Healthy vaginal flora typically consists of primarily lactobacillus species, which are critical for the prevention of colonization by pathogenic microorganisms. Lactobacilli produce antimicrobial substances such as hydrogen peroxide and bacteriocin-like substances, and forms biofilms which competitively inhibit the growth of pathogens.
Oestrogen also has a significant effect on vaginal microbiome. It stimulates the vaginal tissue to produce glycogen, which is metabolised by lactobacilli to produce lactic acid, which is the key factor in maintaining an acidic vaginal pH which is hostile to pathogenic organisms.
Lactobacillus levels increase at puberty, fluctuate throughout the menstrual cycle and decrease after menopause. Vaginal lactobacilli increase during the menstrual cycle before dropping quickly to reach their lowest levels during menstruation. The lactobacilli population recovers and peaks mid-cycle.
The oestrogen decrease seen in menopause is accompanied by the atrophy of the vaginal epithelium and the decline in vaginal lactobacilli count also increases susceptibility to pathogens.
It is well established that microorganisms ascend from the rectum to the vagina, urethra and bladder, this is a potential source of either infection with pathogens or of colonisation with beneficial bacteria.
Clinical trials have shown that a daily oral dose of one billion CFU each of Lactobacillus rhamnosus GR-1 and Lactobacillus reuteri RC-14 and restore and maintain a healthy balance of vaginal microflora. These two strains of lactobacillus have also been shown to persist in the vagina for longer than other strains. A two-week oral course of three strains at a dose of 1 billion CFU each, twice per day, has been shown to result in prolonged colonisation, leading to women with a history of recurrent genitourinary infections remaining free from infection for as much as 16 months.