Selective and enhanced recovery of group A and B streptococci from throat cultures with sheep blood agar containing sulfamethoxazole and trimethoprim.

Sheep blood agar containing 23.75 microgram of sulfamethoxazole and 1.25 microgram of trimethoprim (SXT-BA) per ml was compared with conventional sheep blood agar (SBA) for isolating group A and B streptococci from throat cultures. This selective medium allowed much better recovery of group A and B streptococci and suppressed the growth of the normal flora, including "viridans" streptococci. In an initial study of 700 throat cultures, SXT-BA recovered 42% more group A and 49% more group B streptococci than did SBA. When SXT-BA was introduced into the routine microbiology laboratory and used by a number of medical technologists. SXT-BA recovered 28% more group A and 37% more group B streptococci than did SBA. In addition, the selective medium inhibited 83% of the non-group A and B streptococci that were recovered by SBA.