Environmental (saprophytic) mycobacteria – example: Which organism listed below is a classic example of a saprophytic mycobacterium frequently used in teaching laboratories and molecular work because of its nonpathogenic nature and rapid growth?

Introduction / Context:
Non-tuberculous mycobacteria (NTM) include both environmental saprophytes and opportunistic pathogens. Knowing which organisms are saprophytic helps with risk assessment and with choosing safe model species for laboratory instruction and genetic studies.

Given Data / Assumptions:

• Saprophytes thrive on decaying organic matter and environmental substrates.
• Mycobacterium smegmatis is a rapid grower, nonpathogenic, widely used in research.
• Other listed species have recognized pathogenic potential in humans.

Concept / Approach:
Mycobacterium smegmatis is a classical saprophytic species, prized for its safety and fast growth rate compared with slow-growing pathogenic mycobacteria. In contrast, Mycobacterium marinum causes fish tank granuloma; Mycobacterium chelonae and related rapid growers cause post-procedural infections; Mycobacterium xenopi and Mycobacterium kansasii are associated with pulmonary disease. Therefore, the best example of a saprophytic, teaching-lab-friendly mycobacterium is M. smegmatis.

Step-by-Step Solution:
Identify which species is nonpathogenic and used as a model organism. Exclude species known for human disease (skin or pulmonary). Select Mycobacterium smegmatis. Note that its rapid growth eases culture-based exercises.

Verification / Alternative check:
Research literature and teaching manuals routinely employ M. smegmatis mc^2155 strains for cloning and expression experiments, confirming its saprophytic status and utility.

Why Other Options Are Wrong:
M. marinum, M. chelonae, M. xenopi, and M. kansasii can cause human disease and are not the typical saprophytic classroom organism.

Common Pitfalls:
Equating “rapid-growing” with “nonpathogenic.” Several rapid-growing mycobacteria are opportunists; M. smegmatis is the safer, classic saprophyte.

Final Answer:
Mycobacterium smegmatis.