Vaginal pH measured in vivo: lactobacilli determine pH and lactic acid concentration

Abstract

Background: Lactic acid (protonated lactate) has broad antimicrobial activity. Vaginal lactobacilli produce lactic acid, and are known to confer protection against reproductive tract infections when they are predominant in the vaginal microbiota. Using novel ex vivo methods, we showed that cervicovaginal fluid (CVF) from women with a predominantly lactobacilli-morphotype microbiota contains significantly more lactic acid than previously thought, sufficient to inactivate reproductive tract pathogens. Here, we measured vaginal pH in vivo in 20 women with a predominantly lactobacilli-morphotype (low Nugent score) microbiota. We also investigated the in vitro production of protons (as hydrogen ions) and lactate by vaginal lactobacilli.

Results: The average vaginal pH in these women was 3.80 ± 0.20, and the average lactate concentration was 0.79% ± 0.22% w/v, with pH and lactate concentration tightly correlated for each sample. In vitro, lactobacilli cultured from these CVF samples reached an average pH of 3.92 ± 0.22, but the average lactate concentration was only 0.14% ± 0.06% w/v, approximately five-fold less than in the corresponding CVF samples. When the pH of the cultures was raised, lactate and hydrogen ion production resumed, indicating that production of lactate and hydrogen ions by vaginal lactobacilli is limited primarily by their sensitivity to hydrogen ion concentration (low pH) not lactate concentration.

Conclusions: Some vaginal lactobacilli cultures have a lower limiting pH than others, and limiting pHs in vitro showed good correlation with pHs measured in vivo. The limiting pH of the lactobacilli predominant in a woman's vaginal microbiota seems critical in determining the concentration of antimicrobial lactic acid protecting her.

Keywords: Bacteria; Bacterial vaginosis; Lactic acid; Lactobacilli; Microbiota; Vaginal; pH.

Fig. 1

Vaginal pH measured in vivo in 20 women with Nugent score 0–3, showing stability of recorded values over time. Dashed lines represent women whose cervicovaginal fluid (CVF) contained only L-lactic acid; solid lines represent women whose CVF contained both D- and L-lactic acid

Fig. 2

Vaginal pH measured in vivo, grouped by Nugent score. Open circles represent women whose CVF contained only L-lactic acid; filled circles represent women whose CVF contained both D- and L-lactic acid

Fig. 3

In 20 women with Nugent score 0–3, the vaginal pH measured in vivo correlates very tightly (r² = 0.97) with the lactate concentration of the ex vivo CVF. Open circles represent women whose CVF contained only L-lactic acid; filled circles represent women whose CVF contained both D- and L-lactic acid

Fig. 4

Increase in pH of 20 cervicovaginal fluid samples, probably due to loss of physiological CO₂, showing that the increase is less in samples with lower starting values. Open circles represent women whose CVF contained only L-lactic acid; filled circles represent women whose CVF contained both D- and L-lactic acid

Fig. 5

a The asymptotic pH achieved in vitro by lactobacilli is highly correlated (r² = 0.93) with the vaginal pH of the woman from which the lactobacilli were isolated. Open circles represent women whose CVF contained only L-lactic acid; filled circles represent women whose CVF contained both D- and L-lactic acid. b The concentration of lactate in the supernatants from the in vitro culture correlates more loosely (r² = 0.44) with the lactate content of the CVF samples from which the lactobacilli were isolated. Open circles represent women whose CVF contained only L-lactic acid; filled circles represent women whose CVF contained both D- and L-lactic acid