Antimicrobial Resistance — MCQs

Question 91: Which of the following methods is not suitable for testing anti-tubercular drug susceptibility?

• A. Resistance ratio method
• B. Disc diffusion method (Correct Answer)
• C. Radiometric broth method
• D. Molecular method

Explanation: ***Disc diffusion method*** - The **disc diffusion method** is generally **not suitable** for *Mycobacterium tuberculosis* due to its **slow growth rate** and the **hydrophobic cell wall** that hinders drug diffusion. - This method requires a relatively fast-growing organism to produce a measurable zone of inhibition within a standard incubation period, which *M. tuberculosis* does not. *Resistance ratio method* - The **resistance ratio method** is a conventional susceptibility testing method for *Mycobacterium tuberculosis* that compares the **minimum inhibitory concentration (MIC)** of a drug against the test strain to a susceptible reference strain. - This method is considered reliable for assessing drug resistance in *M. tuberculosis* but can be time-consuming. *Molecular method* - **Molecular methods** (e.g., **PCR-based assays**, **GeneXpert**, **gene sequencing**) are highly suitable for detecting **anti-tubercular drug resistance** by identifying specific mutations in genes associated with resistance (e.g., *rpoB* for rifampicin, *katG* for isoniazid). - These methods offer **rapid results** and can detect resistance even in paucibacillary samples. *Radiometric broth method* - **Liquid culture systems** such as **BACTEC MGIT 960** are rapid and automated methods commonly used for *M. tuberculosis* susceptibility testing. - **MGIT 960** uses a **fluorescence-based** detection system that measures **oxygen consumption** by metabolizing mycobacteria (fluorescence quenching), providing faster results than traditional solid media methods. - The older **BACTEC 460** used a true radiometric method (detecting **¹⁴CO₂** from radiolabeled substrates) but has been largely replaced by non-radiometric systems.