EFFECTS OF MEDIA, ATMOSPHERE, AND INCUBATION-TIME ON COLONIAL MORPHOLOGY OF ARCANOBACTERIUM-HAEMOLYTICUM

Arcanobacterium haemolyticum causes pharyngitis as well as skin and ot her wound infections. Although it is a beta-hemolytic organism, the he molysis is less well defined than that of beta-hemolytic streptococci and may be overlooked in cultures with heavy growth of commensal throa t flora. To determine whether routine throat culture conditions are su fficient to produce recognizable colonies of A. haemolyticum, the morp hology of six distinct strains was studied after various combinations of incubation time, medium, and atmosphere. The agar media, containing 5% sheep blood, were Trypticase soy agar, Columbia agar, and heart in fusion agar. Cultures were incubated in ambient air, 6 to 8% CO2, or a n anaerobic atmosphere. Cultures were compared after 24, 48, and 72 h of incubation for colony size, clarity and size of hemolytic zone, and macroscopic evidence of apr pitting. A minimum of 48 h was needed for expression of beta-hemolysis and pitting. Trypticase soy agar was the superior medium and CO2 was the superior atmosphere for beta-hemolysi s. Agar pitting was not significantly affected by variations in medium or atmosphere. Strains differed in their expression of hemolysis and production of pits at 48 h. After 72 h of incubation, beta-hemolysis a nd pitting were visible in over 96% of culture observations.