Pork contaminated with Salmonella enterica serovar 4,[5],12:i:-, an emerging health risk for humans

Abstract

Salmonella enterica subsp. enterica serovar 4,[5],12:i:- is a monophasic variant of S. enterica serovar Typhimurium (antigenic formula 4,[5],12:i:1,2). Worldwide, especially in several European countries and the United States, it has been reported among the 10 most frequently isolated serovars in pigs and humans. In the study reported here, 148 strains of the monophasic serovar isolated from pigs, pork, and humans in 2006 and 2007 in Germany were characterized by various phenotypic and genotypic methods. This characterization was done in order to investigate their clonality, the prevalence of identical subtypes in pigs, pork, and humans, and the genetic relatedness to other S. enterica serovar Typhimurium subtypes in respect to the pathogenic and resistance gene repertoire. Two major clonal lineages of the monophasic serovar were detected which can be differentiated by their phage types and pulsed-field gel electrophoresis (PFGE) profiles. Seventy percent of the strains tested belonged to definite phage type DT193, and those strains were mainly assigned to PFGE cluster B. Nineteen percent of the strains were typed to phage type DT120 and of these 86% belonged to PFGE cluster A. Sixty-five percent of the isolates of both lineages carried core multiresistance to ampicillin, streptomycin, tetracycline, and sulfamethoxazole encoded by the genes bla(TEM1-like), strA-strB, tet(B), and sul2. No correlation to the source of isolation was observed in either lineage. Microarray analysis of 61 S. enterica serovar 4,[5],12:i:- and 20 S. enterica serovar Typhimurium isolates tested determining the presence or absence of 102 representative pathogenicity genes in Salmonella revealed no differences except minor variations in single strains within and between the serovars, e.g., by presence of the virulence plasmid in four strains. Overall the study indicates that in Germany S. enterica serovar 4,[5],12:i:- strains isolated from pig, pork, and human are highly related, showing their transmission along the food chain. Since the pathogenicity gene repertoire is highly similar to that of S. enterica serovar Typhimurium, it is essential that interventions are introduced at the farm level in order to limit human infection.

FIG. 1.

UPGMA dendrogram of PFGE profiles identified in 148 S. enterica serovar 4,[5],12:i:− and 20 S. enterica serovar Typhimurium strains after digestion with XbaI. At the right of the PFGE profiles, a star indicates the presence of the profile in S. enterica serovar Typhimurium, and a filled circle indicates its presence in S. enterica serovar 4,[5],12:i:−. Profiles were designated X01 to X54. The numbers of isolates belonging to each source (total, pig, pork, and human) are also shown. Designated clusters A to E are indicated by curly brackets. A rectangle highlights the most prominent PFGE profile, X31, also found in an outbreak in Luxembourg (STYMXB.0131; designation according to PulseNet Europe).

FIG. 2.

Phenotypic and corresponding genotypic resistance profiles identified for the 61 S. enterica serovar 4,[5],12:i:− strains tested. At the top of the figure the NRL strain numbers, numbers of isolates with the same genetic resistance profile, and the corresponding phenotypic resistance patterns are indicated. On the left-hand side, the probes for genes related to antimicrobial resistance phenotypes (see Materials and Methods for abbreviations) or to other resistance elements, e.g., integron-associated integrases (int1, int2, and int_SG1) are listed in alphabetical order. The graph shows the hybridization result of each strain. A gray box indicates the presence of the target sequence in the strain; a white box indicates its absence.