A sputum sample from a patient with suspected pulmonary tuberculosis is inoculated onto Lowenstein-Jensen medium. The laboratory informs the clinician that visible colonies may take several weeks to appear. This slow growth of Mycobacterium tuberculosis is best explained by which of the following?
- A A generation time of approximately 14 to 20 hours ✓
- B Requirement of an obligate intracellular growth phase before colony formation
- C Inhibition of growth by malachite green until the dye degrades
- D Dependence on host-derived mycolic acids present only in vivo
Correct answer: A. A generation time of approximately 14 to 20 hours
Explanation
Mycobacterium tuberculosis is among the slowest growing bacteria, with a doubling time of roughly 14 to 20 hours compared with about 20 minutes for E. coli. At this rate, several weeks are needed before colonies become visible on solid media. Malachite green in Lowenstein-Jensen medium suppresses contaminating flora rather than the tubercle bacillus, and the organism grows readily on artificial media, so neither explains the delay.
Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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