Bacteroides

Bacteroides fragilis . From the [1]Zdravotni University

Classification

Higher order taxa:

Bacteria; Bacteroidetes/Chlorobi group; Bacteroidetes; Bacteroides (class); Bacteroidales; Bacteroidaceae; Bacteroides

Species:

Bacteroides caccae; Bacteroides distasonis; Bacteroides eggerthii; Bacteroides fragilis; Bacteroides merdae; Bacteroides ovatus; Bacteroides stercoris; Bacteroides thetaiotaomicron; Bacteroides uniformis; Bacteriodes vulgatus

Description and Significance

Bacteroides are commonly found in the human intestine where they have a symbiotic host-bacterial relationship with humans. They assist in breaking down food and producing valuable nutrients and energy that the body needs. However, when Bacteriodes are introduced to parts of the body other than the gastrointestinal area, they can cause or exacerbate abscesses and other infections.

Genome Structure

The genome of the circular chromosome of many Bacteroides species and strains have been studied; research is being done on sequencing Bacteroides species in order to understand their pathogenic properties. All Bacteroides have G-C composition of 40-48%. Strain NCTC9343 of the species Bacteroides fragilis, for example, is 5,205,140 bp long and has a G-C content of 43.19%. Much of the genome is controlled by sigma factors which respond to environmental factors. A large part of the proteins made by the Bacteroides genome goes to breaking down polysaccharides and metabolizing their sugars (Jian et al. 2003). There have been a total of three genome projects done on two different species of Bacteroides. The three genomes sequenced were that of Bacteroides thetaiotaomicron VPI-5482, Bacteroides fragilis YCH46, and Bacteroides fragilis NCTC 9343. More information on strain NCTC9343 of Bacteroides fragilis can be found at The Sanger Institute. Information and a schematic representation of the Bacteroides thetaiotaomicron VPI-5482 chromosome can be found at TIGR and at NCBI.

Cell Structure and Metabolism

Bacteroides are gram-negative, nonsporeforming, anaerobic, and rod-shaped bacteria. They have an outer membrane, a peptidoglycan layer, and a cytoplasmic membrane. The main by-products of their anaerobic respiration are acetic acid, iso valeric acid, and succinic acid. They are involved in many important metabolic activities in the human colon including fermentation of carbohydrates, utilization of nitrogenous substances, and biotransformation of bile acids and other steroids. Most intestinal bacteria are saccharolytic, which means that they obtain carbon and energy by hydrolysis of carbohydrate molecules. To see a list of metabolic pathways that occur within Bacteroides thetaiotaomicron, visit The Systems Biology Institute.

It is estimated that only about 2% of simple sugars make it past the upper gastrointestinal tract and to the Bacteroides. Thus, simple sugars are probably not Bacteroides main source of energy. However, polysaccharides from plant fibers, such as cellulose, xylan, arabinogalactan, and pectin, and vegetable starches such as amylose and amylopectin, are much more prevalent in the colon. These polysaccharides have also been shown to induce a variety of glucosidase activities from Bacteroides including a b-1,3-glucosidase activity responsible for laminarin degradation and a variety of a and b-1,4 and -1,6 xylosidase and glucosidase activities. A large part of the Bacteroides 4779-member proteome includes proteins that hydrolyze these polysaccharides (Jian et al. 2003).

Bacteroides thetaiotaomicron have been shown to bind to polysaccharides with their outer membrane receptor system (the outer membrane can be seen in the picture to the right) before pulling the polysaccharides into the periplasm for monosaccharide degradation. This technique may help insure that the polysaccharides are not stolen by other intestinal organisms or lost in the intestines by diffusion. Bacteroides polysaccharide utilization genes are thought to be controlled by repressor/inducer mechanisms.

Although Bacteroides are gram-negative, as are most bacteria in the human colon, and live in the same environment as E. coli, the two bacteria are actually less closely related than Bacteroides are to gram-positive bacteria. The Bacteroides definition has specific criteria, some of which are as follows: obligately anaerobic, gram-negative, saccharolytic, contain enzymes of the hexose monophosphate shunt-pentose phosphate pathway, have a DNA-base composition of about 40-48% G-C, and membranes contain a mixture of long-chain fatty acids, mainly straight chain saturated, anteiso-methyl, and iso-methyl branched acids.

Cross section of a Bacteroides showing an outer membrane, a peptidoglycan layer, and a cytoplasmic membrane. From New-asthma

Ecology

Anaerobes make up the majority of bacteria found in the bacterial flora found in the human colon; the most predominant bacteria found are Bacteroides. The colon contains over 400 species of organisms and has more than 10¹¹ organisms per gram of wet weight. Bacteroides by themselves constitute nearly 10¹¹ organisms per gram of feces (dry weight). These anaerobes enhance health of the human host by helping catabolize complex molecules such as fucosylated glycans.

Anaerobes, such as Bacteroides, are though to play a fundamental role in this ecosystem by processing complex molecules into simpler compounds. (These simpler compounds are used by the human host as well as the Bacteroides.) The ability to convert complex polysaccharides into useable compounds might allow Bacteroides to be more competitive than bacteria that must rely on other sources of energy. Furthermore, Bacteroides actually stimulates the gut lining to produce fucosylated glycans. The bacteria also stimulate angiogenesis (formation of blood vessels) in the newborn epithelium, enhancing human uptake of nutrients. Thus, Bacteroides bacteria have a complex and generally beneficial relationship with their host--so long as they are retained within the gut lumen.

Because each species lives inside a certain person, it has to adapt slightly to its surroundings. Today fecal DNA, which is provided by Bacteroides, can identify their hosts in horses and pigs. Many of the human, dog, cat, and gull sequences had very close matches with multiple hosts and thus may not be useful targets for fecal source identification.

Pathology

When Bacteroides escape the gut, they are responsible for many types of infections and abscesses that can occur all over the body including the central nervous system, the head, the neck, the chest, the abdomen, the pelvis, the skin, and the soft tissues. The widely accepted model for abdominal infections goes as follows: disruptions of the intestinal wall, bacterial flora infiltrate the cavity, aerobes (most active part in infection) like E. coli start the preliminary tissue destruction and reduces the oxidation-reduction potential of the oxygenated tissue (low oxidation-reduction potential favors anaerobe growth), anaerobic Bacteroides start to replicate, and then Bacteroides dominate the infection. Along with diarrhea and abscesses, Bacteroides have been known to be involved in cases of meningitis and shunt infections, especially in children. Any tissue not normally colonized with Bacteroides has a possibility of infection when introduced to mucous or other materials containing Bacteroides. Diagnosis and treatment are complicated due to the slow growth of Bacteroides, the increasing resistance to antibiotics, and the polymicrobial nature of an infection with Bacteriodes (most infections deemed Bacteriodes infections can actually involve between 5 and 10 organisms). Bacteroides fragilis, which occurs with the most in clinical infections, along with B. distasonis, B. ovalus, B. thetaiotaomicron, and B. vulgatus are almost universally resistant to penicillins, mostly due to the production of beta-lactamase. These bacteria are prevalent in the gastrointestinal area and are responsible for most intra-abdominal infections, such as perirectal abscesses and decubitus ulcers. In addition, Bacteroides present a huge problem as a source of infection during gastrointestinal surgeries; thus, proper measures and efficient surgical draining must take place in order to lower the risk of infection.