Evaluation of techniques for isolation of group A streptococci from throat cultures

In the first study, selective sulfamethoxazole-trimethoprim blood agar (SXT-BA) and conventional blood agar (BA) plates incubated under CO2 and anaerobically were compared for their ability to recover group A streptococci from throat cultures. Recovery rates were: SXT-BA (anaerobic), 100%; SXT-BA (CO2), 98%; BA (anaerobic), 89.2%; and BA (CO2), 76.5%. Primary plate bacitracin test results could be read on significantly more of the SXT-BA plates. Readability rates were: SXT-BA (anaerobic), 97%; SXT-BA (CO2), 96%; BA (anaerobic), 70.6%; and BA (CO2), 32.4%. A second study compared with the SXT-BA method versus a BA-double-disk (BA-DD) method which utilizes conventional media with addition of a bacitracin differentiation and a sulfamethoxazole-trimethoprim susceptibility disk placed adjacent to one another in the heavy area of inoculation. Isolation rates were: SXT-BA, 100% and BA-DD, 88%. Readability rates for direct bacitracin tests were: SXT-BA, 92% and BA-DD, 76.2%. In our hands, the SXT-BA method was superior for yielding highest isolation rates and for yielding highest readability rates of direct bacitracin test results.