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Streptococcus: hemolysis

The genus Streptococcus is classified primarily on the basis of the hemolytic reaction seen around colonies on blood agar plates.

Gamma-hemolysis refers to colonies that are NOT hemolytic. Gamma-hemolytic Streptococcus are not only non-hemolytic but also tend to be more opaque and somewhat larger than other Streptococcus colonies. The most common gamma-streptococci are called Enterococcus because they are common inhabitants of the GI tract. Some Enterococcus species will grow on MacConkey agar.

Alpha-hemolytic Streptococcus colonies have an area of partial hemolysis surrounding them. This zone of hemolysis often appears as a greenish color due to a chemical change in the hemoglobin.

Beta-hemolytic Streptococcus colonies have a clear zone of hemolysis around them, and the colonies themselves tend to be translucent.

Beyond identification of streptococci by hemolytic reaction, many of the Streptococcus species of veterinary importance are difficult to identify to species and require a competent microbiology lab. Presumptive identification, however, may be made simply based on the source of the specimen. For example, Streptococcus equi is the most likely beta-strep from a horse with swollen or abscessing lymph nodes and this species would almost never be found as a cause of mastitis in cattle.