15.1.2.2: The Streptococci

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Although the Streptococci are Staphylococci are both Gram-positive cocci, Streptococci are quite distinct from Staphylococci. Streptococci do not tolerate exposure to oxygen as well as the other Gram-positive cocci, perhaps due in part to the fact that they are catalase-negative. Also, they do not tolerate high levels of salt. The different groups of Streptococci are often characterized by their hemolysis (ability to degrade red blood cells) on media that contain blood (Figure \(\PageIndex{1}\)). The hemolysis is a result of toxins released by the bacteria. Some of the most pathogenic Streptococci, such as S. pyogenes, are beta-hemolytic, meaning they completely destroy the red blood cells. Some pathogenic Streptococci, such as S. pneumoniae and those in the viridans group, are alpha-hemolytic, only partially degrading the red blood cells which leaves a green tint to the media. Finally, the Enterococci are gamma-hemolytic, not degrading the blood at all. The Enterococci are also more tolerant of salt than most Streptococci, being able to grow in 6.5% salt.

Figure \(\PageIndex{1}\): Types of hemolysis on blood agar plates. Hemolysis is often used to differentiate among groups of streptococci. (Rebecca Buxton. 2005. https://www.asmscience.org/content/e...ery/image.2881)