Increased detection of extended spectrum beta-lactamase producing Salmonella enterica and Escherichia coli isolates from poultry

Abstract

To gain more information on the genetic basis of the rapid increase in the number of isolates exhibiting non-wild type Minimum Inhibitory Concentrations (MICs) for cefotaxime observed since 2003, beta-lactamase genes of 22 Salmonella enterica and 22 Escherichia coli isolates from broilers in 2006 showing this phenotype were characterized by miniaturized micro-array, PCR and DNA-sequencing. Presence and size of plasmids were determined by S1-digest pulsed-field gel electrophoresis and further characterized by PCR-based replicon typing. Transfer of resistance plasmids was tested by conjugation and transformation experiments. To link resistance genes and plasmid type, Southern blot hybridization experiments were conducted. In 42 isolates, five ( bla CTX-M-1, bla CTX-M-2, bla TEM-20, bla TEM-52, bla SHV-2) different extended spectrum beta-lactamase (ESBL)-genes and two ( bla ACC-1, bla CMY-2) AmpC-genes were present. Three of the detected ESBL-genes ( bla CTX-M-1, bla TEM-52 and bla CTX-M-2) were located on similar types of plasmids (IncI1 and IncHI2/P) in both E. coli and Salmonella. Two other detected ESBL- and AmpC-genes bla SHV-2 and bla CMY-2 respectively (on IncK plasmids), were only found in E. coli, whereas the AmpC-gene bla ACC-1 (on non-typable plasmids), and the ESBL-gene bla TEM-20 (on IncI1 plasmids), were only detected in Salmonella. In two isolates, no ESBL- or AmpC-gene could be detected through these methods. The increase in the number of E. coli and S. enterica isolates from the gastro-intestinal tract of broilers exhibiting non-wild type MICs for cefotaxime is mainly due to an increase in IncI1 plasmids containing bla CTX-M-1. The reason for the successful spread of this plasmid type in these species is not yet understood.