You are working up a throat culture. Standard procedure in plating throat cultur

Question

You are working up a throat culture. Standard procedure in plating throat cultures in your lab includes dropping an A disc near the initial inoculum in the first quandrant when streaking the specimen. You examine the blood agar plate and see moderate normal flora and many beta hemolytic colonies that do not grow up to the A disc. The beta hemolytic colonies are catalase negative gram positive cocci. Which pathogen is in this throat culture?

Streptococcus pyogenes

Staphylococcus aureus

Streptococcus agalactiae

Staphylococcus epidermidis

Explanation / Answer

Answer is Streptococcus pyogenes.

S.pyogenes are beta hemolytic due to the production of hemolysins. Streptococci are catalase negative. They do not split hydrogen peroxide into water and nascent oxygen. So there would be no bubbles. Catalase test differentiates Streptococci from Staphylococci. The latter is catalase positive.

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