THE LARGE-SIZED PLASMIDS OF ENTEROHEMORRHAGIC ESCHERICHIA-COLI O157 STRAINS ENCODE HEMOLYSINS WHICH ARE PRESUMABLY MEMBERS OF THE ESCHERICHIA-COLI ALPHA-HEMOLYSIN FAMILY

Most enterohemorrhagic Escherichia coli O157: H7 strains harbor a larg e-sized (90 kb) plasmid designated pO157 and show an enterohemolytic p henotype. In this study the hemolytic activity of E. coli O157:H7 stra in EDL933 was investigated. Curing of strain EDL933 from pO157 resulte d in loss of its hemolytic activity. Py transformation with Tn801-tagg ed pO157 (pSK3), the hemolysin-negative E. coli K-12 strains C600 and DH5alpha became positive for hemolysin production. By transformation o f 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 DH5alpha DNA-hybridization of pO157 and subclones w ith the alpha-hemolysin specific DNA probe was only found under condit ions of low stringency. No hybridization was found with enterohemolysi n I (EHly1) and enterohemolysin II (EHly2) probes. Our results indicat e that a hitherto not described hemolysin belonging to the alpha-hemol ysin family is encoded by the 90 kb plasmid of E. coli O157 strains.