Bacteroidetes contains four orders: Bacteroidales, Cytophagales, Flavobacteriales, and Shingobacteriales. There are more than 700 hundred characterized species across the Bacteroidetes orders. The species in the Bacteroidetes phylum are gram negative nonsporulating rods and be both fermentative and aerobic. The most important order of the Bacteroidetes is the Bacteriodales relative to humans. This order contains a genus, Bacteroides, that is well studied because it is a major component of the human gut.

Bacteroides are the dominant bacteria in the human large intestine and other animals. This genus of Bacteroidales are usually commensals but can occasionally become pathogenic, an example being bacteremia. One of the most important bacteria in the human gut is the Bacteroides thetaiotaomicron. A large portion of B. thetaiotaomicron genome specializes in breaking down complex polysaccharides. This Bacteroides species is responsible for our ability to break down certain plant polymers that we would, otherwise, be unable to do on our own. The human genome does not produce the enzymes that can do this, neither does the human genome encode for these enzymes either.