In parts 1 and 2 of the vaginal pH redux, I have of course spent the majority of my time discussing vaginal acidity. You might have noticed a layer of the conversation beginning to assert itself, though, concerning vaginal microbial communities, or vaginal flora. The interplay between the composition of the vaginal microbial community and vaginal pH is a pretty interesting one – which state drives the other? How much variation do you find among healthy women? What are the conditions under which these communities evolved or asserted themselves?

Dr. Angel Rivera is a microbiologist who is the lead author of a study on baboon and human vaginal microbial communities (he’s lead author on this one too, also worth a read). As it turns out, he also works down the street from me, and his collaborators are anthropology colleagues of mine, Profs. Rebecca Stumpf and Steven Leigh. So for the third and final part of this series, I decided to interview him to learn more about research into the vaginal environment to understand what questions need to be answered, and how research in this field helps us improve human health.


What is your name and title? What is your research about?

My name is Angel J. Rivera. I am postdoctoral associate for the Energy Biosciences Institute at the Institute for Genomic Biology (IGB) at the University of Illinois, Urbana-Champaign and my doctorate is in microbiology.

Until recently I was part of the Host Microbe System theme at IGB were I’ve done research on bacterial communities structure of the vaginal tract in primates (specifically baboon and mangabeys) the differences and similarities between each primate hosts and the relationships, if any, to that of humans.

We examined different aspects of the bacterial communities present in this niche, (1) Community structure in a troop of baboons where conditions where consider homogeneous. (2) The differences between humans and baboons and (3) the presence of antibiotic resistant genes in host that have never been expose to antibiotics before.


Your paper demonstrates very different vaginal microbial communities in baboons and humans. How much of this is due to endogenous variation in pH, and how much to environmental differences?

For some time now changes in the acidity and/or alkalinity of the vaginal tract has been a point of contention when considering “normal” community structures and changes within it as a consequence of pH. You see, this can be considered a classic “chicken or the egg” case. In the case of humans, is the pH low because of the dominant bacteria living there or are the bacteria living there causing the pH to be low? The answer is a little more elusive than we would like to admit. Vaginal tissue cells can produce metabolites (acidic compounds) that can lower pH, however no study has been done where one can see if the tissue produces enough for certain bacteria to colonize and further acidify the environment with their own metabolic byproducts. I believe there are researchers investigating this.


How does variation in pH impact vaginal microbial communities and bacterial infections? What are other health implications?

Vaginal pH represents the first line of defense against undesired bacterial or yeast species. The acidic nature of the vaginal environment prevents the organism from surviving or simply proliferating. Also, the bacterial species that seems to colonize harmoniously in the female tract (Lactobacillus sp.) prefers and maintains such and environment. The added bonus, if you choose to see it that way, is that these same bacteria can provide other mechanism to defend its environment (competition, antimicrobial compounds, etc.) ultimately protecting the host from an unwanted infection.

Conversely, researchers have found that changes in pH may set the stage for a community shift or imbalance that appear to be a precursor for disease states in the vaginal tract. One of the most common diseases in women is bacterial vaginosis. This condition is said to be the result of an imbalance that displaces lactobacilli form the dominance position and allow others, mainly anaerobic bacteria to take over.

One of the consequences of this happening is the danger it presents to pregnant women exhibiting preterm birth complications. As the undesirable bacteria become dominant some of those species can migrate up through the uterus and colonize the amniotic fluid. The precise reasons why this happens are not yet elucidated but researchers are putting special attention on the mechanisms. Treatment, however, is available.

In short, women would be wise to follow healthy practices and try to avoid unnecessary perturbations (douching, fragrant hygiene products, or other irritating chemicals) that may increase their possibility of developing infection.


Tell me one exciting new thing you're working on right now.

I’m currently putting the finishing touches on a manuscript that reports microbial communities comparison between baboons and mangabeys. In our previous studies we found substantial differences in the microbiota of baboons and humans. We reason this could be a result of the evolutionary distance between humans and baboons as they diverged approximately 25 million years ago. If phylogenetic distance explains differences in the vaginal microbiota, then primates with more recent shared ancestry should have bacterial communities that are more similar. We found that although mangabeys and baboons are evolutionarily closely related their vaginal microbiota differ considerably. Even though there are remarkable differences between the vaginal microbial communities in the mangabey and the baboon, these two monkeys still have more similar vaginal flora than either of them do compared to humans.

I consider it fun to do this thought experiment. If we step away and trace the hominid lineage and then try to characterize each or some steps of the lineage perhaps we can see trends that may provide a picture of the microbial-human coevolution.


What else do you think people should know about vaginal pH and microbial communities?

As I mentioned before the cell lining in the vaginal tract epithelium does produce compounds that are of acidic nature setting the stage for those bacteria that prefer this setting and that further acidify the environment. It has been suggested that low pH prevents undesirable microorganisms from establishing dominance and causing adverse effect. There are even some reports that claim viral infections (HIV) could be deterred if conditions are maintained at low pH and certain species of bacteria (lactobacilli) dominate vaginal tract communities. We still have very little understanding on specifics about vaginal tract pH and its relation to “normal” or “healthy” states in women.

As for microbial communities, well these have profound effects in human development, function and health. From the moment we are born to the moment we cease to exist microbes accompany us. I think it is important that people understand this and even consider that our microbes can be personal physiology indicators. Another set of fingerprints, if you may. Let me provide some examples:

  • Recent studies tell us that microbial structures can vary within different ethnicities. Women of white ethnic background have communities dominated by lactobacilli species. However, black and in particular Hispanic women can have a completely different structure where lactobacilli are NOT the dominant organisms yet they are considered to have a “healthy” vaginal tract.
  • In our own studies with baboons we have found that even when environments, diet and life style are very similar (captive troop) their vaginal communities exhibit differences. This has been also observed in humans. The truth is, these observations have been reported since the sixties and seventies but it is now that we have the technologies to more accurately confirm the findings. Interestingly, most if not all of these studies are snap shots of the microbial communities at the moment samples were taken leading us to believe that these communities are comparatively invariant. What I’m trying to point out is that bacterial communities in any environment are fairly dynamic. This is especially true in a niche like the vaginal tract where a significant amount of perturbation (menstruation, hormonal patterns, sexual activity, douching, etc.) occurs over any woman lifetime.
  • Longitudinal studies are underway and preliminary results tell us that vaginal communities of nearly all women are dynamic and exhibit marked changes in the relative abundance of species over time.

Thanks to Dr. Rivera for his time!