Molecular identification in monophasic and nonmotile variants of Salmonella enterica serovar Typhimurium

Abstract

Variant strains of Salmonella enterica serovar Typhimurium, lacking one or both flagellar phases have been widely reported. The monophasic S.1,4,[5],12:i:- variant has emerged worldwide in the past few years and has become one of the most frequently encountered in many countries. In contrast, monophasic S.1,4,[5],12:-:1,2 and nonmotile S.1,4,[5],12:-:- strains are rarely described. This study investigated seven molecular markers to identify and delineate monophasic S.1,4,[5],12:i:- (n = 90), S.1,4,[5],12:-:1,2 (n = 25), nonmotile S.1,4,[5],12:-:- (n = 17) strains, and some serovar Typhimurium strains (n = 124) collected through the French Salmonella network between 2001 and 2010. Three markers were commonly detected in serovar Typhimurium and in all variant strains: STM2757, mdh and fliA-B. Monophasic S.1,4,[5],12:i:- were genotypically confirmed by the absence of the fljB, fljA, and hin genes. Nevertheless, 13 (14.5%) of them were positive for these last three genes, revealing monophasic strains named "inconsistent" as previously described. All nonmotile 1,4,[5],12:-:- strains had the fliC, fljA, fljB, and hin genes and the fliC gene was detected in 88% of monophasic S.1,4,[5],12:-:1,2 strains. The combination of the seven markers detection enables to recognize eight different genotypes within the S.1,4,[5],12:i:- collection, among which the Spanish and the U.S. clones previously described could be distinguished and assigned to a genotype. Based on this molecular approach, 71% of the French S.1,4,[5],12:i:- collection belonged to the Spanish clone, whereas only 2% were assigned to the U.S. clone. This study highlights the usefulness of these molecular markers and genotypes for identifying lineages, especially among the epidemiologically important monophasic S.1,4,[5],12:i:- variant.

Figure 1

Map of the genomic regions carrying the targeted genes fliA, fliB, fliC, STM2757, fljA, fljB, and hin in the Salmonella enterica Typhimurium LT2 genome. Flagellin of the first flagellar phase is encoded by the fliC gene (STM1959). This gene constitutes an operon together with fliA (STM1956) and fliB (STM1958) genes. The expression of this fliCBA operon is under the negative regulation of FljA. The fljA (STM2770) gene forms an operon together with the fljB (STM2771) gene, encoding the flagellin of the second flagellar phase. The expression of this fljBA operon is regulated by the phase-inversion mechanism. The fljBA promoter is localized in the H segment, flanked by inverted sequences, hixL and hixR, between which site-specific recombination occurs carried out by the invertase Hin (STM2772).