Development and evaluation of multilocus variable number tandem repeat analysis for fine typing and phylogenetic analysis of Salmonella enterica serovar Typhimurium.

We identified 16 variable number tandem repeat (VNTR) loci for Salmonella enterica serovar Typhimurium. These VNTRs were evaluated with panels of 183 diverse isolates, 203 closely related isolates and 54 isolates from seven outbreaks. The evaluations revealed that five of the 16 VNTRs had diversity values greater than 0.5, and three (STTR5, STTR6 and STTR10) were hypervariable. The results obtained from the outbreak isolates suggested that the 16 VNTRs were considerably stable in isolates recovered during a normal outbreak time course. Multilocus VNTR analysis (MLVA) based on four most variable VNTRs (MLVA4), exhibited a better resolving power over pulsed-field gel electrophoresis (PFGE) in discriminating among isolates, in particular among the closely-related isolates. An MLVA5, which is based on five VNTRs and has been widely used in many European laboratories, displayed a level of discrimination close to MLVA4. The phylogenetic tree established using the MLVA16 profiles presented four distinct clusters, which were associated with four different phage types. Therefore, MLVA based on four or five highly variable VNTRs is sufficiently powerful to supplement or replace PFGE for outbreak investigation and surveillance of S. Typhimurium infections, and MLVA data based on 16 VNTRs can be useful in establishing clonal structures among isolates.