Microbiome Insights is proud to be the sponsor of Season 3 of Ruairi Robertson's Biomes Podcast.
In this final episode, Ruairi sits down with Dr. Douglas Kwan, an infectious disease specialist at Massachusetts General Hospital and researcher at the Ragon Institute, to explore the complexities of the vaginal microbiome. Dr. Kwan's work spans the intersections of microbiology, immunology, and women's health, with a particular focus on understanding how vaginal microbes influence reproductive health and the risk of acquiring infections like HIV.
Uncovering the Vaginal Microbiome
Dr. Kwan delves into the unique characteristics of the vaginal microbiome, explaining how it differs from the more commonly studied gut microbiome. He discusses the dominance of specific Lactobacillus species, particularly Lactobacillus crispatus, which are associated with favourable reproductive outcomes. The conversation highlights the significance of a low-diversity microbiome in maintaining vaginal health, contrasting the typical preference for diversity seen in gut microbiome studies.
HIV, STIs, and Reproductive Health
A key focus of the episode is Dr. Kwan's research in South Africa, where he investigates the role of the vaginal microbiome in HIV acquisition risk. He explains how certain bacterial communities can increase inflammation and the presence of HIV target cells, thereby raising the risk of infection. The discussion also touches on the broader implications of these findings for other sexually transmitted infections and reproductive outcomes, such as preterm birth.
Challenges and Future Directions
Dr. Kwan and Ruairi discuss the current challenges in treating conditions like bacterial vaginosis (BV), where standard antibiotics often lead to high recurrence rates. They explore the potential of emerging treatments, including probiotics and vaginal microbiome transplants, to create more durable changes in the vaginal environment. Dr. Kwan emphasizes the importance of continued research to develop more effective interventions that can improve women’s health globally.
Conclusion
Dr. Douglas Kwan's insights into the vaginal microbiome underscore the complexity of this underexplored area of microbiome science. This episode sheds light on the critical role the vaginal microbiome plays in women's health and the ongoing efforts to better understand and manipulate it for improved health outcomes.
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Ruairi Robertson: Cool, so thank you very much, Doug Kwan, for agreeing to chat today all about the vaginal microbiome. You're one of the leading experts in that field. Maybe you can introduce yourself, tell us where you're working, and how you got into this exciting field.
Douglas Kwan: Yeah, well, it's a real pleasure to be here and to talk to you. So, I'm an infectious disease doctor at Massachusetts General Hospital here in Boston, but I also do research at the Ragon Institute of MGH, MIT, and Harvard. We're an institute that really studies human immunology and ways to use human immunology to fight and cure diseases. My background is that I'm an MD, PhD. I trained in microbiology and immunology with someone named Dan Littman, who I did my PhD work with. Then, I did clinical training in San Francisco and infectious disease training here at Harvard. After my clinical training, I started a postdoc and eventually established a lab looking at mucosal immunology, particularly how the mucosal immune system interacts with microbes and how that affects human health.
Ruairi Robertson: Great. And how did you get into the niche of the vaginal microbiome? Was it through your clinical work or research practice? Maybe you can tell us a bit more about that.
Douglas Kwan: Yeah, it's interesting because I think a lot of people get into the vaginal microbiome through a microbiology interest, focusing on ecology. But we were different. We got into this through our work in HIV and HIV prevention. We had been doing work in South Africa and other regions of sub-Saharan Africa, looking at the biological factors that affect a woman's risk of acquiring HIV. About 12 years ago, we started a study in a township called Umlazi, just outside of Durban, South Africa, where there are enormously high HIV incidence and prevalence rates among young women. We were trying to better understand what biological factors were affecting HIV risk, particularly within the female genital tract and the mucosal tissues of the genital tract. One of the things we identified was that the vaginal microbiome had a big influence on baseline inflammatory states within those tissues, which then affected a woman's risk of acquiring HIV.
So, that's how we started looking at the interactions between vaginal microbes and the mucosal immune system, to understand how that might affect HIV acquisition risk. Later, more literature emerged showing that these same vaginal microbiome communities that affected HIV risk also influenced other important reproductive outcomes like preterm birth, cervical dysplasia, cervical cancer, and fertility. That broadened our interest toward women's health, particularly for women living in low and middle-income countries.
Ruairi Robertson: That's fascinating. For those listening who have a small to medium knowledge of the microbiome, most only really consider the gut microbiome, which is arguably more complex because it's more diverse. Could you talk about the foundations of vaginal microbiome research, how it can be classified, especially in Western settings, and what these different vaginal microbiome profiles mean?
