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Relative value of selective group A streptococcal agar incubated under different atmospheres

A commercially available selective group A streptococcal agar (ssA) was evaluated for the recovery of group A streptococci (GAS) in comparison with recovery from simultaneous cultures on conventional sheep blood agar (SBA). Both sets of plates were incubated in air, 5% CO2, and anaerobically for 48 h, with a first reading taken at 24 h. A total of 402 (67.0%) GAS were isolated from the 600 specimens that were submitted. Recovery of GAS was significantly greater after 48 h of incubation than after 24 h of incubation for each medium-atmosphere combination. After 48 h of incubation, the sensitivities of GAS detection obtained by each culture technique were as follows: ssA-anaerobic atmosphere, 98.5%; SBA-anaerobic atmosphere, 89.5%; ssA-CO2 atmosphere, 88.0%; SBA-air, 86.5%; SBA-CO2 atmosphere, 82.0%; and ssA-air, 74.6%. There were no cultures positive in air or CO2 which were not positive anaerobically on either medium. The increased sensitivity of detecting positive GAS cultures when incubation was done in an ssA-anaerobic atmosphere for 48 h uncovered patients truly infected with the organisms.