We know we have lots of bacteria in our body, but how do they help to keep us healthy?
We learned in Parts I and II that the vaginal microbiota play a major role in the health of our reproductive system and our risk for infection. To sum up, there are 5 different types of vaginal environments depending on which species of Lactobacillus bacteria is most prevalent, and Types 3 and 4 appear to have a higher coincidence with bacterial vaginosis and persistent HPV infections. Also, remember that Type 4 is the odd duck here: it actually is noted for its lack of lactobacilli and a high amount of strict anaerobic bacteria that are associated with Bacteria Vaginosis (Gardnarella, Megasphera, Sneathia, Prevotella).
So how do these bacteria help protect you from infection? It comes down to what the bacteria produce: lactic acid, hydrogen peroxide, and bacteriocins.
Most people know of lactic acid because it is causes muscle soreness after a very strenuous workout. However it does have other roles in the body other than reminding you that yesterday was Leg Day. Lactic acid is present at high amounts in a healthy vaginal environment. In this symbiotic relationship, the Lactobacillus species consume glycogen (present in high quantities in estrogen-rich cervix and vaginal epithelium) and produce lactic acid, which results in a low pH environment (acidic).
While saying an environment is acidic doesn’t sound good, this acidic environment actually helps protect your body against some STI’s, specifically against growth of some pathogenic species responsible for Chlamydia, Gonorrhea, and Garnerella vaginalis.
The low pH environment may even correlate with protecting your body against HPV infection: a study out of Costa Rica measured the vaginal pH of over 9,000 women and found women with a vaginal pH >5 had a 10-20% increased risk of being HPV+. Potentially this is because the specific protein responsible for the transmission of the human papillomavirus, simply called E5, is particularly susceptible to low pH. So a higher pH vaginal environment may be less capable at clearing the virus.
Another way lactic acid from Lactobacillus species protects your body from HPV infection and neoplasia may be the chemical structure of lactic acid itself.
Lactic acid comes in two forms, a right-handed form (D) and a left-handed form (L), which chemically behave differently within the body and are produced by different bacteria.
D-Lactic acid is made mostly by L. crispatus (Type 1), L. gasseri (Type 2) and L. jensenii (Type 5). On the other hand, L-Lactic acid is made by the vaginal epithelium, L. iners (Type 2), and various anaerobic bacteria (Type 4).
From this we can see that Type 3 and Type 4 vaginal environments have a higher ratio of L- to D-Lactic acid. This is important to note because higher concentrations of D-Lactic acid have been shown to increase mucus viscosity within the cervix/vaginal. One role of mucus within the body is to trap foreign particles. This thicker mucus may be able to stave off viral infection, or at least it increases its potential for trapping viral particles.
The second bacterial byproduct that may be helpful to your vaginal health is hydrogen peroxide – it’s good for more than just cleaning up scrapes and skinned knees, and thankfully we can’t feel its sting this way! We use it to clean up wounds because it has both antibacterial and antiviral properties. In fact studies have shown that vaginal environments with low-rates of hydrogen peroxide-producing bacteria have an increased rate of bacterial vaginosis.
The majority of the lactobacilli produce hydrogen peroxide. Care to take a wager which one doesn’t? If you guessed L. iners, you are right! L. iners, the lactobacilli that dominates in Type 3 vaginas is incapable of producing hydrogen peroxide. It is possible that this is another way that Type 3 vaginal environments is more prone to STIs and growth of anaerobic bacteria.
Lastly, your bacteria protect you by producing proteins that kill bacteria (bacteriacidal) or stop them from reproducing (bacteriostatic). Bacteriocins are a group of bacteriostatic proteins produced by bacteria; Gassericin is a bacteriocin produced by L. gasseri (where it gets its name), L. crispatus, and L. reuteri, which prevent growth of pathogenic bacteria.
Another group of proteins produced by bacteria, called biosurfactants, interrupt bacterial adhesion so that the bacteria can’t stick together and create a biofilm. This biofilm is particularly associated with G. vaginalis and results from an overgrowth of bad anaerobic bacteria. One biosurfactant created by Type 1 dominant bacteria L. crispatus i in particular, LEA (Lactobacillus epithelium adhesions), stops this adhesion of G. vaginalis.
It is important to know that there is currently little evidence showing bacteriocin production by L. iners, and perhaps this is why it is easier for Type 3 environments to change to Type 4.
We know these bacteriocins help maintain healthy environments throughout the body, but studies still need to be done to see how they relate to HPV persistence.
One final point for today: Not all healthy environments are the same.
We’ve mentioned before that some bacteria just don’t play nicely with others. This isn’t just a good bacteria vs. bad bacteria kind of fight; this also happens amongst the good Lactobacillus species. Apparently L. crispatus (Type 1) is rarely seen to coexist with the other bacterial types – it’s either strongly dominant or almost nonexistent. Therefore it is the least likely of all types to change to the unhealthy Type 4 environment. We may even be able to say it’s the “healthiest” of all the types: it has the lowest pH, is negatively correlated with CIN, and is the least likely to be infected by bacterial STIs and some viral STI’s, such as herpes simplex 2, HIV, and HPV.
Hopefully you have started to see the pattern here – you really want to have a vaginal environment that’s either Type 1, 2, or 5, with Type 1 being the most seemingly ideal.
So now we know how your good bacteria can help protect you from infection. Coming up in Part IV we will cover the flip-side: how the bad bacteria affect your body and how they may increase your risk for persistent HPV infection. To finish up the series, Part V will cover prebiotics and probiotics and how they may help in HPV treatment.
Read the full review here, if you would like to find out more.
Mitra, A., Macintyre, D. A., Marchesi, J. R., Lee, Y. S., Bennett, P. R., & Kyrgiou, M. (2016). The vaginal microbiota, human papillomavirus infection and cervical intraepithelial neoplasia: What do we know and where are we going next? Microbiome,4(1). doi:10.1186/s40168-016-0203-0