title: Comparative evaluation of medium and atmosphere of incubation for isolation of Streptococcus pyogenes.
abstract: To evaluate the effects of medium and atmosphere of incubation for the isolation of group A (GA) streptococci from throat cultures, we compared 1098 throat swabs plated on each of three blood agar plates (BAP). Two plates contained trimethoprim-sulfamethoxazole (SXT) and one had no antibiotics. The antibiotic-free medium and one SXT plate were incubated anaerobically (AnO2) at 35 degrees C, whereas the other SXT plate was incubated in air (O2) at 35 degrees C. The BAP-AnO2 identified 85% of 201 GA compared with 96% for SXT-AnO2 and 92% for SXT-O2. GA were confirmed from only 29% of the beta-hemolytic colonies on BAP-AnO2 compared with 53% from SXT-AnO2 and 44% from SXT-O2. SXT, however, appeared to delay recognition of GA; since 94% of positives were found on day 1 with BAP-AnO2 versus 80% and 62% with SXT-AnO2 and SXT-O2, respectively. We conclude that BAP-SXT (AnO2 or O2) is more sensitive and more specific than BAP-AnO2, but that a second day of incubation is required for optimal recovery of GA on the SXT selective media studied.
mesh_terms: D000332:Aerobiosis; D000693:Anaerobiosis; D003365:Costs and Cost Analysis; D003470:Culture Media; D006801:Humans; D010614:Pharynx / Q000382:microbiology; D013297:Streptococcus pyogenes / Q000302:isolation & purification*
chemical_list: D003470:Culture Media