Brucella species are small, coccobacillary, Gram-negative rods that morphologically resemble Haemophilus and Bordetella. They are nonmotile, non–acid fast, and non–spore forming. The cells have a typical Gram-negative structure and the outer membrane contains proteins and two major antigenic variants (A and M) whose relative proportion varies with species and growth conditions. Although DNA homology studies indicate that there is only a single species, medical microbiologists prefer to believe that there are six species, each of which is primarily associated with its own mammalian hosts. Of these, Brucella melitensis (sheep, goats), Brucella abortus(cattle), and Brucella suis (pigs) are the most important in human disease. Their growth is slow, requiring at least 2 to 3 days of aerobic incubation in enriched broth or on blood agar. All species produce catalase, oxidase, and urease, but do not ferment carbohydrates. They are differentiated by carbon dioxide requirements, hydrogen sulfide production, and susceptibility to dyes (thionin and basic fuchsin).