The groundbreaking sequencing of the human genome was announced in 2001. It was undoubtedly a historical achievement in human biology and medicine; however, it gave us only a partial blueprint of our species. The studies of human microbiota (i.e. the communities of microorganisms that populate human body) and microbiome (i.e. the combined genetic profile of all those microorganisms) hold a promise of completing that blueprint.
Until recently, studies of human microbiome have primarily focused on diseased states, but now it is increasingly recognized that indigenous microorganisms have an important function in maintaining human health. One prime example of a finely tuned mutualistic relationship is the human vagina and associated bacterial communities residing there – also known as vaginal microbiome.
But their responsibility does not stop here, as they also have a protective role for preventing the colonization of potentially harmful microorganisms, such as those responsible for yeast infections, bacterial vaginosis, sexually-transmitted infections, as well as urinary tract infections.
Vaginal Microbiome of Healthy Women
Lactobacilli have long held the title of a cornerstone species from the vaginal milieu of reproductive-age women. Their main fermentation product is lactic acid which benefits the host by lowering the vaginal pH. What is peculiar is that the dominance of lactobacilli appears to be unique to human beings; while they are relatively abundant in the human vagina (more than 70% of all bacteria), lactobacilli rarely account for more than 1% of vaginal microbiota in other mammals.
Phylogenetic research conducted in the last decade (primarily by using 16S rRNA gene sequencing) has shown that the composition of vaginal bacterial communities is much more complex than previously imagined. Moreover, the vaginal microbiome has been shown to be highly dynamic, sometimes even going through states without any Lactobacillus species.
These comprehensive studies have also shown how vaginal communities may harbor strikingly different species, with differences most pronounced in diverse ethnic groups. Studies employing next-generation sequencing have made meticulous characterization of the healthy vaginal microbiome much easier, with description of five major types of microbial communities (also known as community-state types).
Community-state types I, II, III and V are characterized by the domination of Lactobacillus crispatus, Lactobacillus gasseri, Lactobacillus iners and Lactobacillus jensenii, respectively, whereas community-state type IV has specifically low number of Lactobacillus spp. and a high number of diverse anaerobic bacteria.
Despite the aforementioned differences in ethnic groups, detailed molecular research endeavors did not find large differences in vaginal microbiome between different age groups (i.e. between adolescents, women in reproductive age and post-menopausal women). Nevertheless, post-menopausal women are characterized with lower estrogen levels that offer less protection from microbial imbalance.
Although all these high-tech methods have substantially improved our knowledge of the vaginal microbiome, certain limitations have to be taken into account. For example, while molecular techniques uncover both viable and non-viable organisms, some fail in adequately detecting minority species, and sometimes reliable differentiation of species within a genus is unsatisfactory.
The Effect of Probiotics on Vaginal Communities
As already mentioned, a distinctive feature of a healthy vaginal microbiome is the relative abundance of lactobacilli in the majority of women. Additionally, since probiotics (which can be defined as beneficial microorganisms taken as dietary supplements) usually contain species in the genus Lactobacillus, their beneficial effects are often exploited within the context of the reproductive tract.
Accordingly, supplementation with oral Lactobacillus fermentum and Lactobacillus rhamnosus has the propensity to restore vaginal flora in up to 82% of women with previous vaginal microbial imbalance. Besides, such probiotics may be used over long periods of time, which is a very important trait (especially following aggressive antibiotic therapy regimens).
Still, there is a need for better definition and understanding of all the factors that may affect both the composition and the dynamics of vaginal microflora – including the role of genetic determinants and physiological responses in health and disease. With that knowledge, manipulating vaginal microbiota will change how clinicians approach this intricate issue of women’s health.