Differentiating M. tuberculosis from M. bovis – TCH susceptibility: Which organism is classically susceptible to thiophen-2-carboxylic acid hydrazide (TCH), a reagent historically used to distinguish members of the M. tuberculosis complex?

Introduction / Context:
Before widespread molecular typing, biochemical reagents such as TCH aided in distinguishing species within the M. tuberculosis complex. Recognizing the classic TCH susceptibility pattern remains a common viva question in microbiology courses.

Given Data / Assumptions:

• M. bovis: TCH susceptible.
• M. tuberculosis: TCH resistant.
• Rapid growers like M. chelonae and M. fortuitum are not in the complex and are not defined by TCH patterns for speciation.

Concept / Approach:
TCH susceptibility is a traditional phenotypic criterion: growth inhibition by TCH indicates M. bovis, whereas resistance suggests M. tuberculosis. While modern labs rely on molecular assays, the biochemical distinction remains testable knowledge for examinations and historical understanding of TB lab methods.

Step-by-Step Solution:
Recall MTBC-specific TCH profiles (bovis susceptible; tuberculosis resistant). Exclude rapid growers and variable NTM. Select M. bovis as the TCH-susceptible organism. Note the complementary use of niacin and nitrate tests in classical workflows.

Verification / Alternative check:
Legacy identification tables consistently document TCH susceptibility for M. bovis as a differentiator from M. tuberculosis.

Why Other Options Are Wrong:
M. tuberculosis is typically TCH resistant; rapid growers and other NTM are not identified within MTBC schemes by TCH.

Common Pitfalls:
Assuming all MTBC members behave identically biochemically; subtle differences underpin older speciation methods.

Final Answer:
Mycobacterium bovis (susceptible).