New research helps determine what is healthy and unhealthy in the vaginal microbiome

Editor’s note: This blog about the vaginal microbiome is a good primer for this coming Monday’s Microbiome Podcast with Jacques Ravel, where we will discuss the vaginal microbiome and women’s health at length.  There will also be a special announcement during this podcast, so anyone interested should be sure to download it Monday, June 1.

The typical ‘healthy’ vaginal microbiome is dominated by a Lactobacillus.  However many women, especially those of African descent, are not dominated by this genus, and instead have a high diversity of bacteria in their vaginal tract.  This low lactobacilli, high diversity phenotype has been associated with many disease states, such as bacterial vaginosis (BV), preterm birth, and higher rates of sexually transmitted disease (STD) transmission.  (We have written about some of these diseases before, and encourage any interested reader to click the ‘vaginal microbiome’ below this story to learn more.)  Vaginal microbiome research is still in its early days though, and it is not clear why vaginal microbiome not dominated by Lactobacillus should lead to these diseases, and if this phenotype, if asymptomatic, should even be considered unhealthy.  New research though, out of Harvard University, shows that this phenotype does lead to inflammation, and that these inflammatory response can affect reproductive health and STD transmission.  They published their study in the journal Immunity last week.

The scientists studied the vaginal microbiomes of a cohort of 146 HIV negative, asymptomatic, black, South African women.  They discovered that 63% of them were not dominated by Lactobacillus, an extremely high percentage, especially compared to their counterparts in developed countries (38% of black women and 10% of white women).  Nearly half of those women were dominated by Gardneralla vaginalis, which is most commonly associated with BV, and a large percentage of the other half were diagnosed with BV after investigation.  This is especially interesting because, as stated before, all of the women in the cohort claimed to be asymptomatic, but as we are learning, many women are unaware that there is anything wrong.  Overall, the women were able to be grouped into 4 specific phenotypes, those dominated by Lactobacillus iners, those dominated by other Lactobacillus crispatus, those dominated by Gardnerella vaginalis, and those with a high diversity including Gardnerella vaginalis, Fusobacterium gonidiaformans, Prevotella bivia, and Atopobium vaginae (note the lack of Lacotbacillus in this high diversity group). 

The scientists discovered that there were no associations between each vaginal microbiome group and the rate of STDs, contraceptive use, or sexual behavior.  This is important in showing that, at least on first pass, these bacterial communities were not the result of these exogenous factors (nor did they cause them, for that matter).  They also discovered that there was only a loose association between inflammatory cells in the vaginas of these women, and whether or not they had an STD.  The loose association was only observed in women with Chlamydia, and the women with the highest levels of inflammation had no apparent STDs.

The fact that STDs were not strongly associated with inflammation led the researchers to hypothesize that the vaginal microbiome community, rather than STDs, were responsible for vaginal inflammation.  Indeed, when they compared the amount of inflammatory cells in each vagina with the different microbiome groups described earlier, they found a strong association between inflammation and the highly diverse microbiome group.   Moreover, when they tracked individual women over time, those women whose vaginal microbiomes shifted to the high diversity group also increased inflammatory responses.  The researchers then took this work a step further, and identified specific bacteria that were associated with the inflammatory response:  Prevotella amnii, Mobiluncus mulieris, Sneathia amnii, and Sneathia sanguinegens.

Finally, the researchers measured genes for specific receptors in the vagina that are known to trigger an immune response.  They discovered that those women with the high diversity vaginal microbiomes upregulated genes for these receptors, which are known to be activated by bacteria.  Making matters worse, specific immune cells that are triggered by these receptors, which are thought to be critically important to HIV transmission, were found in higher abundances in women in the high diversity vaginal microbiome group.

This paper did a really great job showing that a vaginal microbiome that lacks Lactobacillus is indeed an unhealthy state, because it creates a highly inflammatory vaginal microbiome which likely causes or contributes to many other ailments, beyond just the higher rates of HIV transmission that was demonstrated.  Unfortunately, at the moment, there are no easy ways for women to check which vaginal microbiome they have, but that should be changing soon, and we recommend that all of our readers tune into the Microbiome Podcast this coming Monday to hear a big announcement in this area. 

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