Upper respiratory tract infections are among the most frequent infections that doctors have to diagnose and treat. Although viruses are responsible in a high percentage of these infections, bacteria may also be implicated. Streptococus pyogenes (group A streptococci) is the most common cause of bacterial pharyngitis. The presence of large numbers of these organisms in the throat is evidence of a pathogenic process known commonly as "strep throat."
The photographs show a pure culture of S. pyogenes isolated from an overnight culture of a throat exudate. The colonies were grown on a blood agar plate incubated at 36oC in an air incubator. A heavy growth of beta-hemolytic colonies can be observed. Beta-hemolysis is more apparent in the "stabbed" areas of the agar plate. The specimen was collected using a cotton swab and transported to the laboratory in Stuart's transport medium.
Any colony from throat cultures that are suspected as being group A streptocci should be examined microscopically and further tested for susceptibility to Bacitracin, by serological grouping, or by fluorescent antibody detection.
1. Bailey, W. R., and E. G. Scott. 1974. Diagnostic microbiology, 4th ed. The C. V. Mosby Co., St Louis, Mo.
2. Power, D., and P. McCuen. 1988. Manual of BBLR products and laboratory procedures, 6th ed. Becton Dickinson Microbiology Systems, Cockeysville, Md.