Complete or "true" haemolysis observed in blood agar cultures of various bacteria, especially haemolytic streptococci and staphylococci; virtually all of the erythrocytes are destroyed in a relatively wide, regularly circumscribed, circular zone about the colony, thereby resulting in a clear "halo" of transparent agar; the zone of haemolysis is frequently much wider than the diameter of the colony; the degree of change varies with species of erythrocytes, e.g., those of sheep and rabbits are usually more easily haemolyzed than those of man, and so on; the haemolysin acts extracellularly (in the absence of the bacterial cells) and may be quantitatively estimated by means of tube-dilution tests of a bacteria-free filtrate (containing the haemolytic substance) with a suspension of erythrocytes.
(05 Mar 2000)