CAMLPR Microbiology Practice Test

The main clue here is a combination of distinctive pigment production and enzyme activity that points to a specific hospital-associated bacterium. Serratia marcescens is famous for making a brick-red pigment called prodigiosin when it grows, especially at room temperature. This red coloration on blood agar is a classic identifying feature.

Another key trait is DNase production. When the organism grows on a DNase-containing medium, its DNase enzymatically degrades the surrounding DNA, and the resulting test shows a clear halo around the colony. This DNase activity helps confirm the identity alongside the pigment clue.

Serratia marcescens is well known as a frequent cause of nosocomial (hospital-acquired) infections, including urinary tract infections, wound infections, and catheter-associated infections. The combination of brick-red pigment and DNase halo makes it the best match among common hospital pathogens, differentiating it from others like Pseudomonas (which often has a green pigment) or Klebsiella/Enterobacter (which lack the red pigment).