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SUMMARY.Transfer of an Hly plasmid determining production of a haemolysin to a non-haemolytic strain of Escherichia coli increased the virulence of the strain for mice. Injections of non-toxic amounts of a haemolysin, phenylhydrazine, haemoglobin, iron or manganese salts simulated the effect of the Hly plasmid by stimulating bacterial growth. Active or passive immunisation against o! haemolysin protected mice on challenge with haemolytic E. coli by inhibiting in-vivo proliferation of the strain. Protection was eliminated by administration of iron salts at the time of challenge. The Hly plasmid probably acts as a virulence factor by enabling haemolytic strains of E. coli to obtain iron for growth from the lysed erythrocytes of infected animals.

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