By sreeremya.s

Bacteroides thetaiotamicron is a gramnegative ORGANISM. It consists of a 4776 member proteome containing the structural means to bring in and hydrolyze non-digestible polysaccharides as well as an environment sensing mechanism consisting of outer membrane proteins. Initially isolated from fecal matter, Bacteroides thetaiotaomicron has great importance in terms of the study of the symbiotic bacteria-host relationships within the human intestine as well as for its digestive abilities and potential breakdown of digested plants.

Bacteroides thetaiotaomicron is one such bacterial symbiont that is a dominant member of the intestinal microbiota of humans and other mammals. The recent report of the genome sequence of B. thetaiotaomicron1 is the first reported for an abundant Gram-negative organism of the human colonic microbiota and, as such, provides the first glimpse on a genomic scale of the genetic arsenal used by a Gram-negative symbiont to dominate in this ecosystem. The genome has revealed large expansions of many paralogous groups of genes that encode products essential to the organism's ability to successfully compete in this environment. Most noteable is the organism's abundant machinery for utilizing a large variety of complex polysaccharides as a source of carbon and energy.

GENOME STRUCTURE Bacteroides thetaiotaomicron consists of a 6.26 Mb genome

containing 4776 protein coding genes. The genome exists as one circular chromosome made of double stranded DNA. The GC content is 42.8% and 90% of the genome is involved in c Additionally, Bacteroides thetaiotaomicron consists of one circular plasmid (p5482) which is 33,038 bp long, containing 38 genes coding for 38 proteins which function mainly in environmental sensing. The GC content of the plasmid is 47.2% and 83% of the genome is involved in coding for proteinsoding for proteins essential in the binding and import of various polysachharides. Additionally, Bacteroides thetaiotaomicron consists of one circular plasmid (p5482) which is 33,038 bp long, containing 38 genes coding for 38 proteins which function mainly in environmental sensing. The GC content of the plasmid is 47.2% and 83% of the genome is involved in coding for proteins

CELL STRUCTURE AND METABOLISM acteroides thetaiotaomicron is an obligate

anaerobe, a major endosymbiont of the human gut. The bacterium uses various polysaccharides as its source of carbon and energy. B. thetaiotaomicron is able to use amylose, amylopectin, and pullulan (all three forms of starch) in addition to maltooligosaccharides. Thus, the organism digests complex plant materials that human enzymes cannot, and it makes an important contribution to human caloric..

 A key step in the metabolism of the polysaccharides entails their binding to the cell surface before undergoing hydrolysis. This allows for the efficient sequestration of hydrolysis products. Cell associated enzymes are responsible for hydrolyzing the polysaccharides into small fragments which are easily digestible. This binding and cleavage of the large substrate occurs either before or during translocation into the periplasm

The specific starch utilization system of B. thetaiotaomicron consists of a structure of seven sus (starch utilization genes) structural genes (susA to susG). Most of these genes encode proteins involved in the binding and hydrolysis of starch, and are separated into two transcriptional segments (susA and susB to susG respectively). Maltose as well as higher oligomers of starch are important in regulating structural gene expression, as the sus structural genes are only expressed in their presence. The expression of sus genes is regulated by the regulatory proteins SusR and MalR, which are both always expressed regardless of environmental cues. These two regulons make up for all of the genes needed to grow on starch and maltotriose.

ECOLOGY Bacteroides thetaiotaomicron is a major component of the

adult intestine and has been used as a useful model for the study of human-bacterial symbiosis. Its metabolic function for humans is to degrade plant polysacharides, a very essential capability for the human gut. Additionally, it is very important during the postnatal transition between mother's milk and a diet heavily consisting of plant starches. It has been found to stimulate angiogenesis (growth of new blood vessels from pre-existing vessels) within the gut, due to a microbial signal via bacterial sensing Paneth cells. B. thetaiotaomicron benefits its host by providing sufficient absorptive ability for nutrients the microbe helps process. Another postnatal developmental process within the gut mediated by Bacteroides thetaiotaomicron is the formation of the intestinal mucosal barrier, which helps protect the host against pathogenic invasion via the regulation of the expression of species-specific protein antibiotics.