The majority are anaerobic, which means they don’t require oxygen and, of these, species of the genus Bacteroides are among the most common.
Outside the gut, strains of Bacteroides can cause abscesses in the abdomen, brain, liver, pelvis, and lungs, as well as bacteremia, an infection of the bloodstream. But in the colon, they serve important functions, breaking down carbohydrates, producing enzymes specifically designed to deal with different foods, and extracting energy from those foods. One species, B. fragilis, appears to stimulate immune cells called regulatory T-cells, which restrain more aggressive inflammatory T-cells, which can trigger colitis and other disorders.
Researchers are also beginning to tease out the possible relationship between the overall makeup of a person’s gut microbiome and a propensity toward obesity, in which it is suspected Bacteroides may play a role.
In any case, the usefulness and ubiquity of bacteria in the colon probably can’t be overstated. Three-quarters of human feces is water and, of the remaining quarter, one-third is composed of bacteria—or as Giulia Enders, author of Gut: The Inside Story of Our Body’s Most Underrated Organ, describes them, “gut flora that ended their careers in the digestive business and are ready to retire from the workplace.”