Three methods are used for susceptibility testing of MTB; MOP, absolute concentration method, and resistance ratio method. MOP is most frequently used in the United States and Western Europe (Inderlied, 2004). MOP for mycobacterial susceptibility testing was developed in the 1960s by G. Canetti (Pfyffer, 2003). This method was modified with a standard method published by NCCLS in 2003 (Parsons et al., 2004). The preferred medium for this test is Middlebrook 7H10 agar plates because it has a simple composition, is easy to prepare, and allows the early detection and quantitation of colonies. The Lowenstein-Jensen medium is recommended by WHO and IUATLD as an alternative medium (Pfyffer, 2003). The inoculum can be prepared as either a direct or an indirect test. A smear-positive specimen is used as the source of inoculum in the direct test. In the indirect test, the pure culture is used as the inoculum source. Several dilutions of a standardized suspension are inoculated onto suitable agar plates. The number of colony-forming units (CFU) on the drug-containing plates are compared with the number of colony-forming units on a drug-free plate. Strains of tubercle bacilli that exceed greater than 1% growth on drug-containing media, compared with growth on drug-free media, are considered resistant to that agent (Inderlied, 2004).
In the absolute concentration method, the minimum inhibition concentration (MIC) of the agent is detected. The inoculating control media and drug-containing media are used for serial twofold dilutions of each agent. The lowest concentration of antibiotic that inhibits growth of the agent indicates resistance (Sharma and Mohan, 2004).
In the resistance ratio method, MIC of the isolate is shown as a multiple of the MIC of a standard susceptible strain in order to avoid intra- and inter-laboratory variations. Inoculum size should be strictly controlled in both tests. These tests are not suitable for direct sensitivity testing of concentrated clinical specimens (Sharma and Mohan, 2004).
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