First Report of a Foodborne Salmonella enterica Serovar Gloucester (4:i:l,w) ST34 Strain Harboring bla CTX-M- 55 and qnrS Genes Located in IS26-Mediated Composite Transposon.

Extended-spectrum β-lactamases (ESBLs) production and (fluoro)quinolone (FQ) resistance among Salmonella pose a public health threat. The objective of this study was the phenotypic and genotypic characterization of an ESBL-producing and nalidixic acid-resistant Salmonella enterica serovar Gloucester isolate (serotype 4:i:l,w) of sequence type 34 (ST34) from ready-to-eat (RTE) meat products in China. Whole-genome short and long read sequencing (HiSeq and MinION) results showed that it contained bla CTX-M- 55, qnrS1, and tetB genes, with bla CTX-M- 55 and qnrS1 located in chromosomal IS26-mediated composite transposon (IS26-qnrS1-IS3-Tn3-orf-bla CTX-M- 55-ISEcp1-IS26). The same genetic structure was found in the chromosome of S. enterica subsp. enterica serovar Typhimurium strain and in several plasmids of Escherichia coli, indicating that the IS26-mediated composite transposon in the chromosome of S. Gloucester may originate from plasmids of E. coli and possess the ability to disseminate to Salmonella and other bacterial species. Besides, the structural unit qnrS1-IS3-Tn3-orf-bla CTX-M- 55 was also observed to be linked with ISKpn19 in both the chromosomes and plasmids of various bacteria species, highlighting the contribution of the insertion sequences (IS26 and ISKpn19) to the co-dissemination of bla CTX-M- 55 and qnrS1. To our knowledge, this is the first description of chromosomal bla CTX-M- 55 and qnrS in S. Gloucester from RTE meat products. Our work expands the host range and provides additional evidence of the co-transfer of bla CTX-M- 55 and qnrS1 among different species of Salmonella through the food chain.