In a world filled with Clorox and other 99.9%-antibacterial hand sanitizers, we spend an awful lot of time thinking about external bacterial exposure and how bad bacteria may affect our health. What about the bacteria inside your body? There is a constant dynamic interplay amongst an orchestra of our own bacteria, trillions of them actually. In fact, around half of the cells in the body are composed of the bacterial communities of different parts of our body, together called the microbiota, where they help maintain the health of respective environments.
Typically diversity is a good thing: within our bacterial communities it is a sign of a healthy system. However, the vaginal microbiota (VMB) is unusual in that a healthy vagina is typically colonized predominantly by only one or a few kinds of one genus of bacteria: Lactobacilli.
This group of bacteria is amazing in that it plays a protective role in the reproductive tract: producing hydrogen peroxide to fight certain yeast infections and producing compounds (bacteriatocins) that fight colonization of bad bacteria.
Although this one group of bacteria dominates the environment, not all vaginas are the same. Researchers have found there are actually five types of microbial communities, called Community State Types, based on which species of Lactobacillus dominates:
- CST I: dominated by crispatus
- CST II: dominated by gasseri
- CST III: dominated by iners
- CST IV: devoid of Lactobacillus species; high levels of strict anaerobic bacteria which are often associated with bacterial vaginosis (Gardenella, Megasphera, Sneathia, and Prevotella)
- CST V: dominated by jensenli
From this it is pretty easy to see that CST IV is the odd duck of the group. Typically this type of VMB composition is often seen in women who have bacterial vaginosis, a vaginal disease marked by an overgrowth of bacteria and a very distinctive consequential discharge.
Even though there are different types, every woman may go through transitions from one CST to another, however CST III to CST IV is the most common. Each species of Lactobacillus has its strengths and weakness – L. iners is capable of surviving a wider pH range, but seemingly is not as good at inhibiting the growth of these strict anaerobes seen in CST IV. Therefore, as the strict anaerobes multiply and the level of lactobacilli decreases because L. iners can’t fight them off, it results in a power change where the strict anaerobes rule the roost.
This is important to know because, in effect, women with communities dominated by L. iners may be at a greater risk of developing bacterial vaginosis (BV). Moreover, there are studies that show having BV “increases incidence, prevalence, and persistence in HPV and development of [cervical intraepithelial neoplasia].”
So there is this relationship between the CST and HPV. However, it is a bit of a chicken and egg situation. It’s unknown what is the causal relationship – does the predominant type of bacteria make HPV infection and persistence more likely, or does the virus manipulate the bacterial environment? More studies still have to be done in order to understand this interaction.
So what factors influence your VMB composition? Ethnicity is one major factor. Hispanic/Black women tend to have a significantly lower level of lactobacilli than Caucasian/Asian women, though it is not known if this is related to genetic factors or variation in hygiene practices, specifically vaginal douching.
Please, please, please, for the love of all that is good… stop vaginal douching! The thought of it has always made me cringe, but there is good reason behind stopping. It actually disrupts the balance of good bacteria and increases bacterial diversity, thereby putting you more at risk for BV. It’s also been linked to increased risk of HPV infection, CIN, and Cervical Cancer. Currently 22% of American women say they still douche, with African American women twice as likely to do so.
Hormones are another major factor influencing the type of vaginal microenvironment. When women are first born, it’s thought the vaginal environment is sterile, but maternal estrogen helps lactobacilli to colonize soon thereafter. After a few weeks, as estrogen levels decrease, so do lactobacilli levels, resulting in increased bacterial diversification and a CST IV-like environment, which persists until puberty.
At puberty this monotony is broken by a massive increase in estrogen and progesterone, which reduces the bacterial diversity in favor of an environment dominated by lactobacilli. One of group bacteria often found in the diversified environment, Sneathia spp., are associated with miscarriages and preterm labor. Remember, the lactobacilli have a protective component so it makes perfect sense why a body preparing for pregnancy would prefer a microbiota filled with these good bacteria instead of the alternative.
However the level of lactobacilli is not constant throughout the month. Estrogen and progesterone go through cycles, with estrogen peaking first right before ovulation, followed by the progesterone peak after ovulation that sustains the vaginal lining. During these time points, the lactobacilli levels are relatively high – bacterial diversity is low, therefore the vaginal environment is stable. During your period, the levels of both estrogen and progesterone have completely dropped, lactobacilli are depleted and species diversity increases. Women who have gone through menopause have an exacerbated version of this.
It’s not entirely understood how hormone-bacteria interaction exists, but it is thought to relate to the vaginal lining (vaginal epithelium). When estrogen levels increase during the pre-ovulation time period, the vaginal lining increases, prepping to be able to support implantation of a fertilized egg. This maturation is concurrent with increased amounts of glycogen because vaginal cells retain a high level of glycogen compared to other cell types.
Glycogen is a form of glucose made for storage and also happens to be a food support system for Lactobacillus species. An enzyme in the vaginal mucosa, called α-amylase, breaks down this glycogen and turns it to maltose, maltotriose, and maltotetraose – which are metabolized by the lactobacilli.
Because of this hormonal influence, it has been found that contraceptive synthetic hormone use may actually decrease the recurrence, prevalence, and risk of bacterial vaginosis by supporting the good bacteria with a consistent supply of hormones. So while there are pros and cons to hormonal birth control, this is one in their favor it seems. However, what happens to bacterial colonies when they no longer have this constant supply would have to be studied further.
A couple of other factors that influence your microflora are smoking and having sex. Smoking decreases the levels of L. crispatus (the species that dominates in CST I) and has been associated with an increased risk of BV. Women who are CST IV are much more likely to also be smokers.
Having sex alters your bacteria dynamics because, well, the other person has bacteria, too. Sometimes they don’t all play nice together. This is a reason why changing sex partners may result in changes in pH, bacterial vaginosis, odors, urinary tract infections, yeast infections, etc. Another good reason to use condoms!
So to wrap up Part I (no pun intended, I swear!), we have lots of little bacteria in our bodies, which help keep them healthy, and a stable environment of Lactobacilli is a sign of a healthy vaginal microbiota. Certain factors can influence your VMB including ethnicity, hormones, smoking, douching, and recent sex. Decreased levels of Lactobacillus species can either lead to an environment that facilitates abnormal bacterial growth and increases the risk of HPV infection and persistence OR the environment can be a result of it – it’s a very dynamic system with sophisticated interactions that are not yet fully understood.
Coming Up in Part II: The vaginal microbiota and further discussion on HPV/CIN/Cervical Cancer
However, if you want to read ahead a bit, read the full review here.
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