Bacteroides thetaiotaomicron

Bacteroides thetaiotaomicron was first described in 1912 under the name Bacillus thetaiotaomicron and moved to the genus Bacteroides in 1919.[2] The B. thetaiotaomicron type strain VPI-5482 was originally isolated from a healthy adult's human feces.[3] The specific name derives from the Greek letters theta, iota, and omicron; the List of Prokaryotic names with Standing in Nomenclature indicates this as "relating to the morphology of vacuolated forms".[2] The name is used as an example of an "arbitrary" species name in the International Code of Nomenclature of Prokaryotes.[4][5] Bacteroides belong to the Bacteroidaceae family, Bacteroidales order, Bacteroides class, Bacteroidetes phylum, and Bacteroidetes/Chlorobi group.

The genome of B. thetaiotaomicron was sequenced in 2003. It is 6.26 megabases in length, but has a relatively small number of distinct genes, due to many genes coding for proteins that are unusually large compared to other prokaryotes.[6] This genomic feature is shared with another member of the genus with a similar lifestyle, Bacteroides fragilis.[7] The genome is notable for containing very large numbers of genes associated with breaking down polysaccharides, including glycoside hydrolases and starch binding proteins.[6][7] The genome also contains large numbers of genes encoding proteins involved in sensing and responding to the extracellular environment, such as sigma factors and two-component systems.[6][8][9] The B. thetaiotaomicron genome also encodes a large number of small non-coding RNAs,[1] though few have been characterized to date.

Bacteroides thetaiotaomicron is capable of metabolizing a very diverse range of polysaccharides. Its complement of enzymes for hydrolysis of glycosidic bonds is among the largest known in prokaryotes, and it is thought to be capable of hydrolyzing most glycosidic bonds in biological polysaccharides.[7] As a component of the human gut flora, it can use both dietary carbohydrates and those sourced from the host, depending on nutrient availability.[10]

Although it is considered an obligate anaerobe, B. thetaiotaomicron is aerotolerant and can survive, but not grow, when exposed to oxygen. It expresses a number of proteins that scavenge reactive oxygen species such as hydrogen peroxide when exposed to air.[11]

Members of the genus Bacteroides accounts for about a quarter of the microbial population in an adult human's intestine. Bacteroides thetaiotaomicron is one of the most common components of the human gut flora. In a long-term study of Bacteroides species in clinical samples, B. thetaiotaomicron was the second most common species isolated, behind Bacteroides fragilis.[12]

B. thetaiotaomicron is considered commensal or symbiotic, meaning it provides the host with key benefits like digestion.[13][14] B. thetaiotaomicron's proteome that consists of 4779 members creates an apparatus capable of hydrolyzing otherwise indigestible polysaccharides, like amylose, amylopectin, and pullulan.[8] B. thetaiotaomicron has far more glycosyl hydrolases, which are enzymes that catalyze the hydrolysis of the glycosidic bonds of carbohydrates, than any other bacteria present in the human gut. Additionally, 61% of these glycosyl hydrolases are located in the outer membrane or extracellular, suggesting that the digestive capabilities serve the bacteria's host more than anything. [15]The polysaccharides are converted into monosaccharides which can then be absorbed by human cells.

B. thetaiotaomicron is also an opportunistic pathogen and can infect tissues exposed to gut flora.[16] Its polysaccharide-metabolizing abilities make it a food source for other components of the microbiome. For example, while B. thetaiotaomicron expresses sialidase enzymes, it cannot catabolize sialic acid; as a result its presence increases the free sialic acid available for other organisms in the gut. These interactions can contribute to the growth of pathogenic bacteria such as Clostridium difficile, which uses sialic acid as a carbon source.[17] Similar interactions can cause B. thetaiotaomicron to exacerbate pathogenic E. coli infection.[18] These strategies allow B. thetaiotaomicron to thrive in the competitive environment of the human intestine.[19]