intro: Deep in the bowels of our, well, bowels, lurk trillions of microscopic bacteria. But don't be fooled by the big bad "B" word, intractably tied to infections and disease. In fact, these bitty bugs do us a world of good.
"There's a certain 'ick' factor associated with gut bacteria," said Lita Proctor, director of the Human Microbiome Project, a National Institutes of Health initiative to characterize the bugs that colonize our every crevice – outside and in. "People tend to think of them as germs and disease-causing pathogens, but they're actually part of our bodies."
A pretty major part, actually. Healthy adults carry up to five pounds of bacteria in their digestive tracts alone – roughly the weight of a brain.
"They belong there," said Proctor, adding that without gut bacteria, our brains and the rest of our bodies would suffer. "We need them to maintain our health."
Studies on the many roles of these bugs are quickly piling up, the latest suggesting that they can trigger obesity or help prevent it. Mice fed gut bacteria (read: poop) from obese humans gained more weight than those fed bacteria from lean humans, according to the study published today in the journal Science.
"We are walking communities," said Dr. Stanley Hazen, chairman of cellular and molecular medicine at the Cleveland Clinic Lerner Research Institute, who studies the effects of gut bacteria on human health but was not involved in the obesity study. "And yet we have not appreciated this from a medical standpoint really, and are only just starting to see how this appreciation may change the way we think about diseases like heart disease, obesity and diabetes."
Will there come a time when drugs and other therapies target our bacterial buddies instead of our own ailing cells? Proctor and Hazen both say "yes." In the meantime, here are five fascinating facts about the tiny tagalongs in your gut.
quicklist: 1 category: The Ins and Outs of Gut Bacteria title: They Move In at Birth url: text: Your first wave of gut bacteria comes from the wild ride through the birth canal, according to Proctor.
"It's during the event of birth that the initial and important inoculums of microbes happen," she said, adding that babies born by cesarean section might get a different dose of bacteria. "Nature meant for us to be born vaginally, and babies that don't getting that exposure might not start out with the bacteria they're supposed to have."
Breast milk is another important source of gut bacteria, according to Proctor. And it also contains nutrients that feed the bugs, called prebiotics.
"Some breast milk compounds are actually meant to be consumed by the bugs. We can't digest them," Proctor said, adding that breast milk also contains crucial antibodies. "There's a purpose for breast milk."
quicklist: 2 category: The Ins and Outs of Gut Bacteria title: They Fight Off Disease url: text: Gut bacteria are our first defense against "actual germs," according to Proctor.
"They hold down the fort," she said, describing how gut bacteria "outcompete" invading bugs that cause disease. "They themselves produce antimicrobials that can kill invading pathogens."
Studies in mice suggest gut bacteria play an important role in immunity, with mice engineered to be free of all microbes developing abnormal immune systems or none at all.
"Microbes help teach our bodies self from non-self," Proctor said, describing how a confused immune system might lead to allergies and other diseases. "Diabetes, rheumatoid arthritis, certain cancers, gut disease, there's work emerging that suggests an imbalance of microbes. We're not quite sure if it's cause and effect, but we know there's some type of contribution."
quicklist: 3 category: The Ins and Outs of Gut Bacteria title: They Outnumber Us url: text: The microbes in our bodies outnumber our own cells 10 to one, according to Proctor.
"The reason it's not apparent when you look in the mirror is they're much smaller than your own cells and the vast majority of them are living in your gut," she said.
They outnumber us in genes, too. The Human Microbiome Project has identified about 8 million microbial genes, compared with the roughly 20,000 genes uncovered by the Human Genome Project.
"Less than 10 percent of the DNA in each of us is from homo sapiens," said Hazen. "The vast majority is actually bacterial."
quicklist: 4 category: The Ins and Outs of Gut Bacteria title: They Take a Cut of Our Food url: text: After gobbling down the indigestible nutrients in our mothers' breast milk, gut bacteria continue to take a cut of the fruits, vegetables and grains we eat every day. The microscopic meals, called prebiotics, fuel the bacteria, which in turn keep our bodies in balance.
It's "a symbiotic relationship," according to Hazen, who studies how the byproducts of microbial metabolism can influence our risk of disease. "The only thing we know is that long-term dietary patterns can make an impact on the gut microbiome."
According to Hazen's research, "a longstanding diet low in animal products" appears to influence the microbiome for the better. But he says more work is needed to tease out the true relationship between the bugs we house and the food we eat.
quicklist: 5 category: The Ins and Outs of Gut Bacteria title: They're Changeable url: text: Unlike the 200,000 genes we inherit from our parents, our gut bacteria are changeable.
"That's the beauty of it. We can alter them with diet and scientifically-based probiotics," Proctor said, explaining how probiotic supplements and foods, like yogurt, can introduce new bacteria into our guts, potentially easing a range of symptoms.
And let's not forget about the fecal transplant – a procedure that helps restore the crucial gut bacteria balance with bugs from a healthy donor's poop. It's gaining popularity among patients with certain gut disorders and infections, and Proctor said it could hold promise for other diseases, too.
"It's another example of how adjusting a sick microbiome appears to be doable," said Proctor. "It's still quite young in the field, so we're trying to collect data to understand what's happening and develop scientifically-based and carefully designed procedures."
Carefully designed procedures unlike the many DIY fecal transplants demonstrated in YouTube videos, Proctor added, noting that the unsupervised practice could be dangerous.
"They don't really know what they're doing," she said.