Evaluation of Sulfamethoxazole- Trimethoprim Blood Agar Plates for Recovery of Group A Streptococci from Throat Cultures

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We compared the selective blood agar medium of Gunn et al. (J. Clin. Microbiol. 5:650-655, 1977) which contains sulfamethoxazole plus trimethoprim (SXT-BA) to the conventional blood agar surface plate (SBA) and a modified blood agar pour plate plus broth method for the recovery of group A streptococci from throat swabs. The influence of CO2 and ambient air incubation of the SXT-BA and SBA plates was also evaluated. A total of 696 throat swabs from symptomatic children were cultured simultaneously by the five methods and observed after overnight incubation; 204 positive cultures were detected overall. Recovery rates of each individual method were: SXT-BA (CO2), 90.7%; SXT-BA (air), 87.7%; pour plate plus broth, 83.3%; SBA (CO2), 79.4%; and SBA (air) 77%. Approximately one-half of the false-negative cultures in the SXT-BA (CO2) and SXT-BA (air) methods had colony counts of ≥10 to 100 colonies per plate. In contrast, for the SBA (CO2), SBA (air), and pour plate plus broth methods, approximately 70% of the false-negative cultures had colony counts of ≥10 to 100/plate. False-positive cultures obtained by the SXT-BA (CO2) and SXT-BA (air) methods were 11 and 12.7%, respectively—one-half as high as the rates obtained by the remaining methods. Beta-hemolytic streptococci, groups C, F, and G, are inhibited on the SXT-BA plates and were the primary cause of the higher false-positive rates on SBA and pour plate plus broth methods. An additional 3% positive cultures were obtained by incubating SXT-BA (CO2) plates up to 48 h before discarding as negative. We recommend either the SXT-BA (CO2) or the SXT-BA (air) method with up to 48 h of incubation for routine use in throat cultures.

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