Comparison of Multilocus Sequence Typing, Pulsed-Field Gel Electrophoresis, and Antimicrobial Susceptibility Typing for Characterization of Salmonella enterica Serotype Newport Isolates

Abstract

In the United States, multidrug-resistant phenotypes of Salmonella enterica serotype Newport (commonly referred to as MDR-AmpC) have emerged in animals and humans and have become a major public health problem. Although pulsed-field gel electrophoresis (PFGE) is the current "gold standard" typing method for Salmonella, multilocus sequence typing (MLST) may be more relevant to investigations exploring evolutionary and population biology relationships. In this study, 81 Salmonella enterica serotype Newport isolates from humans, food animals, and retail foods were examined for antimicrobial susceptibility and characterized using PFGE and MLST of seven genes, aroC, dnaN, hemD, hisD, purE, sucA, and thrA. Forty-nine percent of the isolates were resistant to nine or more of the tested antimicrobials. Salmonella isolates displayed resistance most often to sulfamethoxazole (57%), streptomycin (56%), tetracycline (56%), ampicillin (52%), and ceftiofur (49%) and, to a lesser extent, to kanamycin (19%), trimethoprim-sulfamethoxazole (17%), and gentamicin (11%). A total of 43 PFGE patterns were generated using XbaI, indicating a genetically diverse population. The largest PFGE cluster contained isolates from clinically ill swine, cattle, and humans. MLST resulted in 12 sequence types (STs), with one type encompassing 62% of the strains. Ten new sequence types and one novel allele type were identified. Furthermore, MLST typing showed that strains closely related by PFGE clustered in major STs, whereas more distantly related strains were separated into two clusters by PFGE. The results of this study demonstrated that the MLST scheme employed here clustered S. enterica serovar Newport isolates in distinct molecular populations, and strain discrimination was enhanced by combining PFGE, antimicrobial susceptibility, and MLST results.

Scientific Opinion on the public health risks of bacterial strains producing extended-spectrum β-lactamases and/or AmpC β-lactamases in food and food-producing animals

“…Outbreaks of CMY-2 producers due to clonal expansion of S. Heidelberg caused by imported live animals, meat consumption and unpasteurized dairy products were described in the EU and the US Folster et al, 2009;Zhao et al, 2008). Human infections by S. Newport lineage II comprises most of MDR-AmpC clones of this serotype, which are linked to ST45 (and single locus variant ST116) and associated with consumption of cattle, bovine, and horse meat or pets treats (Espie and Weill, 2003;Harbottle et al, 2006;Pitout et al, 2003;Sangal et al, 2010). In 1984, a strain of S. Newport with resistance to cephalosporins originating in cattle in the USA, was traced through the food chain to humans (Holmberg et al, 1984).…”

Integron-mediated Multidrug Resistance in a Global Collection of NontyphoidalSalmonellaentericaIsolates

“…mpiib-berlin.mpg.de/mlst) (21)(22)(23)(24). Visualization of PCR products and sequencing of gene fragments was accomplished as described above for integron gene cassettes.…”

Bloodstream Infection Due to CTX-M-15 and TEM-1 Extended-Spectrum β-Lactamase-Producing <i>Salmonella enterica</i> serovar Virchow ST16

“…The isolate was classified as Salmonella enterica serovar Virchow, according to the Kauffmann-White scheme. Its sequence type (ST) was determined to be ST16 by sequence analysis of 7 different housekeeping genes (aroC, dnaN, hemD, hisD, purE, sucA, and thrA) (4). Salmonella enterica serovar Virchow strains belong to the eBurst Group BG9, and the majority of BG9 strains are classified as ST16 in the multilocus sequence typing database (5).…”