betahemolytic

Beta-hemolytic describes bacteria that produce beta-hemolysis on blood agar, visible as clear zones around colonies due to complete lysis of red blood cells. This pattern contrasts with alpha-hemolysis, which shows partial, greenish discoloration, and gamma-hemolysis, which shows no hemolysis.

Most clinically relevant beta-hemolytic organisms are streptococci, especially the lancefield groups A and B, though other genera can exhibit beta-hemolysis. The characteristic clearing results from toxins called streptolysins that perforate erythrocyte membranes. Streptolysin O is oxygen-labile, while streptolysin S is oxygen-stable.

On culture, beta-hemolysis aids preliminary identification. Group A streptococcus (Streptococcus pyogenes) is a common cause of pharyngitis, skin infections, and sometimes rheumatic fever and necrotizing fasciitis. Group B streptococcus (Streptococcus agalactiae) is an important cause of neonatal sepsis and meningitis.

Laboratory tests such as bacitracin susceptibility (the A disk) and the CAMP test help assign Lancefield groups to beta-hemolytic isolates. The appearance of beta-hemolysis can be influenced by medium composition, incubation time, and atmospheric conditions.

While beta-hemolysis is a useful diagnostic feature, it does not by itself determine pathogenic potential; nonpathogenic beta-hemolytic strains exist, and some pathogenic strains may exhibit variable hemolysis.