To help clear up the confusion about how women should ― and shouldn’t ― try to make their vaginas a welcoming neighborhood for the right microbial crowd, we turned to Dr. David Fredricks, a physician-researcher at Fred Hutchinson Cancer Research Center and an expert on the vaginal microbiota. Fredricks’ most recent research on the topic was published this month in the journal mBio .

While the gut microbiome has recently gotten a lot of attention, the vaginal microbiome also plays a powerful role in women's health. The vagina is host to hordes of bacteria and some yeast, and women are probably most aware of them when an overgrowth of the wrong ones causes annoying symptoms. These symptoms can get so bothersome that women can be tempted to turn to any quick fix out there that offers to help ― and there are many. However, many remedies, such as some dietary supplements, may be a waste of money. Others, such as douching, could cause harm.

Entire miniature worlds exist inside our bodies. And for women, those myriad microorganisms that are at home between our legs can have a giant impact.

Bacterial vaginosis affects nearly a third of U.S. women and is linked to a host of conditions ranging from preterm birth to pelvic inflammatory disease.

Bacterial vaginosis affects almost a third of U.S. women at any given time. It is associated with an increased risk of sexually transmitted infections like HIV, gonorrhea and chlamydia, and increased risk of preterm birth and pelvic inflammatory disease, to name a few. And if a woman is HIV positive, having BV makes her almost three times more likely to transmit HIV to her partner, Fredricks said, noting that the role of BV in many of these conditions is still completely unknown.

“Women with bacterial vaginosis tend to have malodorous vaginal discharge, and that’s probably the most common complaint, although there can also be some local irritation,” Fredricks said. “But the key thing about bacterial vaginosis is it’s associated with numerous adverse health outcomes.”

Bacterial vaginosis, an overgrowth of any number of a diverse group of “bad” bacteria in the vagina, can lead to serious health issues. It’s sometimes mistaken for a yeast infection, a fungal infection caused by too much yeast. While yeast infections are often more familiar to women, they are actually less common than bacterial vaginosis.

First, let’s be clear about one important thing: Having the proper mix of vaginal microbes is not just a matter of personal comfort, he said.

A small amount of Candida yeast normally lives in the healthy vagina as well, and most women experience an overgrowth of these microorganisms ― yeast infection, or candidiasis ― at some point in their lives. Interestingly, the bacteria and the yeast in the vagina don’t seem to interact with one another ― they just commingle peacefully like neighbors with houses on the same block. The only known interaction, Fredricks said, is yeast’s tendency to overgrow when bacteria are depleted by antibiotics. (Unfortunately, we don’t yet know how to prevent this, Fredricks said, although yeast infections are treatable with over-the-counter creams or an oral pill.)

“Lactic acid is actually different than other organic acids in that it seems to have a more profound antimicrobial effect than, for instance, acetic acid [vinegar] or other things,” Fredricks said. “And it seems to disrupt the cell wall of bacteria. So by producing this acid it makes it inhospitable for the growth of other bacterial species ― including, it’s thought, the BV-associated bacteria.”

Vaginal species of Lactobacillus bacteria live off of a sugar called glycogen that is produced by cells of the vagina, and they are thought to convert this sugar into lactic acid, which lowers the pH of the vagina and makes the vagina an inhospitable environment for other bacterial species.

Almost all of the bacteria in most women with healthy vaginal microbial communities are species of the genus Lactobacillus. Lactobacilli are familiar to most of us as one of the types of bacteria living in yogurt with live cultures ― but the particular species of Lactobacillus living in dairy products are different than the species that live in the human vagina.

We know that having the wrong vaginal microbiome is downright dangerous, even if it’s not clear why. So what does the right one look like?

There’s also some hints that men might be asymptomatic reservoirs of BV-associated bacteria, Fredricks said. “That’s why we’re also doing partner studies, of men and women, to try and see whether there’s transmission of these bacteria back and forth.”

The researchers hope that some of the biological and behavioral data they’re collecting from these women correlate with the risk of BV development, information that could provide valuable clues about where the bad bacteria are coming from. For example, previous research by his group has found that some ― but not all ― women who go on to develop BV first harbor the bacteria in their rectum or their mouth, so some types of sexual activity might spread bacteria from one location to the other, at least in some women.

These dedicated research volunteers are also keeping daily diaries, Fredricks said, tracking their use of any type of product inside the vagina and the specific sexual activities they engage in, and in what sequence.

“We don’t know what causes these shifts,” Fredricks said, “and to get at that, we’re doing longitudinal studies in women, where we have women without bacterial vaginosis who obtain daily vaginal swabs so we can track what’s happening on a daily basis.”

Wondering what amines smell like? Two associated with BV are called “putriscine” and “cadaverine.” (You’re welcome.)

“It helps to explain some of the symptoms that women with BV have,” Fredricks said. “For example, one of the key features that women with BV have is amine odor, and we think that this amine odor comes from the metabolism of amino acids,” the building blocks of proteins.

