For in vitro susceptibility testing of Mycobacterium tuberculosis, which method is appropriate?

For in vitro susceptibility testing of Mycobacterium tuberculosis, which method is appropriate?

• A. Broth microdilution in Mueller-Hinton broth
• B. Kirby-Bauer disk diffusion
• C. BACTEC method
• D. Schlichter method

Answer: (C)

Testing Mycobacterium tuberculosis susceptibility requires a method that can handle its very slow growth and unique cell wall, using a system that directly measures growth in the presence of drugs. The BACTEC approach fits this need by using a broth-based, radiometric detection of metabolisms to monitor bacterial growth when drugs are present, allowing determination of drug inhibition and resistance patterns with faster turnaround than conventional solid-media methods. This method is validated for TB drug susceptibility testing and can reliably reveal resistance to first-line drugs like isoniazid and rifampin. In contrast, broth microdilution in standard Mueller-Hinton broth isn’t appropriate for TB because it doesn’t use the specialized media and prolonged incubation TB requires, and disk diffusion relies on diffusion through agar, which TB’s slow growth and cell wall complexity make unreliable. The Schlichter method is not the validated standard for routine M. tuberculosis susceptibility testing.

Testing Mycobacterium tuberculosis susceptibility requires a method that can handle its very slow growth and unique cell wall, using a system that directly measures growth in the presence of drugs. The BACTEC approach fits this need by using a broth-based, radiometric detection of metabolisms to monitor bacterial growth when drugs are present, allowing determination of drug inhibition and resistance patterns with faster turnaround than conventional solid-media methods. This method is validated for TB drug susceptibility testing and can reliably reveal resistance to first-line drugs like isoniazid and rifampin. In contrast, broth microdilution in standard Mueller-Hinton broth isn’t appropriate for TB because it doesn’t use the specialized media and prolonged incubation TB requires, and disk diffusion relies on diffusion through agar, which TB’s slow growth and cell wall complexity make unreliable. The Schlichter method is not the validated standard for routine M. tuberculosis susceptibility testing.