Which genus is most likely to yield tan colonies that are tributyrin hydrolysis positive in a sputum culture?

Which genus is most likely to yield tan colonies that are tributyrin hydrolysis positive in a sputum culture?

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

Answer: (C)

Lipase production detected by tributyrin hydrolysis is the key concept here. Tributyrin agar reveals lipase activity when bacteria secrete extracellular lipase; colonies that produce lipase often appear with a distinct appearance on culture, such as tan-colored, smooth colonies in respiratory specimens. Moraxella species are classic respiratory pathogens that can form tan to gray, smooth colonies in sputum cultures. They are associated with lipase activity that yields a positive tributyrin hydrolysis result. This combination—tan colony color plus a positive tributyrin hydrolysis test—fits Moraxella, making it the best match for a sputum isolate with these characteristics. Actinomyces would be anticipated as filamentous anaerobes with sulfur granules and different colony morphology, not typically described by lipase activity on tributyrin agar. Bacillus species can produce a range of colony colors and may be lipase-positive in some contexts, but their appearance and typical lab profile in sputum differ from the tan, lipase-positive pattern described. Rhodococcus often shows pigment production (such as pinkish-orange) rather than tan colonies, and its lipase profile isn’t the defining clue here.

Lipase production detected by tributyrin hydrolysis is the key concept here. Tributyrin agar reveals lipase activity when bacteria secrete extracellular lipase; colonies that produce lipase often appear with a distinct appearance on culture, such as tan-colored, smooth colonies in respiratory specimens.

Moraxella species are classic respiratory pathogens that can form tan to gray, smooth colonies in sputum cultures. They are associated with lipase activity that yields a positive tributyrin hydrolysis result. This combination—tan colony color plus a positive tributyrin hydrolysis test—fits Moraxella, making it the best match for a sputum isolate with these characteristics.

Actinomyces would be anticipated as filamentous anaerobes with sulfur granules and different colony morphology, not typically described by lipase activity on tributyrin agar. Bacillus species can produce a range of colony colors and may be lipase-positive in some contexts, but their appearance and typical lab profile in sputum differ from the tan, lipase-positive pattern described. Rhodococcus often shows pigment production (such as pinkish-orange) rather than tan colonies, and its lipase profile isn’t the defining clue here.