Presumptive identification of group A, B, and D streptococci on agar plate media

Abstract

Several presumptive tests were evaluated for their effectiveness in differentiating streptococci. When the tests were combined into a battery and the resulting reactions were interpreted as patterns, the overall presumptive identification rate was at least 97%. We used the hemolytic reaction, susceptibility to bacitracin and sulfamethoxazole plus trimethoprim (1.25 micrograms plus 23.75 micrograms), and standard CAMP reactions on sheep blood Trypticase soy agar, and bile-esculin and 6.5% NaCl agar tolerance tests with incubation in candle extinction jars. Subsequently, 98.9% of the group A; 95.3% of the group B; 100% of the beta-hemolytic non-group A, B, or D; 92.3% of group D enterococcal; 100% of the group D non-enterococcal; and 92.8% of the viridans streptococci were presumptively identified. We then used the hemolytic reactions, susceptibility of bacitracin and sulfamethoxazole-plus-trimethoprim disks, CAMP disk reactions on sheep blood Trypticase soy agar and bile-esculin and 6.5% NaCl agar tolerance tests with incubation in normal atmosphere. Subsequently, 98.1% of the group A; 98.6% of the group B; 99.2% of the beta hemolytic non-group A, B, or D; 97.5% of the group D entercoccal; 97.6% of the group D non-entercoccal; and 92.4% of the viridans strains were presumptively identified.