The last place we’d expect to find life forms sharing their resources without a clear payoff is the human gut. Of course, there must be a payoff, but the relationships amongst gut bacteria are so complex that we cannot yet see them. What we see is something Tyrrell Conway in Microbe Magazine calls the “restaurant hypothesis.” Certain biofilms of mixed species (such as Bacteroides species) feed off complex sugar chains that others such as E. coli cannot break down. Within the gut lining, different patches of biofilm digest sugar chains into different kinds of sugars–which then E. coli can feed upon. So E. coli bacteria can visit different “restaurants” to consume different meals.
How does Bacteroides get paid for the meal? Hard to say, although I might wildly speculate that because Bacteroides are anaerobes (poisoned by too much oxygen) they benefit from E. coli sucking up the oxygen as it chows down on sweets.
Be that as it may, the Harvard research team of Laurie Comstock has devoted countless research hours to figuring out the various restaurant management teams. The result (from Current Biology 24:40) is a chart like this:
If I may venture to decipher what’s going on here, based on the paper, this is what I come up with:
Bo = Bacteroides ovatus. A super chef, B. ovatus offers a long menu with all kinds of fresh ingredients: inulin (most plants), pectin (from fruits), levans (from onions), xylan (plants and algae). B. ovatus actually provides the enzymes (called hydrolases) needed to break down these ingredients, to the sous-chefs: Bv (B. vulgatus), Pd (P. distatonis) and sometimes Bc (B. caccae). These sous-chefs then break the stuff down further to a mix of simple sugars that end up on E. coli‘s plate.
Bt = Bacillus thetaiotaomicron. (Somebody liked Greek letters.) B. thetaiotaomicron doesn’t service xylan, but it provides enzymes to break down levan for lots more kinds of bacteria. And so forth.
How do the chef bacteria provide the hydrolase enzymes? They wrap them up in packages of outer membrane, called outer membrane vesicles, then share them within the restaurant biofilm.
That’s about as much as my brain can digest for tonight.