A new group of serotype F bacteriophages of has been found which mediates the simultaneous triple-lysogenic conversion of enterotoxin A, staphylokinase and β-lysin. The phages were recovered from methicillin-resistant strains of isolated in Irish hospitals between 1971 and 1988 and from strain PS42-D, which has been used as the propagating strain for the typing phage 42D since before 1965. The molecular mechanism of triple conversion mediated by three of these phages was determined by molecular cloning, restriction endonuclease site mapping and hybridization analysis, and compared with the mechanism of β-lysin and staphylokinase conversion mediated by the serotype F, double-converting phage β 13. The genetic determinants mediating expression of enterotoxin A () and staphylokinase () were cloned from the DNA of the triple-converting phage and expression of the cloned determinants detected in and . The and determinants were closely linked in the phage DNA adjacent to the phage attachment site () in each case and furthermore, the determinant of phage ø 13 was also located near its . The restriction maps of the -, - and -containing DNA regions of the three triple-converting phages were very similar to each other and to the corresponding - and -containing DNA region of phage ø 13. Hybridization analysis using a cloned β-lysin determinant () and cloned -containing DNA fragments as probes demonstrated that β-lysin conversion mediated by the triple-converting phages and phage ø13 was caused by insertional inactivation of the chromosomally encoded determinant by orientation-specific integration of phage DNA following lysogenization.


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