Alphahemolytic

Alpha-hemolysis refers to a pattern of partial hemolysis of red blood cells around bacterial colonies growing on blood agar. This partial lysis produces a greenish discoloration of the surrounding medium, caused by the oxidation of hemoglobin to methemoglobin (with possible contributions from biliverdin). The characteristic appearance helps distinguish certain streptococcal species from those that cause complete clearing (beta-hemolysis) or no hemolysis (gamma-hemolysis).

Organisms commonly described as alpha-hemolytic include members of the viridans group streptococci and Streptococcus pneumoniae. Viridans streptococci are a diverse group that colonize the human oral cavity and can be associated with dental caries and endocarditis, while Streptococcus pneumoniae is a leading cause of pneumonia, meningitis, and other respiratory infections. In clinical microbiology, alpha-hemolysis is used alongside other phenotypic tests to help differentiate among Gram-positive cocci and to guide identification.

Laboratory interpretation involves observing the hemolysis pattern on blood agar after incubation, typically in 5% CO2. Alpha-hemolysis is contrasted with beta-hemolysis, which shows complete clearing around colonies, and gamma-hemolysis, which shows no hemolysis. Additional tests, such as optochin sensitivity and bile solubility, are often employed to distinguish Streptococcus pneumoniae (typically optochin sensitive and bile soluble) from viridans streptococci (often optochin resistant and bile insoluble). While useful, alpha-hemolysis is one of several characteristics considered in microbial identification.