Although the human digestive tract is like an unobstructed tube, when it comes to the bacteria that live inside it, there are distinct territories within the tube that host specific populations of microorganisms. Researchers have been investigating what it could mean for human health when a type of bacteria that is common in one zone migrates to another region.
What they have noticed is that people with conditions such as inflammatory bowel disease and colon cancer, among others, have higher intestinal concentrations of bacteria that are usually found primarily in the mouth.
An example of a type of mouth bacteria that is showing up in these cases is Klebsiella pneumoniae. K. pneumoniae is normally found in the mouth, intestines and on the skin of healthy people. When immune function is not up to par, K. pneumoniae can cause opportunistic infections which can range from mild to severe and impact almost every part of the body.
In research published in the journal Science in October 2017, investigators found opportunistic intestinal overgrowths of K. pneumoniae appeared to trigger heightened activity among T helper 1 (TH1) immune cells in the intestines of special germ-free mice. The overactive TH1 cells created inflammatory changes similar to those that occur with inflammatory bowel disease .
These findings are preliminary and may not apply to people in the same way they do for mice, but what would it mean if they do apply to people?
One possibility is that unique probiotic products would be developed for people with bowel conditions to restore microbial balance and get rid of pesky overgrowths. Another possibility is that oral care products might be developed to tweak the microbial balance in the mouth to reduce the risk of overgrowth of an organism such as K. pneumoniae further along the digestive tract.
Instead of trying to deal with overgrowths and infections by using antibiotics, the future might find us using different bacteria to recolonize areas of overgrowth or infection, thereby avoiding medication side effects and problems associated with antibiotic resistance.
 Atarashi, Koji, et al. “Ectopic colonization of oral bacteria in the intestine drives TH1 cell induction and inflammation.” Science 358.6361 (2017): 359-365.