File:Group A streptococcus19.jpeg

Magnified 100x, this 1977 photograph depicted a Petri dish filled with trypticase soy agar medium containing 5% defibrinated sheep's blood, i.e., blood agar plate (BAP). After having been inoculated by streaking the surface of the BAP with Group A Streptococcus pyogenes (GAS) bacteria, the dish was incubated in a carbon dioxide enriched atmosphere at 35oC for 24 hours. The culture grew bacterial surface colonies. The characteristic color changes, i.e., a colorless region surrounding each colony in which the red blood cells in the blood agar medium had been destroyed, or "hemolyzed", indicated that these bacteria were indeed beta-hemolytic in nature.