Characterization of Escherichia coli hemolysins conferring quantitative differences in virulence.

Recombinant plasmids encoding hemolysins (hly) isolated from four different Escherichia coli strains were found to be very similar by restriction endonuclease fragment analysis within the hemolysin region. Each of the four recombinant plasmids were used to transform a nonhemolytic fecal strain of E. coli. The comparative virulence of the transformants was tested in a rat model of peritonitis. Despite the physical similarity among the hemolysin recombinant plasmids, each conferred different normally avirulent fecal E. coli strain. Reciprocal exchange of similar restriction endonuclease fragments enabled the construction of hybrid hemolysin determinants with two hemolysins of disparate extracellular hemolysin production and relative virulence levels. The hybrid plasmids were introduced into the standard avirulent fecal strain and tested in the rat peritonitis model. The region conferring quantitative differences in extracellular hemolysin production and virulence was found to be within a region of less than 1 kilobase where the hly c cistron is encoded as well as the probable transcription initiation ares for the entire hemolysin operon. A 750-base pair (bp) AvaI fragment from this region was isolated from a virulent hemolysin recombinant and inserted into a common AvaI site present in an avirulent hemolysin plasmid. This insertion resulted in an increase in the amount of extracellular hemolysin activity and the associated virulence when in the fecal E. coli background.