Epidemiologic subtyping of Escherichia coli serogroup O157 strains isolated in Ontario by phage typing and pulsed-field gel electrophoresis

Phage typing and DNA macrorestriction fragment analysis by pulsed-field gel electrophoresis (PFGE) were evaluated for use in the epidemiological subty ping of Escherichia coli serogroup O157 strains isolated in Ontario, Canada . Among 30 strains isolated from patients with sporadic cases of infection, 22 distinct XbaI macrorestriction patterns were identified and 17 strains exhibited unique PFGE patterns, In contrast, phage typing identified only s even different phage types and 17 strains belonged to the same phage type. A total of 25 phage type-macrorestriction pattern combinations were identif ied among the strains from patients with sporadic cases of infection. PFGE subtyping differentiated between unrelated strains that exhibited the same phage type, and in one group of strains, phage typing differentiated betwee n strains of the same PFGE, subtype, Both typing procedures correctly ident ified outbreak-related isolates as belonging to the same type in four separ ate outbreaks. Each outbreak strain was characterized by a distinct macrore striction pattern, while phage typing subdivided the outbreak strains into only three different types. A small percentage of outbreak-related isolates had PFGE patterns that differed slightly (one or two DNA fragment differen ces) from that of the outbreak strain. On the other hand, each isolate from the same outbreak belonged to the same phage type as that of the outbreak strain. We conclude that phage typing and PFGE fingerprinting represent com plementary procedures for the subtyping of E. coli serogroup O157 and that the combined use of these procedures provides optimal discrimination.