Bacitracin differentiation for presumptive identification of group A beta-hemolytic streptococci: comparison of primary and purified plate testing.

The detection of group A beta-hemolytic streptococci by bacitracin susceptibility was examined, with the presumptive identification on primary blood agar plates compared with that on pure subcultured plates. A total of 638 group A streptococcus isolates was recovered from 4,632 throat swab specimens. Only about half of these isolates were identifiable on the primary plate, compared with 96% identifiable on pure subcultured plates. The unreliable results with bacitracin disk testing on primary plates were due to (1) too few colonies present in the vicinity of the disk or (2) colonies overgrown by oropharyngeal flora.