SUMMARY. The influence of haemolysin production on virulence was studied in an experimental mouse model. Urinary strains of can be divided into three virulence groups by determining their kinetics in the mouse kidney after intravenous injection. Virulent strains of groups II and III were more often haemolytic than avirulent group-I strains. Haemolytic virulent strains often caused haemo-globinuria in the mice, and killed the mice more rapidly than did non-haemolytic virulent strains. No relationship was found between α-haemolytic activity and virulence in wild-type haemolytic strains. When haemolysin production was reduced or eliminated by treatment with actinomycin-D or rifampicin, six out of seven group-II strains tested gave the same results as avirulent group-I strains. However, the kinetics in the mouse kidney of four haemolytic group-III strains tested was not changed after reduction or elimination of haemolysin production; only a small decrease in toxicity was observed. It is concluded that haemolysin production by is a decisive virulence factor in most of the mouse-nephropathogenic group-II strains, but not in the virulent group-III strains.


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