Alphahemolysis

Alpha-hemolysis is a pattern of partial hemolysis of red blood cells around bacterial colonies on blood agar, producing a greenish halo. It contrasts with beta-hemolysis, which is complete lysis with a clear zone, and gamma-hemolysis, which shows no lysis. The green coloration results from partial oxidation of hemoglobin to methemoglobin.

The term is most commonly applied to certain streptococci. Streptococcus pneumoniae is characteristically alpha-hemolytic. Many members of the viridans group streptococci—such as Streptococcus mitis, S. oralis, S. sanguinis, and S. mutans—are also typically alpha-hemolytic, though some strains may be nonhemolytic or variably hemolytic.

Clinical relevance centers on the organisms’ roles in human disease and normal flora. Alpha-hemolytic streptococci are part of the normal oral microbiota; S. pneumoniae is a major pathogen causing pneumonia, meningitis, and otitis media. Viridans streptococci are common causes of dental caries and can contribute to subacute endocarditis in individuals with damaged heart valves.

In the laboratory, alpha-hemolysis helps distinguish streptococcal species from beta-hemolytic and non-hemolytic organisms. Additional tests differentiate S. pneumoniae from viridans: optochin susceptibility (S. pneumoniae is optochin sensitive) and bile solubility (S. pneumoniae is bile soluble).