Primary plate identification of group A Streptococcus on a selective medium. Efficiency in an office practice
M. H. Roe, P. R. Tolliver, P. L. Lewis and J. K. Todd
The efficiency of group A Streptococcus detection and identification in an
office practice was studied using a selective blood agar plate containing
sulfamethoxazole and trimethoprim and primary plate bacitracin disk
speciation. All results were confirmed by conventional bacteriologic and
immunologic techniques the next day by a reference laboratory. In all,
1,591 cultures were processed, of which 156 (10%) could be confirmed the
next morning to have group A Streptococcus by primary disk susceptibility
interpretation. Bacitracin-resistant beta-hemolytic colonies, which could
be immunologically confirmed as group A Streptococcus grew from only three
cultures (0.1%). Fifty-six (3%) of the cultures had too few beta-hemolytic
colonies to determine bacitracin susceptibility, of which 47 were later
proved to be group A Streptococcus. On all preliminary negative cultures,
1% demonstrated group A Streptococcus after incubation for an additional 24
hours. If a selective blood agar plate with primary bacitracin disk
susceptibility speciation is used in an office laboratory setting, 99% of
all cultures can be accurately interpreted within 24 hours of incubation,
providing that those plates with limited growth of beta-hemolytic colonies
are thereafter immunologically tested for group A Streptococcus antigen.
