Vaginal flora

Vaginal flora, vaginal microbiota or vaginal microbiome are the microorganisms that colonize the vagina. They were discovered by the German gynecologist Albert Döderlein in 1892 and are part of the overall human flora. The amount and type of bacteria present have significant implications for an individual's overall health. The primary colonizing bacteria of a healthy individual are of the genus Lactobacillus, such as L. crispatus, and the lactic acid they produce is thought to protect against infection by pathogenic species.

The primary colonizing bacteria of a healthy individual are of the genus Lactobacillus (90–95%), the most common being L. crispatus, L. iners, L. jensenii, and L. gasseri. Since the first description of lactobacilli by Döderlein, lactobacilli have been generally considered the gatekeepers of the vaginal ecosystem. Lactobacilli have been shown to inhibit in vitro growth of pathogenic microorganisms, e.g. Bacteroides fragilis, Escherichia coli, Gardnerella vaginalis, Mobiluncus spp., Neisseria gonorrhoeae, Peptostreptococcus anaerobius, Prevotella bivia and Staphylococcus aureus. It is generally accepted that this is achieved mainly through the action of lactic acid. Moreover, lactobacilli normally help to prevent long-term colonization of the vagina by adhering to vaginal epithelial cells. This usually reduces pathogens from infecting to the vaginal epithelium.

Next to lactic acid production and competition for adherence, other antagonistic mechanisms include hydrogen peroxide (a broad-spectrum antimicrobial) and bacteriocins (target-specific antimicrobials) production.

Low pH is generally accepted to be the main mechanism controlling the composition of the vaginal microflora. Although the lactic acid produced by lactobacilli contributes to the vaginal acidity, it is still not proven to be the primary source of low vaginal pH, but the fact remains that most lactobacilli thrive best at a pH < 3.5 . Normal vaginal pH is considered to be under 4.5 with a range of 3.8 to 4.4.

Production of hydrogen peroxide (H₂O₂) is a proposed mechanism for bacterial antagonism in vitro, inhibiting growth of microorganisms via direct interaction or via human myeloperoxidase. However, in vivo, the role of peroxide is uncertain. Under optimal anaerobic growth conditions, physiological concentrations of lactic acid inactivated the BV-associated pathogens without affecting the vaginal lactobacilli. Although the hydrogen peroxide production of lactobacilli has been considered as an important antimicrobial component, contributing to the colonization resistance provided by lactobacilli, and although there seems to be a link between H₂O₂-producing lactobacilli and normal vaginal microflora, recent data do not support this role for H₂O₂.

Experimentally, hydrogen peroxide-producing lactobacilli have been shown to inactivate HIV-1, herpes simplex virus type 2 (HSV-2), Trichomonas vaginalis, G. vaginalis, P. bivia and E. coli. O'Hanlon and Baeten found that 96% of Lactobacillus species from a healthy vaginal ecosystem produced H₂O₂ (L. jensenii and L. vaginalis produce the highest levels of H₂O₂), whereas only 6% of the lactobacilli recovered from women with BV produced H₂O₂. In agreement with this, L. iners, most frequently associated with disturbed vaginal microflora, is a poor producer of H₂O₂. Vaginal colonization by H₂O₂-producing lactobacilli has been associated with a decrease in the occurrence of bacterial vaginosis (BV). However, more recently O'Hanlon et al. demonstrated that cervicovaginal fluid and semen have a significant H₂O₂-blocking activity and they later demonstrated that physiological concentrations of H₂O₂ below 100 μM fail to inactivate any of the 17 tested BV-associated bacteria, e.g. A. vaginae, G. vaginalis, Mobiluncus spp., P. bivia, P. corporis, Mycoplasma hominis, even in the presence of human myeloperoxidase, known to increase the microbicidal activity of H₂O₂. Only supraphysiologic concentrations of exogenous H₂O₂ (0.34% w/v, 100 mM) were sufficient to inactivate BV-associated bacteria at which concentration it more potently inactivated vaginal lactobacilli (L. crispatus, L. gasseri, L. iners and L. jensenii). A concentration of 100 mM H₂O₂ is approximately 50-fold higher than lactobacilli are capable of producing even under optimal aerobic, low-antioxidant conditions, and approximately 5,000-fold higher than the estimated H₂O₂ concentration in vivo. Even more remarkable, the addition of only 1% vaginal fluid blocked the microbicidal activity of 1 M H₂O₂. Possible explanations may be that cervicovaginal fluid and semen contain proteins, glycoproteins, polysaccharides, lipids, and other molecules with the potential to react with and inactivate H₂O₂. In addition, the vagina is hypoxic most of the time, whereas lactobacilli require oxygen to produce hydrogen peroxide. It is also remarkable that catalase, which provides bacteria protection against toxic H₂O₂, is absent in lactobacilli, and as such they would be unprotected against their own H₂O₂ production. Consequently, hydrogen peroxide production may not be the major mechanism of defense.

Vaginal lactobacilli produce antimicrobial peptides, i.e. bacteriocins such as lactocin 160 and crispasin. with inhibitory activity ranging from narrow (closely related Lactobacillus species) to broad (diverse groups of bacteria, including G. vaginalis and P. bivia), and bacteriocin-like substances, with a broader spectrum of activity than bacteriocins (e.g. a heat-resistant peptide produced by Ligilactobacillus salivarius subsp. salivarius CRL 1328). Several studies have indicated that the activity of bacteriocins is favored by low pH.

The inhibitory substances produced by vaginal Lactobacillus is a primary factor in protecting the vaginal microbiota, with organic acids, bacteriocins, and hydrogen peroxide. These act synergistically against infection by pathogens. Not all Lactobacillus spp. and not all strains within one Lactobacillus species exhibit all 3 mechanisms. Lactobacillus species differ in premenopausal women, i.e. L. crispatus, L. jensenii, L. iners, L. gasseri (and possibly Limosilactobacillus vaginalis), as assessed through cultivation-dependent and cultivation-independent techniques. Vaginal lactobacilli have been shown to display a pronounced vaginotropism, and their pili act as ligands for attachment to receptors of vaginal epithelial cells. The limited number of Lactobacillus spp. found in the human vagina is remarkable, which leads to the possibility that there are host factors that select for specific organisms, that these species have unusual characteristics that allow them to successfully colonize the vagina, or both . However, the vaginotropism, does not only apply to this selected group of lactobacilli that stand for a healthy vagina, but also for the bacterial species associated with BV. The microbiota detected in the human genital and gut econiche do not appear to grow outside their host and probably are likely to rely on the close contact between parents and their children for transmission, e.g. mother to neonate transmission of genital microflora, most probably also with gut microflora homogenously distributed over the baby's body including skin, the oral cavity, nasopharynx, and feces.