Multiple-locus variable-number tandem-repeat analysis for discriminating within Salmonella enterica serovar Typhimurium definitive types and investigation of outbreaks

The discriminatory power of multiple-locus variable-number tandem-repeat analysis (MLVA) needs to be evaluated for all Salmonella enterica subspecies enterica serovar Typhimurium (S. Typhimurium) phage types so that the power of this methodology is understood and results can be interpreted correctly during outbreak investigations. We evaluated the ability of MLVA to characterize four definitive phage types (DT) problematic in New Zealand. MLVA discriminated between DT104 isolates although there was very limited variation in the MLVA profiles for isolates with an RDNC phage type (reacts but does not conform to a recognized Typhimurium phage pattern) first observed in New Zealand's Enteric Reference Laboratory in May 2006. Most DT101 isolates had indistinguishable MLVA profiles or profiles that differed at one or two loci. This was also observed in DT160 isolates. MLVA may not identify all common-source outbreaks although it provided valuable data when applied to case isolates from two S. Typhimurium outbreaks.