Continuous medical education activities; Answers to Case No. 3

Abstract

The considerations were Listeria, Corynebactrium, Erysipelothrix and Arcanobacterium species. The isolate was catalase-negative, which rules out Corynebacteria and Listeria species. The Gram-positive card of the Vitek 2 system can identify Erysipelothrix species, which eliminated that genus. At this point, a reverse CAMP test was performed to determine if the microorganism might be Arcanobacterium hemolyticum (A. hemolyticum). Arcanobacterium hemolyticum typically inhibits the beta hemolysis of Staphylococcus aureus on blood agar, hence resulting in a positive reverse CAMP test. The patient's isolate neither inhibited nor enhanced the β-hemolysis of S. aureus and was not A. hemolyticum. Additional biochemical tests were performed. The mystery microorganism is a tiny, Gram positive rod that is strongly β-hemolytic, catalase negative, ferments xylose and demonstrates no reaction on the reverse CAMP test. What is your diagnosis? (To be continued…).