INHIBITION OF STREPTOCOCCAL GROWTH BY HUMAN BLOOD AGAR

Abstract

It is generally accepted that the ideal culture plate medium for Group A beta-hemolytic streptococci (GABHS) is 5% sheep's blood agar (SBA), with human, rabbit or horse blood being acceptable substitutes. During a recent study of acute pharyngitis conducted in Costa Rica, because of a relative unavailability of sheep's blood, human blood agar plates were utilized. After 29 of 30 throat cultures were negative for GABHS in spite of excellent growth of other organisms, sheep's blood agar was obtained, and a standard GABHS strain (J174A4) from the American Type Culture Collection was reconstituted. Sheep and human blood agar plates were inoculated, then incubated both aerobically and anerobically. After 24 hours of incubation, poor colony growth with minimal hemolysis was noted on both the aerobic and anerobic human blood plates, with excellent growth and beta hemolysis on SBA. Frozen samples from the 30 original throat cultures were subsequently grown on SBA and typed by means of latex agglutination. Four specimens in addition to the original positive culture were confirmed as positive for GABHS. One was suspicious but could not be isolated for confirmation. Preliminary data suggest that human blood agar is not an acceptable substitute for SBA as some lots may contain factors which may inhibit growth of GABHS.