Brief overview of the normal vaginal microbiota:

Some parts of the female reproductive system -- specifically the vulva and vagina -- are characterized by a normal microbial flora (microbiota) that includes dense populations of various species of bacteria and fungi. Some generalizations can be made about what constitutes the vaginal microbiota of a healthy, post-menarchal/pre-menopausal, sexually active, non-pregnant female.

The vaginal microbiota is dominated by the bacterium Lactobacillus, which is ideally suited to the vaginal environment. As facultative or obligate anaerobes, they thrive in a low-oxygen environment such as the vagina. [The vagina normally has only 2% of the oxygen content of outside air.] Lactobacillus bacteria are also acidophilic (acid-loving), and therefore easily tolerate the acidic pH of the vagina. Their optimum pH for growth is pH=6, but their range of tolerance is from pH=3.5 to pH=6.8. [The vagina has a pH=4 for most of the menstrual cycle, but increases to near-neutrality for two days at the onset of menstruation.] In fact, these bacteria secrete lactic acid and thus contribute to the acidity of the vagina. They adhere well to the vaginal cells and very rarely cause infections in humans. These bacteria feed on secretions from the vaginal mucosa, as well as sloughed vaginal epithelial cells.

Also among the normal vaginal microbiota are bacteria such as Gardnerella vaginalis, and fungi such as Candida albicans. Both of these microbes frequently occur in healthy individuals, but either of them can cause infections of the vagina and vulva (i.e., vaginitis, vaginosis, vaginal candidiasis, and/or vulvovaginitis) when their populations grow too large. It has been suggested that these infections often occur when Lactobacillus bacteria do not dominate the microbiota (for example, when antibiotics kill the bacteria), and accordingly, when the vaginal pH rises. Indeed, Gardnerella vaginalis growth is inhibited below pH=4.5, but it grows well between pH=6.0 and 6.5. Similarly, Candida albicans grows well between pH=5.1 and 6.9. It is important to note that Candida albicans can survive a low, acidic pH environment (pH=2), but its growth is inhibited under such acidic conditions.

Aside from antibiotics, certain behaviors such as frequent douching can decrease the vaginal population size of Lactobacillus and increase the numbers of Gardnerella vaginalis. Douching fluids contain antimicrobial compounds that may affect these bacteria. Douching has been associated with an increased risk of vaginosis, HIV acquisition, pelvic inflammatory disease, cervical cancer, and pre-term delivery of babies by pregnant women.

In summary, it appears that the maintenance of “normal” vaginal health may involve conditions that encourage dense vaginal populations of Lactobacillus while discouraging dense vaginal populations of Gardnerella vaginalis and Candida albicans. Supporting this observation, probiotic therapy, in which Lactobacillus are consumed orally or applied directly to the vagina, has been successful in increasing the number of Lactobacillus in the vagina, and helping patients with vaginitis or vaginosis.

Reference: Written by G. Pryor, 2006. Details paraphrased from the book "Microbial Inhabitants of Humans: Their Ecology and Role in Health and Disease." Michael Wilson. 2005. Cambridge University Press, Cambridge, UK.