The rupture of red blood cells and release of hemoglobin into the surrounding fluid, observable in vitro on blood agar and occurring in various physiological and pathological conditions.
Explanation
Hemolysis refers to the lysis of erythrocytes (red blood cells), leading to the release of hemoglobin and other intracellular contents into plasma or culture medium. In vitro, hemolysis can occur when blood specimens are exposed to mechanical stress, extreme temperature, hypotonic solutions or inappropriate handling. In vivo, red cell destruction may result from immune reactions, infections, genetic defects, toxins or physical damage. Examples include hemolytic anemia due to autoantibodies, incompatibility reactions following mismatched transfusions, malaria, certain bacterial toxins, and congenital conditions like glucose‑6‑phosphate dehydrogenase deficiency or hereditary spherocytosis. Hemolysis can lead to anemia, jaundice from bilirubin accumulation and, in severe cases, kidney injury due to free hemoglobin.
Some bacteria produce exotoxins called hemolysins that lyse red blood cells and other host cells. In the laboratory, bacterial hemolysis is assessed by culturing organisms on agar supplemented with sheep or horse blood. Patterns of hemolysis on blood agar are useful in identification. Alpha hemolysis is partial lysis accompanied by greenish discoloration due to oxidation of hemoglobin to methemoglobin; it is characteristic of Streptococcus pneumoniae and many viridans streptococci. Beta hemolysis is complete clearing of the agar around colonies, seen with Streptococcus pyogenes, Streptococcus agalactiae, Staphylococcus aureus and Listeria monocytogenes. Gamma hemolysis denotes absence of hemolysis. The hemolysins responsible include pore‑forming cytolysins such as streptolysins O and S and staphylococcal alpha toxin.
Clinical and Laboratory Examples
Beta‑hemolytic Streptococcus pyogenes, the causative agent of strep throat and necrotizing fasciitis, completely lyses erythrocytes on blood agar, producing a clear zone. Streptococcus pneumoniae produces alpha hemolysis, giving colonies a greenish halo. In malaria caused by Plasmodium species, intracellular parasites rupture red blood cells during their life cycle, leading to periodic fevers and anemia. Hemolytic disease of the newborn arises when maternal antibodies against fetal red blood cell antigens cross the placenta and cause hemolysis. G6PD deficiency predisposes red cells to oxidative damage from certain drugs or fava beans, triggering hemolytic episodes. In laboratory settings, controlled hemolysis is used in osmotic fragility and complement fixation tests and to lyse erythrocytes prior to leukocyte analysis.
Recognizing hemolysis patterns aids in microbial identification, and understanding mechanisms of red cell destruction informs diagnosis and management of hemolytic disorders.
Related Terms: Hemolysin, Anemia, Streptococci, Blood agar, Complement