Drug-Resistant Salmonella Typhimurium DT 120: Use of PFGE and MLVA in a Putative International Outbreak Investigation

Isolates of Salmonella enterica serovar Typhimurium belonging to definitive phage type (DT) 120 (Salmonella Typhimurium DT 120) from simultaneous outbreaks of infection in the England and Denmark have been compared on the basis of antibiogram, pulsed-field gel electrophoresis (PFGE), and multiple locus variable number tandem repeat analysis (MLVA). Isolates from England had the resistance profile (ampicillin, streptomycin, sulfamethoxazole, and tetracycline), MLVA profiles 2-4-4-0-2, 2-4-5-0-2, and 2-4-0-0-2, and the PFGE type STYMXB.0083. Representative isolates from the Denmark outbreak were resistant to ampicillin only (A) and had the MLVA type 2-12-6-0-2 and the PFGE type STYMXB.0010. These results demonstrated that outbreak isolates from England and Denmark were not identical. Subsequently, comparison of outbreak isolates with contemporary animal isolates showed that an isolate with the same PFGE type and a similar MLVA type had been isolated in England before its identification in Denmark. These results confirmed the usefulness of MLVA in international outbreak investigations of multiresistant Salmonella Typhimurium and have demonstrated how new molecular strategies may be used to supplement existing methods such as PFGE to enable the accurate and rapid comparison of isolates from different countries. The data also indicate that MLVA proves a useful method for detection of specific Salmonella Typhimurium DTs from human and veterinary sources.