The large-sized plasmids of enterohemorrhagic Escherichia coli O157 strains encode hemolysins which are presumably members of the E. coli alpha-hemolysin family.

Most enterohemorrhagic Escherichia coli O157:H7 strains harbor a large-sized (90 kb) plasmid designated pO157 and show an enterohemolytic phenotype. In this study the hemolytic activity of E. coli O157:H7 strain EDL933 was investigated. Curing of strain EDL933 from pO157 resulted in loss of its hemolytic activity. By transformation with Tn801-tagged pO157 (pSK3), the hemolysin-negative E. coli K-12 strains C600 and DH5 alpha became positive for hemolysin production. By transformation of recombinant plasmids carrying a 11.9 kb BamHI fragment and a 5.3 kb SalI fragment of pSK3 hemolytic activity is revealed when transformed in E. coli C600 or DH5 alpha DNA-hybridization of pO157 and subclones with the alpha-hemolysin specific DNA probe was only found under conditions of low stringency. No hybridization was found with enterohemolysin I (EHly1) and enterohemolysin II (EHly2) probes. Our results indicate that a hitherto not described hemolysin belonging to the alpha-hemolysin family is encoded by the 90 kb plasmid of E. coli O157 strains.