Comparative Evaluation of Selective and Nonselective Culture Techniques for Isolation of Group A Beta-Hemolytic Streptococci

Abstract

A new selective blood agar medium, Strep A Isolation Agar (SI) from Remel (Lenexa, KS), was compared with Becton Dickinson's Streptococcus Selective Agar (SA) (Becton Dickinson Microbiology Systems, Cockeysville, MD) and with a nonselective Columbia Blood Agar (CB) (Difco, Detroit, MI). Throat swabs from patients with acute pharyngitis were cultured with the use of a single swab to inoculate each of the three plates in a specific order, rotating in three-week cycles. Plates were examined (each medium by a different technologist) after 24 and 48 hours of incubation at 35 degrees C in 5% carbon dioxide, and beta-hemolytic streptococci were serogrouped with the use of coagglutination. The positivity rate was significantly greater for SI (25%) and SA (26%) than for CB (18%) (P less than 0.001). The respective rates of Group A streptococcal detection by SI, SA, and CB were 91%, 95%, and 67%, respectively. However, a feature associated with the use of SI or SA, in contrast to CB, was delayed identification of isolates by 24-48 hours because of small colony size, slower growth rate, and inability to serogroup colonies taken directly from primary culture plates. Recovery of non-Group A beta-hemolytic streptococci occurred with CB (12%) greater than SI (8%) greater than SA (6%). SI is superior to a nonselective medium, such as CB, and is equal to SA for recovery of Group A streptococci from throat cultures.