The research published by his team this month revealed two distinct metabolic signatures of BV-associated bacteria ― notable differences emerged there, too: Almost two-thirds of the named chemical metabolites the team detected in the vaginas of women with BV were different than those in women without BV, and they differed across multiple metabolic pathways.

In BV, a vaginal microbiome dominated by a couple species of sugar-eating Lactobacilli switches to one dominated by diverse, protein-eating bacteria. Research in Fredricks’ Lab published in 2012 characterized these BV-associated bacteria (including several that his team were the first to link to the disease ), finding that most women with BV had some combination of 24 non-Lactobacillus species.

But as for the bacteria? Fredricks said that we just don’t know what causes the microbiome of a particular vagina to shift over from one dominated by a couple species of Lactobacillus to one dominated by BV-associated bacteria.

Besides use of antibiotics, several other factors ― including diabetes and HIV infection ― are known to cause an overgrowth of yeast within the vaginal microbiome.

So what’s a gal to do?

It’s frustrating to have such big gaps in our knowledge about the vaginal microbiome, especially given that bacterial vaginosis is so persistent in many women and is linked to so many poor outcomes.

But, from Fredricks’ perspective, there is one clear takeaway for women seeking a healthy vaginal microbiome: “Don’t douche.”

Almost one of five U.S. women of reproductive age douched in a 12-month period, according to recent data from the Centers for Disease Control and Prevention, and douching and other types of vaginal washing are even more common among women in many other countries, as reported in an article this month in The Atlantic magazine.

Women commonly douche in attempts to keep the vagina healthy and conform to perceived societal expectations for hygiene.

“It’s a cultural practice,” Fredricks said. “And what happens is [women’s] mothers did it so they feel they should do it, and they feel it’s associated with cleanliness.” So why should women run, not walk, away from this common practice? Many reasons, Fredricks said.

“Numerous studies have shown the deleterious consequences of douching. It increases your risk of BV. It increases your risk of pelvic inflammatory disease. If you’re pregnant, it increases your risk of preterm birth. It may increase your risk of cervical cancer. There are a lot of negative associations that have been linked with douching. “No one’s ever shown any benefit to douching,” He said.

Even douches of plain water and of vinegar fall into the no-go category, he said. Research shows that water douches temporarily wash out Lactobacillus, he said, and even though our good Lactobacilli produce acid, vinegar is just not the same.

“All acid is not created equal, and acetic acid cannot substitute for lactic acid,” Fredricks said.

So douching is out. What else?

Unlike in the gut, where there’s a link between diet and the microbes living there, “there is no clear association between diet and the vaginal microbiota,” Fredricks said. So while there are numerous reasons to eat healthfully, influencing your vaginal microbiome is not one of them, as far as we know.

What about probiotics? These are products, including oral supplements, containing live bacterial species. Here the picture gets murkier.

“Curiously, and I don’t really understand this, there have been studies that have looked at oral probiotics with some of these gut Lactobacillus species [species of these bacteria that will live in the gut but not the vagina], and they’ve had more of an impact than studies that have used direct inoculation into the vagina with things like Lactobacillus crispatus” ― one of the vaginal species of Lactobacillus.

Fredricks referred to a 2006 randomized, controlled clinical trial that found the addition of a probiotic containing two gut species of Lactobacillus ― L. rhamnosus and L. reuteri ― to standard antibiotic therapy for BV reduced the risk of recurrent disease.

Fredricks scoffed at some of the ingredients listed in some products designed to improve the health of the vaginal microbiome (“That doesn’t make any sense,” he said of one product purporting to contain “yeast-digesting enzyme”) but he did say this:

“I would say, if I was going to pick one probiotic intervention that we actually have scientific evidence to suggest that it actually has some benefit, it would be Lactobacillus reuteri with Lactobacillus rhamnosus,” he said, although he noted that larger randomized, controlled trials are needed to confirm this benefit.

How taking an oral probiotic containing gut bacterial species could influence the growth of distinct vaginal species is certainly a head-scratcher, but it underscores how much more we have to learn about the microorganisms that call our bodies home and the influence they have over our health.

Fredricks’ research right now on these microscopic creatures is laying a foundation that he hopes will lead to future advances in modulating the vaginal microbiome to reduce health risks associated with BV, like preterm birth and HIV transmission.

“If BV is associated with increased risk of HIV transmission to men ― and this is what hasn’t been proven ― if you could potentially modulate the microbiota, then you might decrease the risk of HIV transmission,” he said. “So again, it’s part of the mystery of what we’re trying to sort out.”

Fredricks’ team is currently recruiting study participants to help them better understand the vaginal microbiome. In the meantime, ladies, while this mystery is being unraveled, see your doctor if you have any concerns about your vaginal health and ― please ― set down that little plastic squeeze bottle for good.

Susan Keown is a staff writer at Fred Hutchinson Cancer Research Center. Before joining Fred Hutch in 2014, Susan wrote about health and research topics for a variety of research institutions, including the National Institutes of Health and the Centers for Disease Control and Prevention. Reach her at skeown@fredhutch.org.