4.4 Biochemical properties

4.2 Cellular morphology and staining

characteristics:

Bacterial Cattle Diseases

disease.

Mycobacteria are nonmotile, noncapsulating, and nonspore forming rods measuring 0.2–0.6 μm by 1.0–10 μm with a slender, straight or slightly curved shape. The cell wall of mycobacteria contains a hydrophobic lipid layer, which

dimycocerosates, isoprenoid lipids, glycerophospholipids, lipoarabinomannan, and trehalose mycolates and lipoglycans, which give the organism some unique

1. Growth requires complex organic media, containing long-chain free fatty acids

2. The hydrophobic lipid layer causes poor penetration of nutrients, hence the slow growth of the organism and the long incubation period of

3. The poor penetration of chemical agents makes the organism difficult to stain by ordinary procedures. Once stained, they resist decolorization even by weak mineral acids such as 3% hydrochloric acid in ethanol, hence the name acid-fast bacilli (AFB). Mycobacteria have a cell wall structure characteristic of Gram-positive bacteria but they cannot be stained by this method although

5. The presence of fatty acids in the cell wall causes cells to aggregate in a pattern referred to as "cording," observed in stained smears and in broth cultures, due to resemblance to strands of rope cords, and in solid media, growth resembles

6.The lipid layer plays a role in resistance to the host's immune system [25–27]. Paradoxically, the biosynthesis site of some of the lipid components is also the

Mycobacteria are obligate aerobes and require complex organic media for

Middlebrook 7H10, and Middlebrook 7H11 or liquid media such as Modified Middlebrook 7H9 broth are used. Like other MTBC members, M. bovis is a slow grower. On solid media, colonies are detectable 3–6 and up to 12 weeks of incubation at 37°C weeks depending on the concentration of inoculum [28]. Colonies are small, raised, rounded, off-white (bluff) in color, wrinkled surface, and with irregular margins [29]. Addition of pyruvate is reported to stimulate growth of M. bovis and glycerol, which favors growth of M. tuberculosis, is said to inhibit it [28, 30]. Other findings however, indicate that M. bovis can grow satisfactorily in media containing either substance [5]. Members of MTBC group, including M. bovis, are inhibited by paranitrobenzoic acid (PNB), a criteria used to differentiate the group from MOTTs [31, 32]. Growth in liquid media is faster since the organism is surrounded by the media and access to nutrients is more efficient. Growth appears as clumps or "cords." Addition of egg yolk to the growth medium enhances growth, due to the presence of

growth. Solid media such as the egg-based Lowenstein-Jensen, (L-J),

4.The organism is highly resistant to disinfectants and most antibiotics,

includes mycolic acids, phosphatidylinositol, mannosides, phthiocerol

necessary for the synthesis of the lipid layer.

they may stain weakly Gram-positive.

site of action of anti-TB drugs [26].

4.3 Cultural characteristics

66

occasioning lengthy treatment of infection.

that of fungi (myco = Greek = means "fungus").

M. bovis exhibits strain variation in biochemical characteristics, which can be summarized as follows [28, 34, 35]:


The main limitation of biochemical tests is that sufficient amounts of bacterial cells as well and several weeks of incubation are required. The other limitation is that unknown species of mycobacteria cannot be identified. The availability of more rapid methods such as molecular methods has therefore diminished the use of biochemical tests.

## 4.5 Environmental, chemical, and drug resistance

In general, mycobacteria are inactivated by prolonged exposure to heat, direct sunlight, and dry conditions. They are killed by temperatures of 65°C and above for at least 30 minutes and UV light but are resistant to freezing for prolonged periods. Under ordinary temperatures, M. bovis can persist in slurry and soil for at least 6 months and can survive for long periods in buildings and transport vehicles under dark, cold, and moist conditions [29, 36, 37]. The high lipid and wax content makes mycobacteria less susceptible to many chemical agents and disinfectants. Chemicals such as quaternary ammonium compounds, hexachlorophene, and chlorhexidine have bacteriostatic effect while formaldehyde vapor, chlorine compounds, 70% ethanol, hydrogen peroxide alkaline glutaraldehyde, and 5% phenol have bactericidal effect. Although treatment of infected animals is not normally practiced, M. bovis is resistant to most antibiotics but sensitive to the drugs used in treatment of M. tuberculosis (rifampin, isoniazid, streptomycin (STR), and ethambutol). M. bovis subsp. bovis is resistant to pyrazinamide (PZA), a first-line TB treatment drug in humans [38]. This characteristic is relevant in the management of infection in humans and also useful in differentiating M. bovis from M. tuberculosis. Multi-drug resistant strains of M. bovis have been reported in many countries [39].
