1. A family of obligate anaerobic (microaerophilic species may occur), non-spore-forming bacteria (order Eubacteriales) containing gram-negative rods that vary in size from minute, filterable forms to long, filamentous, branching forms; pronounced pleomorphism may occur. Motile and nonmotile species occur; motile cells are peritrichous. Body fluids are frequently required for growth. Most species ferment carbohydrates, often with the production of acid; gas may be produced in glucose or peptone media. These organisms occur primarily in the lower intestinal tracts and mucous membranes of warm-blooded animals. They may be pathogenic. The type genus is Bacteroides.

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