A new study published in the journal PLOS Biology has revealed diverse microbial populations can facilitate vaginal colonization of pathogens, which in turn can prolong the characteristics of bacterial vaginosis, a condition characterized by an imbalance of vaginal microbiome. The study findings also provide evidence linking oral sex with bacterial vaginosis.
What is bacterial vaginosis?
Bacterial vaginosis is a common condition of vaginal microbial imbalance (vaginal dysbiosis), affecting about 29% of women in the United States. In women with bacterial vaginosis, the number of beneficial bacteria reduces, and the colonization of pathogenic bacteria increases. Although the condition is mostly asymptomatic, it can put a woman at higher risk of developing sexually transmitted diseases and urinary infection. In pregnant women, bacterial vaginosis can increase the chance of preterm birth.
Although the exact etiology of increased pathogen colonization is not known, it is generally believed that a reduction in ‘good bacteria’ level may facilitate the entry and colonization of pathogens inside the vagina.
The current study hypothesis
In the current study, the researchers hypothesized that the presence of sialidase activity in the vaginal fluid of women with bacterial vaginosis is responsible for pathogen colonization and maintenance of vaginal dysbiosis.
In this context, previous studies have shown that in bacterial vaginosis, an abundant bacterial species (Gardnerella vaginalis) generate sialidases to induce the secretion of sialic acids from mucosal components and cell surface glycoproteins. Sialic acids play an essential role in maintaining bacterial growth, survival, and virulence. According to the current study hypothesis, in a condition like bacterial vaginosis, bacterial population with sialidase activity mutually facilitates the persistence of bacterial species that do not have sialidase activity.
Mutually beneficial relationship between diverse bacterial populations in the vagina
Fusobacterium nucleatum is a Gram-negative bacterium predominantly found in the human mouth. This bacterium does not have endogenous sialidase activity and is known to colonize with bacterial population that has sialidase activity. As hypothesized by the researchers, F. nucleatum may utilize sialic acids as a nutritional source in the presence of exogenous sialidases produced by sialidase-positive bacteria.
Using both in vivo and in vitro experimental models, the researchers observed that F. nucleatum cannot utilize glycan-bound sialic acids because of the absence of sialidase activity. However, upon colonization with sialidase-producing vaginal bacteria, F. nucleatum can get nutritional benefits from sialic acids. Moreover, the researchers found that F. nucleatum maintains a mutually beneficial relationship with sialidase-producing bacteria by triggering the characteristics of vaginal dysbiosis, such as increased sialidase activity and enrichment of Gardnerella vaginalis. The maintenance of vaginal dysbiosis, in turn, facilitates the persistence of susceptible bacterial population in the vagina.
Besides the synergistic relationship between microbial populations, antagonistic interactions do exist to maintain vaginal homeostasis. Lactobacilli (good bacteria) that are present in high amounts in women without bacterial vaginosis induce a lactic acid-enriched and low pH condition in the vagina, which in turn facilitates the removal of F. nucleatum.
The link between oral sex and vaginal dysbiosis
Because F. nucleatum is present predominantly in the mouth, it is generally considered that women acquire this bacterium in the vagina via oral sex. Moreover, oral sex contact is known to be a potential risk factor for bacterial vaginosis.
The current study findings indicate that vaginal exposure to F. nucleatum increases the growth of Gardnerella vaginalis, even if very low numbers of F. nucleatum are introduced. According to the researchers, oral sex can increase the chance of mouth-to-vaginal transmission of the bacterial population that can subsequently facilitate the maintenance of vaginal dysbiosis.
The current study significance
The current study provides valuable information about bacterial symbiosis through ‘metabolite cross-feeding’ that can actually facilitate the colonization of pathogenic bacteria in the vagina, as well as provide a suitable condition to maintain vaginal dysbiosis.
Moreover, the current study findings clarify why having bacterial vaginosis increases a woman’s susceptibility to vaginal colonization of pathogenic bacteria that are responsible for intrauterine infection and other health complications.