Answer to May 2013 Photo Quiz

Answer: Listeria monocytogenes Bacteremia Following an overnight incubation at 35°C, the subcultures from the positive-blood-culture bottles grew small colonies of weakly beta-hemolytic organisms on sheep blood agar plates. The Gram stain morphology of these colonies showed small coccobacilliary Gram-positive rods. Given the catalase positivity, along with the colony morphology, Listeria sp. was suspected. Further testing of the organism demonstrated characteristic motility in both hanging drop and semisolid-agar preparations. A CAMP test displayed synergistic hemolysis between the organism and a β-lysin-producing Staphylococcus aureus strain, and the identification of Listeria monocytogenes was confirmed by analysis of a 500-bp 16S rRNA gene sequence.