Which mycobacterium is described as rapidly growing and grows on MacConkey agar in 5 days?

• A. Mycobacterium bovis
• B. Mycobacterium chelonei
• C. Mycobacterium kansasii
• D. Mycobacterium fortuitum

Answer: (D)

Rapidly growing non-tuberculous mycobacteria form colonies on routine media in about a week or less, which helps distinguish them from slower growers that take several weeks. Among the mycobacteria listed, fortuitum is a classic example of a rapid grower that can produce colonies on MacConkey agar within roughly five days. The slow-growing pathogens, such as bovis and kansasii, take much longer to appear on standard media, so they wouldn’t be described as growing on MacConkey in five days. While other rapid growers exist (like chelonae), the description emphasizing growth on MacConkey within five days most clearly matches fortuitum.