Tight economic times remind us of two important lessons: our biggest challenge is often just finding a way to make a living, and our ability to make a living depends heavily on the people around us.
These lessons are especially pertinent in bacterial communities, where ‘tight economic times’ are the norm. Of the many challenges bacteria face in making a living, perhaps none are more important than generating ATP, maintaining redox balance, and acquiring carbon and nitrogen to synthesize primary metabolites. So persistent and fundamental are these challenges that bacteria devote extraordinary effort to them, as illustrated here.
The ability of bacteria to make a living depends on the rest of their community. While much more is known about certain forms of bacterial signaling such as quorum sensing, the process by which a microbial species detects the levels of a specific metabolite to monitor its own abundance (Bassler and Losick, 2006
), it is easy to forget that the most fundamental way in which bacteria communicate is by importing the substrates for metabolism and exporting its end products. If your neighbor eats starch and produces 5 mM succinate, you will probably take notice. If you can turn the succinate into butyrate and generate ATP in the process, your chances of thriving are even better. Of course, your neighbor will likely sense the decreased succinate and increased butyrate, and respond accordingly. A pair of bacterial species carrying out housekeeping metabolism is thus an important example of systems biology at work, with the proviso that the system does not end at the cell membrane.
In this review, we will travel down a carbohydrate catabolic pathway common to many species of Bacteroides, highlighting the interspecies interactions established (often inevitably) at its key steps. Primary fermenters, like the Bacteroides, are the gateway through which carbohydrates enter the network of syntrophic links (inter-species metabolic interactions) within a gut microbiota. (). Carbohydrates consumed by gut microbiota are typically oligo- or polysaccharides derived from diet, host mucosal secretion, or other resident (or dietary) microbes. In the simplest manifestation, Bacteroides and other glycophagic (carbohydrate-eating) species, degrade the complex carbohydrates to their component monosaccharides, which in turn are metabolized through the sequential action of (i) one of three prototypic glycolytic pathways to yield phosphenolpyruvate (PEP), followed by (ii) conversion of PEP to fermentation end-products such as an organic acid, solvent, or alcohol. Below we discuss factors that influence the operation of these pathways and the resulting spectrum of secreted by-products and end-products which serve as the currency of syntrophy within the gut. By focusing on a these pathways, our goal is to offer illustrative examples and highlight existing questions rather than provide an exhaustive account of what is known, and we apologize in advance to our colleagues whose work has been omitted inadvertently and due to space constraints.
Simplified illustrative schematic of some trophic networks within the intestinal microbiota