Douglas Kwan: Sure. It's quite interesting. If you look at most women in a place like the U.S. who are white, 90% of them will have a vaginal microbiome that consists of a single species of Lactobacillus, often Lactobacillus crispatus. However, in our cohort in South Africa or other cohorts of women of reproductive age in sub-Saharan Africa, at least 50% of them have a vaginal microbiome that's comprised of a mixed community of diverse bacteria called anaerobes, which thrive in low oxygen environments, and they have very little Lactobacillus.
Globally, as people started looking at different populations, they found a finite number of bacterial communities. One is dominated by a single species of Lactobacillus, often Lactobacillus crispatus or Lactobacillus iners. Then there are mixed anaerobic communities with low Lactobacillus. These are the basic community states, which have been further classified into community state types to define them better. Overall, women with a Lactobacillus-dominant community, particularly Lactobacillus crispatus, tend to have more favorable reproductive outcomes than women with diverse anaerobic-dominant communities with low Lactobacillus.
Ruairi Robertson: That contrasts with what we know about the gut microbiome, where diversity and richness are seen as beneficial. In the vaginal microbiome, however, dominance by one strain, particularly crispatus, is considered good. Could you explain why that is? At a basic level, these Lactobacillus species produce a lot of lactic acid, right? What other mechanisms make them beneficial?
Douglas Kwan: Exactly. You’re right; ecologically, we often think that diversity equals a healthier ecosystem because it creates resilience. But in the vaginal microbiome, it's the opposite. Domination by a single species of Lactobacillus, particularly Lactobacillus crispatus, is better. This suggests some selection for those communities. Lactobacillus crispatus does a lot of healthy things, like producing lactic acid, which maintains a low pH and helps prevent less favorable bacteria from colonizing. They also produce compounds like bacteriocins or bacteriostatic compounds that inhibit the growth of less favorable bacteria. Lactobacillus crispatus is associated with a low general inflammatory state in the female genital tract, which we believe is favorable for reproductive outcomes.
Ruairi Robertson: And what defines the colonization of a particular species? Are there genetic or environmental factors that determine that environment, allowing one species to colonize? How is it different from the gut, where nutrients constantly pass through and there's continuous exposure to different things?
Douglas Kwan: The truth is, no one really knows for sure, which is still a mystery in the field. It's an important question because if you know that Lactobacillus crispatus is optimal, you want to create an intervention that generates a Lactobacillus crispatus-dominant community in a durable way. To do that, we need to understand what factors determine community composition in the female genital tract, and that's something we still don't fully understand.
There is variation in geography. For example, Lactobacillus crispatus-dominant communities are more common in developed countries than in low and middle-income countries, suggesting environmental factors at play. Whether genetics, diet, stress, or other factors influence it remains uncertain. We do know these communities are resilient. You can’t easily perturb them with antibiotics, as treatment for bacterial vaginosis (BV) has high recurrence rates, suggesting durability in these communities.
Ruairi Robertson: Do we know anything about the evolution of the vaginal microbiome over time? For instance, in the gut microbiome, we know that with industrialization, we're losing microbial diversity compared to traditional hunter-gatherer lifestyles. Has the vaginal microbiome changed over decades, centuries, or millennia?
Douglas Kwan: That's a really interesting question, and we don’t really know. I would say the gut microbiome field is ahead of where we are with the vaginal microbiome. The gut is more complex, and although we can draw learnings from it, the vaginal microbiome has its own unique characteristics. So, many of these fascinating questions are things we still need to answer to better understand how to leverage these communities to improve health.
Ruairi Robertson: Yes, that would be great. Could you tell us about your specific work? It’s fascinating how having a certain vaginal microbiome profile can make someone more or less susceptible to acquiring an infection, particularly in your research on HIV. But you mentioned that this might also be the case for other sexually transmitted infections (STIs). Could you explain how a certain vaginal microbiome makes someone more or less prone to HIV or other STIs?
Douglas Kwan: Sure. But first, let me take a step back to explain what the HIV epidemic looks like in the area where we're working. In KwaZulu-Natal, South Africa, HIV prevalence is incredibly high. South Africa has one of the highest HIV incidence and prevalence rates in sub-Saharan Africa, and KwaZulu-Natal is among the highest within the country. In 2019, a group in Durban did a door-to-door survey in this region, testing people for HIV. They found that while less than 1% of 14-year-old girls were infected, by the time they reached 24 years old, over 60% were infected. This testing covered nearly 20,000 people within a community of 40,000, showing that in some areas, HIV remains unabated.
