Genotypic and phenotypic characterization of Salmonella enterica subsp. enterica serovar Typhimurium monophasic variants isolated in Thailand and Japan.

Monophasic variants of Salmonella enterica serovar Typhimurium isolated in Thailand and Japan were characterized to elucidate the genetic basis of the monophasic phenotype, genetic relatedness, and antimicrobial resistance. A total of 20 Salmonella isolates agglutinated with anti-O4 and anti-H:i serum and not agglutinated with either anti-H:1 or anti-H:2 serum were identified as monophasic variants of Salmonella serovar Typhimurium because they harbored IS200, specific to this serovar, and lacked the fljB gene. An allele-specific PCR-based genotyping method that detects a clade-specific single nucleotide polymorphism indicated that seven swine isolates and one human isolate from Thailand were grouped into clade 1; five isolates from layer chicken houses and layer chicken feces from Japan were grouped into clade 8, together with two Salmonella serovar Typhimurium isolates from chicken houses in Japan; and five isolates from swine feces from Thailand and two isolates from layer chicken feces from Japan were grouped into clade 9. Multilocus sequencing typing demonstrated that sequence type (ST) 34 isolates were solely grouped into clade 9. Clade 1 and 8 isolates were assigned as ST19. Pulsed-field gel electrophoresis revealed multiple types within each of the clades. The presence of antimicrobial resistance genes and plasmid replicon type, of the clade 1 and 9 isolates were comparable to those reported for epidemic strains of monophasic variants. Our results suggest that monitoring monophasic variants of serovar Typhimurium is important for understanding of the spread of these variants in Thailand and Japan.