Vaginal health lactic acid bacteria
Lactobacilli in the vagina produce a wide range of compounds including lactic acid, short chain fatty acids (SCFAs) and hydrogen peroxide (H2O2). These substances have antimicrobial, immunomodulatory and other potential beneficial properties. Lactobacilli also produce protective factors against bacterial vaginosis (BV) associated bacteria and other urogenital pathogens, such as group B Streptococcus (GBS) that causes neonatal morbidity and mortality and Candida albicans, which can cause vulvovaginal candidiasis in women.
In healthy women, the vaginal microbiota is typically dominated by one or more species of lactobacillus (L. crispatus, L. gasseri, L. iners or L. jensenii)1,2. However, many women have a vaginal microbiome that is dominated by non-Lactobacillus species, especially in Black and Hispanic women1,3,4. BV results from deficiency of the lactobacillus population, which is associated with an increase in anaerobic bacteria such as Gardnerella and Atopobium, leading to symptom formation such as a fishy smell or discharge, increased vaginal pH and itching. It has been suggested that the lack of Lactobacillus in the vagina may also contribute to the higher incidence of bacterial infections and STIs in these groups4,5.
Studies in vitro indicate that the combination of vaginal microbiota organic acids (i.e. lactic acid, acetic acid and succinic acid) produced when lactobacilli dominate the microbiota are potent HIV-1 virucidal agents at physiological concentrations and a low pH. However, this virucidal activity is not seen when the same mixture of vaginal microbiota organic acid is tested at pH typical of BV conditions, probably due to the lower inherent virucidal potency of SCFAs and succinic acid when they are at concentrations significantly above their pKa values.
The mechanism of virucidal action of the Lactobacillus-derived organic acids may involve a direct effect on viral surface and fusion proteins or by altering the integrity of the viral lipid envelope. Further investigation is needed to determine the specific antiviral mechanisms of lactic acid.
The potent virucidal activity of lactic acid against BV-associated pathogens and the ability to induce a host immune response through alterations in epithelial cell function, suggest that a lactic acid-based probiotic would be useful in treating BV in women. However, a number of clinical trials have shown varying results and it is important to test the efficacy of lactic acid-based gels, pessaries and rings in robust and sufficiently powered clinical trials using standardised endpoints. Moreover, it is important to identify the exact species and the functional repertoire of the Lactobacillus used in these studies, as different members of the same genus have distinct colonisation stability and physiologic characteristics43,44. Further work is also required to evaluate the effectiveness of probiotics containing the specific species of Lactobacillus that predominate in the vagina of women with BV. This may require the use of novel delivery systems such as e-gels to ensure that the desired Lactobacillus strains reach the target site. Furthermore, it is essential to address concerns about the emergence of resistant strains of the intended Lactobacillus probiotic, which are a common complication of oral administration of probiotics45,46.