Due to metabolism of glucose into lactate by the vaginal microbiome, the vaginal pH is acidic. This is typically pH 4.5 or less in Lactobacillus-based CSTs, and above pH 4.5 (but still acidic) for the mixed CST. After the menopause, vaginal pH tends to become less acidic.^3,8

An acidic environment can deter growth of other microorganisms. Increased pH reduces the protective effect of the vaginal fluids increasing the risk of infection. Other protective effects of lactobacilli include adhering to the vaginal surface and blocking receptor sites and adhesion for pathogens, as well as supporting epithelial strength by promoting tight-junction proteins.^1,9,10

Lactobacilli can secrete bacteriocins (proteins which inhibit pathogen growth) and hydrogen peroxide, another antimicrobial, and can increase the susceptibility of pathogens to attack by the immune system.^1,9

Gut bacteria which metabolise oestrogen (the oestrobolome) can also indirectly affect the vulvovaginal microbiome by releasing oestrogen from conjugated forms which is then reabsorbed into the blood system. This free oestrogen stimulates higher glycogen production in the genitals affecting mucus production and epithelial thickening, boosting the protective activity against viruses such as HPV.¹

Lower levels of circulating oestrogen, such as after the menopause can deplete lactobacilli, allowing growth of other anaerobes and increasing microbiome diversity.⁹