Molecular epidemiological investigation of a diffuse outbreak caused by Salmonella enterica serotype Montevideo isolates in Osaka Prefecture, Japan

In Osaka Prefecture, Japan, three foodborne outbreaks were caused by Salmonella enterica serotype Montevideo in rapid succession between September 2007 and May 2008. Further, Salmonella Montevideo was also isolated from several sporadic diarrhea patients and asymptomatic carriers examined during approximately the identical period. To investigate the relatedness of the isolates, we performed antimicrobial susceptibility testing, pulsed-field gel electrophoresis (PFGE) analysis, and multiple-locus variable-number tandem repeat (VNTR) analysis (MLVA) for 29 Salmonella Montevideo isolates obtained in this region between 1991 and 2008. Although antimicrobial susceptibility tests had low discriminatory power, PFGE patterns revealed 17 unique types with <90% similarity in combined analyses involving XbaI and BlnI. Moreover, we detected three VNTR loci that were useful to genotype Salmonella Montevideo isolates, with our method ultimately classifying the isolates into 11 MLVA types based on differences in repeat unit number in each examined locus. Six isolates obtained from patients of two separate foodborne disease outbreaks, one sporadic patient, and three different carriers between 2007 and 2008 had nearly identical PFGE patterns and were classified into the identical MLVA type; further, the isolates with this PFGE and MLVA pattern appeared only at that time between 1991 and 2008. These data strongly suggest that genetically identical Salmonella Montevideo strains may have caused the 2007 and 2008 outbreaks in Osaka Prefecture. Our results demonstrate that PFGE using XbaI and BlnI is useful for discriminating between Salmonella Montevideo isolates, even within a limited area, and reconfirm that continuous epidemiological surveillance for bacterial intestinal infections such as salmonellosis may be useful to not only monitor changes in the genetic diversity of isolates, but to also detect diffuse outbreaks.