Growth on Lowenstein-Jensen agar is most characteristic of which organism?

• A. Streptococcus pneumoniae
• B. Staphylococcus aureus
• C. Mycobacterium tuberculosis
• D. Escherichia coli

Answer: (C)

Lowenstein-Jensen medium is a specialized, egg-based culture medium that contains malachite green to suppress competing bacteria and provide nutrients favored by mycobacteria. Mycobacteria have a unique, waxy cell wall rich in mycolic acids, which makes them acid-fast and slow-growing. Because of these traits, this medium is used specifically to isolate and propagate mycobacteria, including Mycobacterium tuberculosis. Growth on this medium appears slowly—with colonies that are typically dry, buff-colored, and take weeks to emerge—reflecting the organism’s slow replication and special cell wall.

Streptococcus pneumoniae and Staphylococcus aureus, on the other hand, are commonly grown on enriched or selective media like blood, chocolate, or mannitol salt agar, not Lowenstein-Jensen. Escherichia coli grows readily on standard bacteriological media such as MacConkey or LB broth, not typically on LJ medium.