Which culture techniques are used to identify group A streptococcal (GAS) infections?

Updated: Sep 07, 2018
  • Author: Zartash Zafar Khan, MD, FACP; Chief Editor: Pranatharthi Haran Chandrasekar, MBBS, MD  more...
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GAS grow readily on routine media, but they can be isolated more easily using selective media that inhibit the growth of normal pharyngeal flora. Most laboratories inoculate throat swabs on 5% sheep blood agar containing trimethoprim-sulfamethoxazole. A bacitracin disk that contains 0.04U of bacitracin is also placed at the initial inoculation of the swab.

After overnight incubation at a temperature of 35-37°C, beta-hemolytic colonies that grow despite inhibition of the antibiotic disk are presumed to be composed of GAS.

Cultures that are negative for GAS after 24 hours are held for another overnight incubation and reexamined.

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