A few years ago, a microbiologist named Gregor Reid danced before an audience of students, post-docs, and scientists at a conference held by the American Society of Microbiology.
The song he grooved to was “Sub-Culture,” a synthesizer-infused meditation on loneliness by the 1980s British band New Order. But the talk he delivered—after he finished dancing—was, in a sense, about companionship.
His presentation had the monumental title, “The Vaginal Microbiome’s Role in Humanity.” And his argument was this: Our native microbes are passed from generation to generation much like human culture; the microbes of the human vagina are paramount to our survival; and in this era of burgeoning research on the human microbiome generally, we need to pay more attention to them.
“To not place a huge focus on the vaginal microbiome is like putting human survival at risk,” he said.
The human microbiome consists of the microbes living on and within the human body. Most of these bugs inhabit the large intestine. There, a few pounds’ worth of bacteria, yeasts, archaea, and even viruses help digest food, calibrate our metabolic and immune function, and hold off would-be invaders.
A special subset—the vaginal microbiome—inhabits the vagina. And successful human reproduction, it turns out, owes an immense debt to this microbial community.
A healthy vaginal microbiome produces lactic acid and hydrogen peroxide, which maintain a level of acidity that keeps troublemaking microbes at bay. When the vaginal community becomes unbalanced, on the other hand, acidity decreases. The wrong microbes may then invade or, if they’re already present, bloom.
This disturbance can cause bacterial vaginosis—not really an infection, but an out-of-whack ecosystem. It sounds like a trifling problem, and half of women with vaginosis may display no obvious symptoms. But this minor-seeming imbalance can have major consequences.
Vaginosis increases the risk of contracting secondary infections, from herpes to HIV. But even on its own, the microbial shift may prompt low-grade inflammation that can derail reproduction. It can prevent fertilization in would-be mothers, prompt spontaneous abortion in pregnant women, and increase the risk of preterm birth later in pregnancy.
If the vaginal microbiome were suddenly to shift across the entire human population, it's not unreasonable to predict that humanity would go extinct.
The prevalence of bacterial vaginosis is startlingly high, although it’s not clear whether the rate has always been high or has been changing. The Centers for Disease Control and Prevention estimates that nearly 30 percent of American women suffer from the condition, many unawares. Among African-American women in some studies, prevalence surpasses 60 percent.
Many factors affect the vaginal ecosystem—smoking, stress, diet, the number of sexual partners, and obesity. One of the most direct ways to upset the vaginal microbiome may be douching (more on this later). Recent studies show that the healthy state varies by ethnicity, complicating easy characterizations of “normalcy.”
Yet the consequences of vaginosis can be devastating. In a study of 1,950 urban women in Philadelphia, for example, vaginosis in the first trimester more than doubled the risk of spontaneous pregnancy loss in the second. In Belgium, vaginosis more than quintupled the risk of early preterm birth.
This relationship with preterm birth, defined as labor before 37 weeks in a 40-week pregnancy, is troubling. One in eight American children is born prematurely. These numbers began rising in the 1980s and, recent signs of a decline notwithstanding, they’ve remained stubbornly high ever since. Preterm birth is the leading cause of infant mortality in the nation. By one estimate, the costs to society surpass $26 billion yearly.
Vaginosis-related microbes have been implicated in roughly one-quarter of all preterm births. For the most vulnerable group of children, those born extremely preterm, or before 25 weeks, the number perhaps doubles.
Vaginosis “doesn’t get the attention it deserves, because it’s not a sexy STD,” says Deborah Nelson at Temple University in Philadelphia, who led the Philadelphia study.
Among the urban, mostly minority women Nelson works with, more than 60 percent suffer from the imbalance. This same population has an elevated risk of preterm birth. “It’s worrisome that young women in North Philly that have high levels of BV just don’t understand how hazardous it is,” Nelson says.
How do microscopic microbes provoke such major disasters? The immune system, scientists now realize, assists in labor. Essentially, the inflammatory response that repels invading germs also helps expel the infant. So any stimulus that prompts inflammation during pregnancy—be it from acute infection, microbial imbalance, or even gum disease, some studies show—can trigger labor prematurely.
How to treat vaginosis during pregnancy remains controversial, not least because antimicrobials may further perturb an already disturbed ecosystem. Yet according to one recent metanalysis of five studies involving 2,346 women, antibiotic treatment can lower the risk of preterm birth.
Inflammation during fetal development can have lifelong consequences for the child. That lesson has recently come to light in studies of asthma. In a condition called “chorioamnionitis,” bacteria sneak past the cervix into the uterus, inflaming the placenta. Vaginosis seems to increase the risk of chorioamnionitis, which is not quite a classic infection—like measles or the flu, say—but rather native microbes in the wrong part of the body. A pregnant mother may not notice, in fact. But the effects can be insidious.
In 2008, scientists following a cohort of nearly 1,100 Boston mothers and their children announced that premature birth increased the risk of wheezing at 6 years of age by 70 percent. The real kicker emerged when they separated children of mothers who had suffered from chorioamnionitis during pregnancy. For those children, the risk of asthma was more than four times as high.
A Finnish group has since replicated the finding among 15-to-17-year-olds, implying that the consequences of prenatal inflammation persist into adolescence and probably adulthood.