Vagina is susceptible to infections by pathogenic bacteria on account of its proximity to the anus. Vaginal discomfort (vaginitis and vaginosis) represents the most common cause of visits to the gynecologist.1 The human vagina is inhabited by an array of microorganisms with over 50 species having been recovered from the vaginal tract. Lactobacilli are the most common, especially in healthy women. The composition of the microbiota can change rapidly, and can result in infection or in a state in which pathogenic organisms coexist with other commensals.2

Bacterial vaginosis (BV) is the commonest urogenital infection seen among premenopausal women. It is marked by a reduction in the lactobacilli population and the presence of Gram-negative anaerobes, or in some cases Gram-positive cocci, and aerobic pathogens.2

Several factors affect the ability of lactobacilli to adhere to epithelial cells and colonize the vagina. These include hormonal changes (particularly estrogen), vaginal pH, and glycogen content.2

A vaginal protection system needs to be in place in order to prevent the proliferation of pathogenic microorganisms.1 The ideal vaginal pH of women of reproductive age is 3.5 to 4.5.1 Women with a vaginal pH > 5.0 or > 4.5 and a Gram stain score of 9–10 have been found to have significantly increased rates of preterm birth and birthweigth <2500 g or <1500 g.3 It is, therefore, important to maintain the ideal vaginal pH in order to prevent such complications.

The vaginal protection system thus involves maintenance of a 3.5 to 4.5 acidic pH that makes the conditions unfavorable for the proliferation of potentially pathogenic organisms and provides a suitable environment for acidophilus bacilli. Acidophilus bacilli metabolize under such conditions and produce large amounts of lactic acid by glucose anaerobic fermentation. Lactic acid keeps its acidic pH and prevents uncontrolled growth of other bacteria present in the vagina.1

The characteristic pH of women of reproductive age, i.e., 3.5 - 4.5, is achieved on account of the presence of sufficient amount of lactic acid in the vagina. This molecule thus plays a key role in maintaining a medium that favors the growth of lactobacilli in the vagina.1

Maintaining the ideal pH by means of acidifying the vagina with lactic acid seems to be a potential therapeutic intervention to prevent and treat infections.

References

1. Haya J, García A, López-Manzanara C, et al. Importance of Lactic Acid in Maintaining Vaginal Health: A Review of Vaginitis and Vaginosis Etiopathogenic Bases and a Proposal for a New Treatment. Open Journal of Obstetrics and Gynecology 2014;4:787-799.
2. Cribby S, Taylor M, Reid G. Vaginal Microbiota and the Use of Probiotics. Interdiscip Perspect Infect Dis. 2008; 2008: 256490.
3. Hauth JC, Macpherson C, Carey JC, et al. Early pregnancy threshold vaginal pH and Gram stain scores predictive of subsequent preterm birth in asymptomatic women. Am J Obstet Gynecol. 2003 Mar;188(3):831-5.