Detection of Group A Streptococcus: comparison of solid and liquid culture media with and without selective antibiotics

Eight hundred thirty-seven consecutive throat cultures from pediatric patients were cultured comparatively on conventional sheep blood agar and selective agar containing 1.25 micrograms/ml trimethoprim and 23.75 micrograms/ml sulfamethoxazole. In two sequential studies the sheep blood agar-sulfamethoxazole-trimethoprim plates were significantly (P less than 0.01) more sensitive for Group A Streptococcus (Strep-A) detection than conventional sheep blood agar plates or Todd-Hewitt broth using the fluorescent antibody-antigen detection technique. The selective plates inhibited normal upper respiratory flora but not Strep-A. An incubation time of 18 hours was significantly better than one of 4 hours for the detection of Strep-A antigen in cultures incubated in Todd-Hewitt broth alone or Todd-Hewitt-sulfamethoxazole-trimethoprim broth, resulting in a 24% increase in positives. These studies demonstrate that Strep-A detection in throat swab specimens, by either culture or antigen detection, will be influenced by the use of selective antibiotics and incubation time.