In some human diseases, the wrong mix of bacteria seems to be the trouble.
The particular assortment of microbes in the digestive system may be an important factor in the inflammatory bowel condition known as Crohn's disease.
Research involving more than 1,500 patients found that people with Crohn's disease had less diverse populations of gut microbes.
"[This] basically for the first time identifies what might be the bacterial changes in patients with Crohn's disease," says Ramnik Xavier, of Masssachusetts General Hospital in Boston, who led the work.
More than a million Americans suffer from Crohn's, which seems to start when an overreactive immune system causes abdominal pain, diarrhea, bleeding, weight loss and other symptoms. Many patients have to take powerful steroids (which can have serious side-effects), and some have parts of the digestive tract surgically removed.
Mounting evidence has suggested that microbes living in the gut might contribute to the problem. So Xavier and his colleagues compared the species of bacteria in more than 447 Crohn's patients to the mix of microbes in more than 221 healthy people.
In their paper published in the journal Cell, Host and Microbe, the researchers detailed the clear difference they discovered: The patients with Crohn's seemed to have too many of the sorts of bacteria that rile immune systems.
In addition to having less diversity in their gut microbes, Xavier says, the Crohn's patients had fewer bacteria that have been associated with reduced inflammation and more bacteria associated with increased inflammation. (The findings were confirmed in 800 Crohn's patients from other studies.)
Interestingly, children whose doctors had tried to treat their Crohn's symptoms with antibiotics before they were properly diagnosed had a mix of microbes that was the most out of whack.
"We may have to revisit the use of antibiotics in [these] patients with early-onset Crohn's disease," Xavier says.
Instead, doctors might eventually do better to identify and prescribe treatments that mimic the helpful bacteria, he says, along with foods or other pharmaceutical agents that reduce or counteract the harmful bacteria.
"There's the possibility that we might be able to identify [some] sort of super-probiotics that might be able to correct the gut back to the healthy state," Xavier says.
UCLA pathologist Jonathan Braun, who studies microbial ecology, says the paper offers important first insights into illnesses beyond Crohn's. "Other diseases are thought to be driven at least in part by bacteria," he says, such as some inflammatory and autoimmune disorders. Bacteria may also be involved in obesity.
Humans should work harder to understand bacteria, Braun says, "and live with them when they're helping us, or get them to serve us better when they are causing harm."