I reckon that for most of us the microbe we associate with "human intestinal flora" is Escherichia coli. We'd be quantitatively wrong though, because E. coli and its near relatives the enterobacteriaceae together make up less that 0.5% of the microbiome. Like Escherichia and Neisseria gonorrhoeae, "hard to spell, easy to catch", Bacteroides thetaiotaomicron looks like a mouthful but actually breaks down easily: theta-iota-omicron. I think that's a great name, like a college fraternity, but I know of no other species named after a clatter of Greek letters. Rather than being a homage to someone called Thio, the name was first recorded more than 100 years ago by Arcangelo Distaso, a microbiologist at the Institut Pasteur in Paris, in a 1912 paper "Sur le putréfaction de la paroi instestinale de l'homme".
We should really all pay more attention to B. thetaiotaomicron because it is a major component of our gut bacteria and performs a vital task for any of us who eat either plants or fungi, which is all of us except for some extreme Masai and Inuit who eat only meat. B.thio has a genome coding for 4,800 proteins, so it's a little more biochemically capable than E.coli but it is particularly rich in enzymes that can process the complex polysaccharides that are a major component of plant and fungal cell walls. That makes more digestible sugars available for us and presumably reduces the bulk of the stool. The presence of these bacteria is therefore really important for the weaning transition when we take infants off the breast [or more likely off formula milk in Ireland] and start to shovel rice- and wheat-based pap into their gaping maws. Without B.thio there will be colic, sleep deprivation, ill-temper, battery and divorce. A recent paper in Nature, suggests that Bacteroides spp. may have gotten a toe-hold in the human gut when we started to incorporate a lot of yeast [bread and beer] in the diet.
Because it is there, B.thio is an 'opportunistic pathogen', it can be responsible for what Distaso was studying in his 1912 paper which can be translated as "pathogenesis of the human gut wall". This is particularly so in cases where abdominal wounds and peritonitis have made a hole in the gut wall. It's a problem because B.thio is resistant to many of the antibiotics: because of its biochemical toolbox, it just sees antibiotics as more food. But its normal function and normal interaction with the cells of the intestinal epithelium shines a light on the complexity and subtlety of our relationship with the bugs inside. B.thio for example secretes metabolites that encourage the development of vascularisation of the epithelium, so that the goodies created by its digestive enzyme scan be more effectively carried away to the liver. It also seems to play a role in the development of the mucosal surface which is a key physico-chemical barrier keeping potential pathogens at bay. So little known, so much to learn.