Early Vaginal Microbiome
Research
The vaginal microbiome is simpler than the microbiome of other body
sites―a few species tend to dominate the vaginal microbiome, and all
vaginal microbiomes can be categorized by a few enterotypes, or
community profile. As such, there is a relatively uniform framework for
characterizing the vaginal microbial community. Community state types
(CSTs) were originally introduced in 2011 as a way of categorizing the
composition of the vaginal
microbiome5.
The prevailing literature finds that there are five principal CSTs―four
are marked by the dominance of Lactobacillus species and one is
marked by the absence of this dominant member. This fifth CST, dubbed
the “Diverse CST,” consists of a lower prevalence ofLactobacillus along with a greater prevalence and number of other
anaerobic microbes. The original analysis introducing CSTs noted that
healthy women of different races and ethnicities exhibited different
proportions of the five CSTs, but that, importantly, all CSTs were
common in healthy women across all racial and ethnic groups.
Later literature further simplified characterization of the vaginal
microbiome into a narrower concept - Lactobacillus dominated
communities versus diverse ones7.
The presence of Lactobacillus tends to correlate with lower rates
of sexually transmitted infections
(STI)3,
lower rates of pregnancy
complications8,
and lower diagnosed cases of Bacterial Vaginosis
(BV)9.
The plausible biological mechanism by which Lactobacillus exerts
its protective effect is through the digestion of glycogen in the
vaginal lining into lactic acid, creating a highly acidic vaginal
environment. This acidic environment is hostile to the growth and
proliferation of many pathogens that cause STIs and BV associated
bacteria (BVAB). The proposed mechanism is difficult to experimentally
test as even closely related animal models are physiologically different
from
humans10,
and it can be unethical to experimentally test this in humans. As such,
studies on the human vaginal microbiome are almost entirely
observational.
Observational studies of the human microbiome often fail to account for
many clinical and lifestyle factors, such as comorbid conditions, birth
control
method11,
and recent sexual
contact12,
that have the potential to moderate the relationships observed between
the vaginal microbiome and reproductive health risks. For example, it is
largely unknown how comorbid conditions impact the vaginal microbiome
despite their impact on the gut microbiome being
known13–16.
Further, sexual practices, most recent sexual contact, and birth control
are not always accounted for despite being known to impact the vaginal
microbiome3,11,17,18.
When CSTs were introduced, it was noted that Black and Hispanic women
were more likely than women of other ethnicities to fall into the
Diverse
CST5.
The women in this study were healthy with no reported vaginal symptoms.
As such, the authors cautioned against assuming that the absence ofLactobacillus indicated an unhealthy state.