The human vagina is inhabited by a range of microbes belonging to over 50 species. Among the plethora of species, Lactobacilli are the most common, particularly in healthy women. The microbiota in the Uro-Genital tract can change its composition rapidly, for reasons that are not fully clear. This can lead to an infection or to a state in which organisms with pathogenic potential coexist with other commensals. The most common urogenital infection in premenopausal women is bacterial vaginosis (BV), a condition characterized by a depletion of the lactobacilli population and the presence of Gram-negative anaerobes, or in some cases Gram-positive cocci, and aerobic pathogens.

Treatment of BV traditionally involves a course of antibiotics like metronidazole or clindamycin, however, the recurrence rate remains high, and this treatment is not designed to restore the depleted lactobacilli populations. Antimicrobial treatment of urogenital infections is not always effective, and problems often remain due to bacterial and yeast resistance to antibiotics, recurrent infections, and side effects. Recurrent infections may also be due to the elimination of the commensal organisms in the vagina by the antibiotics, thereby increasing the susceptibility to get re-colonized by pathogens. This is one of the main reasons for considering the use of probiotics: to replenish the commensal microbes as a way to lower the risk of re-infection.

Probiotics are defined as ‘live microorganisms which, when administered in adequate amounts, confer a health benefit on the host’. Lactobacilli are the most common organisms used as probiotics. Since the vaginal microbiota of women with BV has been found to contain a reduced number of lactobacilli in comparison with healthy women, lactobacilli administered orally or intra-vaginally have been tested for their effectiveness in colonizing the vagina and treatment of women with BV or at least preventing its recurrence. In vitro studies have shown that Lactobacillus strains can disrupt BV and yeast biofilms and inhibit the growth of urogenital pathogens. The use of probiotics to populate the vagina and prevent or treat infection has gained considerable interest.

The species that are present in the vaginal mucosa vary between premenopausal women and those who have gone through menopause. The microbiota of healthy premenopausal women is generally dominated by the Lactobacillus species. Postmenopausal women are also more susceptible to urogenital infections, supporting the theory that colonization of the vagina by commensal lactobacilli serves as a protection from these pathogens.

There is strong evidence that the absence of vaginal lactobacilli is associated with the development of BV. Alvarez-Olmos et al. found a negative association between BV and the presence of lactobacilli in the vaginas of females aged 14 – 18 years (p 0.034). Similarly, vaginal lactobacilli were isolated from 73.7% of 825 women without BV, and from 29.8% of 131 women with BV (p <0.001), from 74.3% of 2729 women without BV, and from 38.4% of 445 women with BV (OR 0.7, 95% CI 0.6–0.8), in a study of pregnant women aged >18 years, and from 96% of 28 women without BV, and from 52% of 67 women with BV (p <0.001). These findings demonstrate the relationship between a healthy vaginal microbiota with a high number of lactobacilli and the health benefit it confers to the host by preventing the development of vaginal infections.

In addition, apart from the association between an absence of vaginal lactobacilli and BV, recurrent BV can also result in low concentrations of vaginal lactobacilli.

The mechanisms whereby lactobacilli function as anti-infective defenses may involve the production of antimicrobial factors, like the maintenance of a vaginal pH of ≤4.5, and the secretion of certain bio-surfactants which alter the surface tension of the surrounding and reduce the ability of a wide range of pathogens to adhere and colonize. In addition, lactobacilli have been shown to bind (co-aggregate) to some pathogens and this may be a means to block their adhesion, and subsequently inhibiting them through the production of antimicrobial substances and preventing their spread to other areas of the urogenital tract.

So in order to have a healthy vaginal flora, a probiotic formulation containing a high content of probiotic strains is required to restore the depleted lactobacilli populations in the vagina prevent against infections.