Molecular characterization of cephalosporin-resistant Salmonella enterica serotype Newport isolates from animals in Pennsylvania

Abstract

Multidrug-resistant (MDR) strains of Salmonella enterica serotype Newport have been described for many years. However, the recognition of Newport strains with resistance to cephalosporin antibiotics is more recent. Plasmid-mediated CMY-2 AmpC beta-lactamases have been identified in Salmonella in the United States, and the bla(CMY-2) gene has been shown to be present in Salmonella serotype Newport. This organism is currently undergoing epidemic spread in both animals and humans in the United States, and this is to our knowledge the first description of the molecular epidemiology of this Salmonella strain in animals. Forty-two isolates were included in this study. All isolates were characterized by pulsed-field gel electrophoresis, plasmid analysis, and antibiogram. Four pulsed-field profiles with XbaI were observed. Plasmid analyses showed that although the majority of isolates harbored a single plasmid of 140 kb, this plasmid was not identical in all strains. All isolates showed the presence of the bla(CMY) gene by PCR. Integrons were detected in 16 of the 42 isolates; a fragment of approximately 1,000 bp, amplified with the intI-F and aadAI-R primers, confirmed the presence of the aadAI gene cassette within an integron in these 16 isolates. The potential for coselection of the bla(CMY) gene, if located on an MDR replicon, may not be dependent on any particular antibiotic but rather may be the result of more general antimicrobial use. If this replicon is mobile, it is to be expected that similar MDR strains of additional Salmonella serotypes will be recognized in due course.

FIG. 1.

PFGE profiles of S. enterica serotype Newport isolates 0007-33, 0008-122, 0008-254, and 0102-163, representing PFPs 1, 3, 5, and 12, respectively. Isolates were digested overnight with the restriction enzyme XbaI and run on a 1% agarose gel at 14°C for 20 h at 6 V cm⁻¹ with pulse times from 5 to 50 s. A dendrogram was generated with BioNumerics software and shows that all isolates are highly related, with similarity coefficients greater than 75%.