"We know that this environment can house hundreds of species, so why the competition within the same species?" Lee says. "There certainly isn't a lack of space or nutrients, but this was an extremely robust and consistent finding when we tried to essentially 'super-colonize' the mice with one species."
To explain the results, Lee and the team developed what they called the "saturable niche hypothesis." The idea is that by saturating a specific habitat, the organism will effectively exclude others of the same species from occupying that niche. It will not, however, prevent other closely related species from colonizing the gut, because they have their own particular niches. A genetic screen revealed a set of previously uncharacterized genesa system that the researchers dubbed commensal colonization factors (CCF)that were both required and sufficient for species-specific colonization by B. fragilis.
But what exactly is the saturable niche? The colon, after all, is filled with a flowing mass of food, fecal matter and bacteria, which doesn't offer much for organisms to grab onto and occupy.
"Melanie hypothesized that this saturable niche was part of the host tissue"that is, of the gut itselfMazmanian says. "When she postulated this three to four years ago, it was absolute heresy, because other researchers in the field believed that all bacteria in our intestines lived in the lumenthe center of the gutand made zero contact with the hostour bodies. The rationale behind this thinking was if bacteria
|Contact: Brian Bell|
California Institute of Technology