Development of a pink to red color indicates the presence of nitrite, demonstrating the ability of the organism to reduce nitrate to nitrite. If no color results, the organisms may have reduced nitrate beyond nitrite (as in the conventional nitrate test). Add a small amount of powdered zinc to the negative tube. The development of a red color indicates that unreduced nitrate was present in the tube and the organism was nitro-reductase-negative.
By the ability of the catalase enzyme to remain active after heating, a measure of the heat stability of the enzyme (heat-stable catalase test). When heated to 68° C for 20 minutes, the catalase of M. tuberculosis, M. bovis, M. gastri, and M. haemophilum becomes inactivated.
Ttoeen 80 Hydrolysis. The commonly nonpathogenic, slow-growing scotochromogens and nonphoto-chromogens produce a lipase that is able to hydrolyze TWeen 80 (the detergent polyoxyethylene sorbitan monooleate) into oleic acid and polyoxyethylated sorbitol, whereas pathogenic species do not. TWeen 80 hydrolysis is useful for separating species of photochro-mogens, nonchromogens, and scotochromogens. Because laboratory-prepared media have a very short shelf life, the CDC recommends the use of a commerdal TWeen 80 hydrolysis substrate (Difco Laboratories or Remel Laboratories) that is stable for up to 1 year.
Tellurite Reduction. Some species of mycobacteria reduce potassium tellurite at variable rates. The ability to reduce tellurite in 3 to 4 days distinguishes members of M. avium complex from most other nonchromogenic spedes. All rapid-growers reduce tellurite in 3 days.
Arylsulfatase. The enzyme arylsulfatase is present in most mycobacteria. Test conditions can be varied to differentiate different forms of the enzyme. The rate by which this enzyme breaks down Phenolphthalein disulfate into Phenolphthalein (which forms a red color in the presence of sodium bicarbonate) and other salts helps to differentiate certain strains of mycobacteria. The
BOX 45-4 Target Organisms for Commercially Available Nucleic Acid Prohs
M. tuberculosis complex M. avium complex M. avium M. intracellular M. kansasii M. gordonae
3-day test is particularly useful for identifying the potentially pathogenic rapid-growers, M.'fortuitum and M. chelonae. Slow-growing M. marinum and M. szulgai are positive in the 14-day test (Figure 45-7).
Growth Inhibition by Tbiophene-2-Carboxyiic Acid Hydrazide (TCH). This test is used to distinguish M. bovis from M. tuberculosis, because only M. bovis is unable to grow in the presence of 10 mg per mL TCH.
Other Tests. Other tests (see Table 45-11) are often performed to make more subde distinctions between species. It is not cost effective for routine clinical microbiology laboratories to be able to perform all the procedures necessary for definitive identification of mycobacteria. Thus, those that require further testing can be forwarded to regional laboratories.
Was this article helpful?