Listeria monocytogenes is characteristically positive for which test?

• A. CAMP test
• B. Catalase test
• C. Esculin hydrolysis
• D. Motility at 37°C

Answer: (A)

The key idea is the CAMP reaction, a synergistic hemolysis test used to help identify Listeria monocytogenes. Listeria produces a CAMP factor that interacts with the beta-hemolysin produced by Staphylococcus aureus, enhancing the destruction of red blood cells where the two organisms meet. On a blood agar plate, this shows up as an arrowhead- or Crescent-shaped zone of increased beta-hemolysis at the junction with S. aureus, pointing toward the Staph streak. This distinctive pattern makes the CAMP test a classic and characteristic identifier for Listeria.

Catalase positivity is common among many Gram-positive organisms and isn’t specific to Listeria. Esulin hydrolysis is more relevant for distinguishing enterococci and some streptococci, not the unique CAMP-associated interaction of Listeria. Motility at 37°C is typically negative for Listeria; the organism is more motile at room temperature, so this test isn’t a reliable positive indicator at body temperature.