Gamma (γ)-hemolysis refers to the absence of hemolytic activity when bacteria grow on blood agar; red blood cells around the colony remain intact and there is no zone of clearing or discoloration.
Explanation
Blood agar is used to assess the ability of bacteria to lyse erythrocytes. When incubated under appropriate conditions, some bacteria secrete hemolysins that partially or completely degrade red blood cells. Two classic patterns, alpha (α) and beta (β), are associated with partial greenish discoloration and complete clearing of the medium, respectively. Gamma (γ)-hemolysis is not a form of hemolysis at all; it denotes the lack of any detectable erythrocyte lysis. Colonies exhibiting γ-hemolysis grow on the agar surface without altering the appearance of the surrounding red blood cells. This pattern is common among Enterococcus species, formerly classified as group D streptococci, and among commensal staphylococci. Enterococcus faecalis and Enterococcus faecium are part of the normal intestinal flora and are characteristically nonhemolytic on blood agar, though they can cause urinary tract infections, endocarditis and other opportunistic infections. Staphylococcus epidermidis, a coagulase‑negative staphylococcus found on skin, also displays a γ pattern. The absence of hemolysis helps differentiate these organisms from pathogenic β‑hemolytic streptococci and Staphylococcus aureus in clinical diagnostics. However, nonhemolytic isolates may still produce other virulence factors such as adhesins and bile salts tolerance, so γ‑hemolytic does not imply nonpathogenicity.
Examples and important facts
Enterococcus faecalis shows γ‑hemolysis on sheep blood agar and grows in 6.5 % sodium chloride broth, distinguishing it from the group D streptococci that lack NaCl tolerance. Staphylococcus epidermidis and Staphylococcus saprophyticus, coagulase‑negative staphylococci, form opaque colonies without hemolysis. Streptococcus bovis (recently reclassified as Streptococcus gallolyticus) is typically nonhemolytic and may be associated with colon carcinoma and endocarditis. Lactococcus lactis, used in cheese production, and Leuconostoc species also exhibit γ patterns. Recognition of γ‑hemolysis thus aids in the preliminary identification of Gram‑positive cocci during routine culture workup.
Gamma‑hemolytic organisms do not lyse red blood cells on blood agar, but they may still be clinically significant. Their nonhemolytic phenotype distinguishes them from α‑ or β‑hemolytic pathogens and guides further biochemical testing.
Related Terms: Hemolysis, Blood agar, Alpha hemolysis, Beta hemolysis, Enterococcus