Practical evaluation of molecular subtyping and phage typing in outbreaks of infection due to Salmonella enterica serotype typhimurium.

Identification and control of food-poisoning outbreaks due to salmonellosis depend on prompt microbiological diagnosis and subtyping to identify the causative strain. In Australia, Salmonella enterica subspecies enterica serotype typhimurium (S. typhimurium) is responsible for 40-70% of cases of human salmonellosis. Phage typing is the usual method of subtyping S. typhimurium, but on its own, has limitations. We compared it with three molecular subtyping methods using 100 isolates of S. typhimurium, representing four different phage types (PT 1, 9, 126 and 135) and comprising 74 isolates from three presumed outbreaks, 25 isolates from sporadic cases of salmonellosis and S. typhimurium ATCC 10428 (phage type 126). The isolates were divided into 11 subtypes by IS200 restriction fragment length polymorphism (RFLP) typing, four each by ribotyping and pulsed-field gel electrophoresis (PFGE) and 17 distinct strains using a combination of phage and molecular typing. Isolates from two presumed outbreaks were resolved into multiple strains, possibly explaining the failure to identify a common source for either during the original investigations. IS200 RFLP analysis was the most discriminatory and reproducible typing method. Several strains were identifiable within and shared between phage types 1, 9 and 126. Phage and IS200 RFLP typing together, would provide improved definition of S. typhimurium outbreaks.