Compared with those of other mammals, the human vagina is unique.
As warm, moist canals exposed to all sorts of things including penises, babies and dirt, most mammalian vaginas harbor a diverse mix of bacteria.
However, for many women, one or another species of Lactobacillus has become the dominant bacterial resident.
WHAT THESE BACTERIA DO
Lactobacillus bacteria pump out lactic acid, which keeps the vaginal environment at a low, acidic pH that kills or discourages other bacteria, yeast and viruses from thriving.
There are even hints that certain Lactobacillus species reinforce the mucus in the vagina that acts as a natural barrier to invaders.
BRIEF HISTORY OF VAGINAL BACTERIA
Although no one knows for sure, researchers speculate that human vaginas gained their Lactobacillus protectors around 10,000–12,000 years ago when humans began fermenting milk and eating foods like yogurt and cheese, which are full of the bacteria.
Certain Lactobacillus may have expanded their territory to colonize the vagina – travelling the short distance from the anus to the vaginal opening.
There, they found their perfect environment, a low-oxygen chamber that, during a woman’s reproductive years, has an abundant supply of the sugars Lactobacillus feed upon.
For the most part, we’ve been happily cohabitating ever since, but it’s a delicate balancing act.
DIFFERENT FROM WOMAN TO WOMAN
Researchers are realizing that all Lactobacillus bacteria – long thought to keep vaginas healthy – are not created equal.
In 2011, a study found five different types of bacterial community.
Four of these were dominated by different Lactobacillus species.
But the fifth contained a diverse mix of microbes (including Gardnerella, Sneathia, Eggerthella and Mobiluncus species), many of which have been associated with bacterial vaginosis.