour. BMC Infectious Diseases 2010;10:81[Abstract]
- Hickey R, Abdo Z, Zhou X, et al. Effects of tampons and menses on the composition and diversity of vaginal microbial communities over time. BJOG 2013;120:695-706.[Abstract]
- Vodstrcil LA, Twin J, Garland SM, et al. The influence of sexual activity on the vaginal microbiota and Gardnerella vaginalis clade diversity in young women. PLoS ONE 2017;12(2):e0171856 [Abstract]
- Mändar R. Microbiota of male genital tract: Impact on the health of man and his partner. Pharmacological Research 2013;69(1):32-41.[Abstract]