gammahemolysis

Gamma-hemolysis, or gamma-hemolysis, refers to a pattern seen on blood agar in which bacterial colonies do not cause lysis of red blood cells. The surrounding medium remains unchanged in color and clarity, in contrast to alpha-hemolysis (partial, greenish discoloration) and beta-hemolysis (clear zones from complete lysis).

Mechanism and interpretation: Gamma-hemolysis indicates the organism either does not produce hemolysins active under the assay conditions or does so at undetectable levels. It is described as non-hemolytic. The absence of visible hemolysis does not necessarily reflect absence of pathogenic potential, as other virulence factors may be present.

Clinical relevance: Gamma-hemolysis is a common finding among various Gram-positive cocci, including many Enterococcus species and certain viridans group streptococci. In clinical diagnostics, this pattern is one piece of the taxonomic puzzle and is interpreted alongside other biochemical and morphological tests to identify the organism and assess its clinical significance. It is not diagnostic on its own since non-hemolytic strains may be part of normal flora.

Laboratory notes: To observe gamma-hemolysis, inoculate a sample onto blood agar plates (typically 5% sheep blood) and incubate at 35–37°C for 18–24 hours (extended incubation may be used). Inspect colonies for the absence of hemolysis around them.

See also: Alpha-hemolysis, Beta-hemolysis, Blood agar.