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.