Bacteroides fragilis is an anaerobic, Gram-negative, pleomorphic to rod-shaped bacterium. It is part of the normal microbiota of the human colon and is generally commensal, but can cause infection if displaced into the bloodstream or surrounding tissue following surgery, disease, or trauma. Habitat Bacteroides fragilis resides in the human gastrointestinal tract and is essential to healthy gastrointestinal function such as mucosal immunity and host nutrition. As a mesophile, optimal growth occurs at 37 °C and a pH around 7. Morphology Cells of B. fragilis are rod-shaped to pleomorphic with a cell size range of 0.5-1.5 x 1.0-6.0 μm.B. fragilis is a Gram-negative bacterium and does not possess flagella or cilia making it immotile. However, it does utilize peritrichous fimbriae for adhesion to other molecular structures. B. fragilis also utilizes a complex series of surface proteins, lipopolysaccharide chains, and outer membrane vesicles to help survive the volatile intestinal micro-environment. Metabolism & mutualism in the gut microbiome B. fragilis is an aerotolerant, anaerobic chemoorganotroph capable of fermenting a wide variety of glycans available in the human gut microenvironment including glucose, sucrose, & fructose. B. fragilis can also catabolize a variety of biopolymers, polysaccharides, and glycoproteins into smaller molecules which can then be used and further broken down by other microbes. Fatty acids produced by the fermentation of carbohydrates can serve as a source of energy for the host. Cytochrome bd oxidase is essential for oxygen consumption in B. fragilis and can allow other obligate anaerobes to survive in the now oxygen-reduced microenvironment. Animals lacking gut bacteria require 30% more caloric intake to maintain body mass. Environment-sensing systems The complex environmental-sensory system allows B. fragilis to survive/adapt in the ever-changing human gut microbiome. This system is composed of/designed to effectively handle: Bacteriocins: B. fragilis intestinal isolates secrete high levels of bacteriocin proteins and are resistant to other bacteriocins secreted by other closely related isolates. This mechanism is believed to reduce the level of intra-specific competition. Bile salt resistance: Utilizes enzymes such as bile salt hydrolase to resist the degrading effects of bile salts. Detergent activity of bile salts can permeabilize bacterial membranes which can eventually lead to membrane collapse and/or cell damage. Oxidative Stress Response: Proteins such as catalase, superoxide dismutase, & alkyl hydroperoxide reductase protect the organism from harmful oxygen radicals. This permits growth in the presence of nanomolar concentrations of O2. Antibiotic resistance Member of the genus Bacteroides are characterized with