Scientists have found that strength in numbers doesn't hold true for microbes in the intestines. A minority population of the right type might hold the key to regulating good health.
Researchers watched for immune response as isolates of species of bacteria, normally associated with healthy zebrafish, were introduced one at a time and in combination into previously germ-free intestines of the fish.
In a telling sequence, one bacterial species, Vibrio, drew numerous neutrophils, which indicated a rapid inflammatory response in one fish. Another species, Shewanella, inserted into a separate germ-free fish barely attracted an immune response. In a third germ-free fish, both species were introduced together and assembled with a ratio of 90- percent Vibrio to 10-percent Shewanella.
The inflammatory response in the third fish was completely controlled by the low-abundance species.
Low counts of a bacterial species generally have been discounted in importance, but slight shifts in the ratios of abundant microbe populations have been thought to have roles in obesity, diabetes and inflammatory bowel diseases such as Crohn's disease.
That thinking is now changing, author said. "The contribution of each bacterium is not equal. There is a per-capita effect that needs to be considered."
The keystone - an important participant that functions to regulate a healthy microbiota - may reside in low-abundant bacterial species. The research team found through additional scrutiny that these species secreted molecules - for now unidentified - that allowed them to dampen the immune response to the whole community.
The research is published in the journal Cell Host & Microbe.