What is the basis for presumptive identification of a strain as a group A Streptococcus (GAS)?

Updated: Sep 07, 2018
  • Author: Zartash Zafar Khan, MD, FACP; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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Answer

Presumptive identification of a strain as a group A streptococcus can also be made on the basis of production of the enzyme L-pyrrolidonyl-beta-naphthylamide (PYRase). Among the beta-hemolytic streptococci isolated from throat culture, only group A isolates produce PYRase, which can be identified on the basis of the characteristic color change (red) after inoculation of a disk on an agar plate followed by overnight incubation.

When cultured on blood agar plates, the production of a characteristic zone of complete hemolysis (beta hemolysis) is another important clue to the classification of S pyogenes (see the image below). (For example, Streptococcus pneumoniae generates a zone of only partial hemolysis [alpha hemolysis].)

Streptococcus group A infections. Beta hemolysis i Streptococcus group A infections. Beta hemolysis is demonstrated on blood agar media.

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