Mycobacterium bovis is a slow-growing (16- to 20-hour generation time) aerobic bacterium and the causative agent of tuberculosis in cattle (known as bovine TB). It is related to Mycobacterium tuberculosis, the bacterium which causes tuberculosis in humans. M. bovis can jump the species barrier and cause tuberculosis-like infection in humans and other mammals. Bacterium morphology and staining The bacteria are curved or straight rods. They sometimes form filaments, which fragment into bacilli or cocci once disturbed. In tissues they form slender rods, straight or curved, or club-shaped. Short, relatively plump bacilli (rods) in tissue smears, large slender beaded rods in culture. They have no flagella or fimbria, and no capsule. Mycobacterium tuberculosis group bacteria are 1.0-4.0 µm long by 0.2-0.3 µm wide in tissues. In culture, they may appear as cocci, or as bacilli up to 6-8 µm long. The bacteria stain Gram-positive, acid-fast. The cell wall contains as high as 60% lipid, giving the mycobacteria their hydrophobic characteristics, slow growth, and resistance to desiccation, disinfectants, acids and antibodies. (Mycobacterium family). They are not easy to stain with analine dyes; although they are Gram positive, confirming this may be difficult. Ziehl–Neelsen staining results in stain pink with hot carbol fuscin and then resist decolourisation with 3% hydrochloric acid in 95% alcohol (i.e. they are acid-alcohol fast); following washing, the slide is counterstained with e.g. methylene blue. They are nonspore-forming. Culture and biochemical features Growth requirements M. bovis is a facultative intracellular parasite. For in vitro growth, special culture media are required; for example, Dorset's egg medium incorporates egg yolk, phosphate buffer, magnesium salts, and sodium pyruvate; amino acids may be added, but glycerol is not included, as it is inhibitory. It is inhibited by glycerine. Culture generally requires several weeks at 37 °C to reach colonies visible to the unaided human eye. It is strictly aerobic, and grows at 37, but not at 25 °C. Optimal growth occurs at 37-38 °C. The species does not reduce nitrate or niacin, and is resistant to pyrazinamide. It is sensitive to thiophene-2-carboxylic acid hydrazide. Appearance of colonies Initially (after 3–4 weeks), its minute, dull flakes, begin to thicken to form dry, irregular masses standing high above the culture medium surface. Eventually, confluent growth is seen over the whole culture surface, forming a rough, waxy blanket, becoming thick and wrinkled and reaching up the sides of the container. Colonies are yellow when first visible,