A Mycobacterium species recovered from an AIDS patient showed negative results for niacin and Tween 80 hydrolysis. What is the most likely identification?

The identification of the Mycobacterium species as the most likely candidate in this scenario is based on its negative results for both niacin production and Tween 80 hydrolysis.

Mycobacterium avium-intracellulare complex (MAC) is well known for its negative reaction to both tests. In clinical microbiology, Mycobacterium avium, a member of the MAC, is frequently isolated from immunocompromised patients, particularly those with AIDS. The inability to produce niacin and hydrolyze Tween 80 distinguishes MAC from other Mycobacterium species.

In contrast, Mycobacterium gordonae typically shows a positive niacin test and a positive result for Tween 80 hydrolysis. Mycobacterium bovis is also niacin positive, and while it may vary in terms of Tween 80 hydrolysis, it does not align with the expected test results for the organism in question. Mycobacterium kansasii is known for its niacin production and would therefore not be consistent with the negative reactions observed here.

Thus, the distinct biochemical characteristics associated with Mycobacterium avium-intracellulare complex make it the most appropriate identification given the test results.