Which test would you perform to identify Mycobacterium tuberculosis?

The acid-fast bacilli (AFB) stain is the most appropriate test to identify Mycobacterium tuberculosis. This bacterium has a unique cell wall structure that contains mycolic acid, which renders it resistant to common staining techniques like the Gram stain. The AFB stain specifically targets these mycobacteria, allowing for visualization under the microscope. When a specimen is stained using this method, Mycobacterium tuberculosis appears as bright red bacilli against a blue background, confirming its presence.

In comparison, the Gram stain is not effective for mycobacteria because it does not penetrate their thick, waxy cell wall. The culture and sensitivity test, while important for confirming the diagnosis and determining antibiotic susceptibility, does not provide immediate identification of the organism itself. The coagulase test is used primarily for the identification of Staphylococcus aureus and is unrelated to mycobacterial identification. Thus, the AFB stain is the definitive method for identifying Mycobacterium tuberculosis in clinical specimens.