Hemolyysin

Hemolysin is a general term for a substance that lyses red blood cells (RBCs). In microbiology, bacterial hemolysins are toxins or enzymes secreted or expressed on the cell surface that damage erythrocyte membranes. Hemolysins contribute to virulence by disrupting host cells and liberating nutrients such as iron.

Most bacterial hemolysins act by one of several mechanisms. Pore-forming toxins insert into lipid membranes to form pores, causing cell lysis; examples include Staphylococcus aureus alpha-hemolysin and streptolysins O and S from Streptococcus pyogenes. Other toxins are cholesterol-dependent cytolysins, such as listeriolysin O from Listeria monocytogenes, and some Clostridium toxins that degrade membrane phospholipids (phospholipases).

In the clinical lab, hemolysis is assessed on blood agar plates, where colonies may produce alpha, beta, or gamma hemolysis as indicators of the toxin's activity. Alpha-hemolysis yields a greenish zone due to partial hemolysis; beta-hemolysis yields a clear zone from complete lysis; gamma-hemolysis shows no lysis.

Notable hemolysins include Staphylococcus aureus alpha-hemolysin (Hla) and beta-hemolysin, Streptococcus pyogenes streptolysin O and S, Listeria monocytogenes listeriolysin O, and Clostridium perfringens alpha-toxin (a phospholipase).

Therapeutic or diagnostic relevance: hemolysins are targets for antibodies and vaccines and serve as diagnostic markers in microbiology. Their activity is influenced by environmental factors, and many are regulated by quorum sensing and host signals.