This urgency drives our work. Understanding the biology of HIV acquisition can help us find better ways to prevent it. For us, diverse anaerobic-dominant communities were acquiring HIV at over four times the rate of women with Lactobacillus crispatus in our study. We also found that women with these high-risk, diverse communities had higher levels of general inflammation, indicated by inflammatory cytokines and chemokines secreted by immune cells in the genital tract.
Women with these diverse communities had increased numbers of activated CD4 T cells, which are the first cells HIV infects to establish an infection. We hypothesize that these inflammatory bacteria increase biological risk for HIV by increasing the number of these target cells. Additionally, these bacterial communities degrade the protective mucus in the genital tract, break down physical barriers created by epithelial cells, and degrade antibodies that help fight infection, making the person more susceptible to infections, including HIV and other STIs.
Ruairi Robertson: Are these mechanisms the same for STIs that might be more common in Western settings, like chlamydia or syphilis? Do we know how the vaginal microbiome plays a role in their acquisition?
Douglas Kwan: It's not entirely clear. The increased HIV target cell frequency may be more specific to HIV, but other factors, like decreased physical barriers, likely apply to multiple infections, including chlamydia and gonorrhoea.
Ruairi Robertson: Pregnancy is somewhat related to this, as you've alluded to. Even though it's not the specific focus of your work, it's still an interesting field. There's evidence that certain vaginal microbiome profiles increase a woman's risk of preterm birth, independently of having an STI. Could you tell us what we know from the literature about that and how that process works?
Douglas Kwan: Absolutely. It's true that these same communities that increase the risk for HIV also seem to increase the risk for preterm birth, and they're associated with increased inflammation within the genital tract. The mechanism likely involves inflammatory bacteria being sensed by the immune system in the genital tract tissues, contributing to an increased risk of preterm birth. There's older literature showing similar immune signatures associated with preterm birth and these inflammatory vaginal bacteria. This realization broadened our scope beyond just HIV to other important outcomes. We're now conducting a pregnancy study in the same region to explore how the vaginal microbiome in African women impacts the risk of preterm birth. Ultimately, the goal is to figure out how to alter the vaginal microbiome in a durable way to improve health outcomes.
Ruairi Robertson: There's a barrier in the vaginal environment, similar to the gut, where you don't want microbes passing through and reaching the fetal barrier. Are there specific microbes more likely to cross that barrier, triggering infection or inflammation?
Douglas Kwan: There are candidate anaerobes in these mixed communities of anaerobic bacteria that might do that. However, we’re not yet at the level where we can identify specific culprits. It's challenging to isolate what’s caused by a specific member within a complex community relative to the whole community or a subset of it. This challenge exists in other microbiomes too, like the gut, where complex communities make it hard to pinpoint the role of a single species or strain.
Ruairi Robertson: As far as I’m aware, there is some cross-feeding or interaction between species within the vaginal microbiome. Some species depend on each other, contributing to the ecosystem. Could you elaborate on that?
Douglas Kwan: Yes, like any ecosystem, there are interdependencies. We know this from early on when looking at bacterial vaginosis (BV), which involves mixed anaerobic-dominant communities. Some individual members of those communities are resistant to standard antibiotic treatment for BV. However, treatment can still collapse those communities even without targeting every member, suggesting that hitting a subset can disrupt the community. The challenge, however, is durability, with high recurrence rates after treatment.
Ruairi Robertson: Current treatments for bacterial vaginosis are based on metronidazole, an antibiotic. But you're saying that in many women, it doesn't have a long-term effect?
Douglas Kwan: That's right. As an infectious disease doctor, I find it concerning that we tell patients there's a 50% chance of recurrence within a few months. Yet, the treatment recommendations for BV haven’t changed in 40 years. We still use the same drugs despite knowing they're not very effective. This highlights the need for better interventions and strategies, especially now that there's broader awareness of how vaginal bacteria affect various important outcomes.
Ruairi Robertson: That leads nicely to my next question: What could these potential interventions be? We often think about probiotics and prebiotics in the context of the gut microbiome. Is there any evidence that oral probiotics could colonize and change the vaginal microbiome?
Douglas Kwan: Interestingly, at least in the U.S., you can find a whole shelf of remedies in grocery stores that claim to target the vaginal microbiome. Most of them are oral and don’t work at all. The Lactobacillus strains they contain aren't the ones typically found in the vagina; they come from dairy or animals, like Lactobacillus acidophilus or Lactobacillus reuteri. Even oral Lactobacillus crispatus may not work well. There is some evidence that strains can be shared between the gut and the female genital tract, but oral delivery of probiotics hasn’t been effective at changing the vaginal microbiome.
