Characterization of salmonella enterica serovar typhimurium isolates from pigs and pig environment-related sources and evidence of new circulating monophasic strains in Spain

Abstract

A total of 117 Salmonella enterica serovar Typhimurium and 59 monophasic Salmonella Typhimurium (S. Enterica serovar 4,[5],12:i:-) strains isolated between 2008 and 2012 from pig, wild bird, rodent, and farm environment samples from the northeast of Spain were characterized by phage typing, antibiotic susceptibility testing, and multiple-locus variable-number tandem repeat analysis in order to evaluate their phenotypic and genetic relatedness. In Salmonella 4,[5],12:i:-, the most prevalent phage types were U311 (40.7%) and DT195 (22%), which did not correspond with the so-called Spanish clone and generally showed a different resistance pattern (ASSuT). Antibiotic resistance was found in 85.8% of the isolates, with 94.1% of them displaying multidrug resistance. Multiple-locus variable-number tandem repeat analysis identified 92 different profiles, six of them shared by both serovars. The minimum spanning tree showed one major cluster that included 95% of the Salmonella 4,[5],12:i:-isolates, which came from different animal sources, geographic locations, and time periods, suggesting high clonality among those Salmonella strains and the ability to spread among pig farms. Overall, isolates of Salmonella 4,[5],12:i:-were more similar to European strains than to the well-characterized Spanish clone. The spread of these new strains of Salmonella 4,[5],12:i:-would likely have been favored by the important pig trade between this Spanish region and other European countries. The overall high prevalence of multidrug resistance observed in these new strains should be noted.