Respiratory CAS Practice Exam

Beta hemolysis on blood agar is defined by a clear zone around the colonies, reflecting complete destruction of red blood cells in the surrounding area. This clearing happens because certain bacterial toxins, hemolysins, lyse all the RBCs in the agar, so the red color of hemoglobin is lost in that zone. By contrast, partial lysis produces a greenish tint (alpha hemolysis) due to partial hemoglobin breakdown, and no lysis (gamma hemolysis) leaves the agar unchanged.