A sputum culture yields moderate growth of gram-negative diplococci forming tan colonies that are tributyrin hydrolysis positive. Which genus is most likely?

• A. Actinomyces sp.
• B. Bacillus sp.
• C. Moraxella sp.
• D. Rhodococcus sp.

Answer: (C)

A key idea here is matching appearance on the slide with a supportive biochemical trait to pin down the genus. Gram-negative diplococci recovered from sputum point toward Moraxella or Neisseria, and among the options, Moraxella fits that morphology best. Moraxella catarrhalis typically forms tan to gray, smooth colonies on blood agar, and it has lipase activity that hydrolyzes tributyrin. This tributyrin hydrolysis positive result helps distinguish it from Neisseria species and other non-diplococcal organisms. The other options are not gram-negative diplococci—Actinomyces is gram-positive and filamentous, Bacillus is gram-positive and rod-shaped, and Rhodococcus is gram-positive—so Moraxella is the most likely genus.