Alpha-haemolysin: An additive virulence factor in Escherichia coli

Haemolytic Escherichia coli, including human intestinal and extraintestinal as well as porcine enterotoxigenic and edema disease isolates, and Proteus morganii strains were studied for their virulence. Hly+ wild-type strains and Hly+ transconjugants were more virulent than Hly+ derivatives as shown in mice and chick embryos. This enhanced virulence seems to be connected with the ability of diffusible alpha-hemolysin production because clones producing only non-diffusible, beta-hemolysin behaved as non-hemolytic ones. Hemorrhagic lung symptoms and haemoglobinuria were frequently observed after parenteral challenge of mice with alpha-haemolytic clones. Though the Hly- clone exhibited a high resistance against blood clearance, the number of circulating bacteria was significantly higher in the case of the alpha-haemolytic clone. A causal connection between this phenomenon and the leukocidin activity of alpha-haemolysin is suggested.