Origins of vaginal acidity: high D/L lactate ratio is consistent with bacteria being the primary source

The origin of the lactic acid that acidifies the vagina is not well established. It is widely accepted that during times of high oestrogen (during the neonatal period and again during a woman's reproductive years) large amounts of glycogen are deposited in the vaginal epithelium and that the glycogen is anaerobically metabolized to lactic acid. What is not established is whether lactic acid is primarily produced by vaginal bacteria or by vaginal epithelial cells. Human cells can make only L-lactate, while bacteria can produce both D- and L-, thus the D- to L-lactate ratio can indicate the relative contribution of bacterially derived lactic acid.

In this study, we used chiral HPLC to examine the percentages of D- and L-lactate in vaginal secretions, in primary cultures of bacteria from these vaginal secretions, and in cultures of lactobacillus isolates of vaginal origin.

We found that in most vaginal secretion samples, >50% of the lactic acid was the D-isoform (mean 55%, range 6-75%, n = 14).

Our results thus support the hypothesis that vaginal bacteria, not epithelial cells, are the primary source of lactic acid in the vagina.