A patient culture showed gram-negative rods and Dnase +. What is the most likely organism?

• A. Proteus vulgaris
• B. Klebsiella pneumoniae
• C. Serratia marcescens
• D. Enterobacter cloacae

Answer: (C)

The identification of the organism based on the culture results reveals that the organism is a gram-negative rod that is DNase positive. Serratia marcescens is known for being a gram-negative, non-lactose fermenting bacterium that can produce a characteristic red pigment at certain temperatures. Furthermore, Serratia marcescens is notably DNase positive, which aligns with the laboratory finding in the culture.

This combination of characteristics—being a gram-negative rod and exhibiting DNase activity—is a key diagnostic trait for this organism. While other options include gram-negative rods as well, they do not share the DNase positive attribute. Proteus vulgaris and Klebsiella pneumoniae are typically reported as DNase negative, and while Enterobacter cloacae can show some variability, it is less commonly associated with strong DNase production compared to Serratia marcescens. Therefore, the most likely organism based on the culture results is Serratia marcescens.