Tandem repeat analysis for surveillance of human Salmonella Typhimurium infections

Abstract

In Denmark, as part of the national laboratory-based surveillance system of human enteric infections, all Salmonella enterica serotype Typhimurium isolates are currently subtyped by using phage typing, antimicrobial resistance profiles, and pulsed-field gel electrophoresis (PFGE). We evaluated the value of real-time typing that uses multiple-locus variable-number tandem-repeats analysis (MLVA) of S. Typhimurium to detect possible outbreaks. Because only a few subtypes identified by PFGE and phage typing account for most infections, we included MLVA typing in the routine surveillance in a 2-year period beginning December 2003. The 1,019 typed isolates were separated into 148 PFGE types and 373 MLVA types. Several possible outbreaks were detected and confirmed. MLVA was particularly valuable for discriminating within the most common phage types. MLVA was superior to PFGE for both surveillance and outbreak investigations of S. Typhimurium.

Figure 1

Pulsed-field gel electrophoresis (PFGE), multiple-locus variable-number tandem-repeat analysis (MLVA), phage types, and number of isolates. *PFGE patterns were sorted using the Pearson correlation in BioNumerics 4.0. †Types are shown when present 6× and when >4 isolates had identical MLVA type within each PFGE type. ‡Phage types are only shown when <2 isolates within each MLVA type had the same phage type.

Figure 2

Monthly occurrence of pulsed-field gel electrophoresis (PFGE) types (A) and multiple-locus variable-number tandem-repeat analysis (MLVA) ypes (B) within Salmonella Typhimurium isolates with phage type DT104 over the 2-year study period. All PFGE and MLVA types that occurred <4× were included in other types.

Figure 3

Monthly occurrence of pulsed-field gel electrophoresis (PFGE) types (A) and multiple-locus variable-number tandem-repeat analysis (MLVA) types (B) within Salmonella Typhimurium isolates with phage type DT12 over the 2-year study period. All PFGE and MLVA types that occurred <4× were included in other types.