Ruairi Robertson: Do you think it theoretically could be possible, or is it just that the strains studied so far haven't been effective at colonizing? Or is the pathway itself not good enough?
Douglas Kwan: The strains used so far have certainly not been the right strains or even the right species. Oral treatment might have value, but I think vaginal treatment would be more effective. However, combining vaginal and oral treatments might be the most effective strategy. The gut may act as a reservoir that reseeds the genital tract, so targeting both areas simultaneously could be beneficial. Unfortunately, most current probiotics don’t use the right strains of Lactobacillus, especially Lactobacillus crispatus. So, this is an area where we're just starting to explore what might work best.
Ruairi Robertson: So, are there no currently available therapeutics or products on the market, like topical antibiotics, that have good evidence or clinical trials showing they manipulate the microbiome?
Douglas Kwan: There is topical metronidazole, which has similar high recurrence rates as oral metronidazole. However, some new developments are happening. One product, called Lactin-V, is a vaginal probiotic consisting of a single strain of Lactobacillus crispatus. Although not FDA-approved yet, clinical trials have shown it moderately reduces the recurrence of BV when used with metronidazole. So, there’s some evidence that a Lactobacillus crispatus vaginal probiotic can have a benefit, but it’s not a complete solution. There's still a lot of room for improvement.
Ruairi Robertson: One potential future treatment that has been trialed is the vaginal microbiome transplant, similar to a fecal microbiome transplant. Could you talk to us about that and its potential efficacy?
Douglas Kwan: Sure. The concept of vaginal microbiome transplant comes from the success of fecal microbiome transplants in the gut, especially for treating Clostridium difficile (C. diff) infections, which have high recurrence rates when treated with antibiotics. Fecal transplants have been successful in displacing the C. diff microbiome with a healthier one.
Based on that idea, and the understanding that vaginal probiotics may not be highly effective or durable, some researchers started trying vaginal microbiome transplants. This involves taking vaginal secretions from a healthy woman with a Lactobacillus crispatus-dominant community and placing them in the vagina of a woman with a BV-like community.
The first study on this was small, with only five participants and no placebo control, but it provided intriguing data suggesting some efficacy. However, participants required multiple doses and courses of antibiotics. This small trial suggested it was worth exploring further, and now there are several groups, including ours, looking into this.
Here in Boston, we have a vaginal microbiome transplant study with an OB-GYN named Carolyn Mitchell. A company called Freya is also conducting a study in Ireland with Laura Ensign from Johns Hopkins. More data is forthcoming, but unlike gut microbiome transplants, vaginal microbiome transplants probably aren't a scalable solution because it’s harder to collect enough material from vaginal secretions to treat many people. However, these studies might help us identify the factors needed to durably colonize the vaginal microbiome.
Ruairi Robertson: Do we know what those factors are? Are they nutrients for the bacteria, antibodies, or is it still a mystery?
Douglas Kwan: No one really knows yet. Those are all potential candidates, but it’s still a mystery.
Ruairi Robertson: Are there potential therapies that could modulate the vaginal environment if it's so hard to change? For example, certain nutrients that help Lactobacillus crispatus grow or something that could be provided if it’s hard to introduce a probiotic?
Douglas Kwan: Yes, or possibly used as an adjunctive therapy to probiotics. This is an interesting area that people are starting to explore. We've done some work looking at unsaturated long-chain fatty acids, which seem to promote the growth of Lactobacillus crispatus and inhibit the growth of less favourable bacteria like Lactobacillus iners and anaerobes.
We’re considering using these fatty acids to promote Lactobacillus crispatus colonization, possibly alongside metronidazole or a live biotherapeutic product. So, the strategy might involve giving Lactobacillus crispatus and then supporting it with something that encourages its growth while inhibiting other bacteria. We're planning a first-in-human clinical trial to explore this strategy’s effectiveness. But bacteria alone may not be enough, so adjunctive strategies will likely be necessary to achieve durable colonization.
Ruairi Robertson: Wow, it sounds like you have multiple lines of inquiry to create new formulations that will hopefully change vaginal microbiomes in states where they're unfavourable, such as in BV, preterm birth, and HIV acquisition. I think we'll leave it there, but I'd like to thank you very much, Doug Kwan, for a fascinating conversation. I look forward to hearing the results of your future research.
Douglas Kwan: Thanks so much. It was a pleasure.