Abstract
Escherichia coli carrying blaCTX–M–1 mediating resistance to extended-spectrum cephalosporins was recently described as a new genotype in Norwegian broiler production. The aim of this study was to characterize these isolates (n = 31) in order to determine whether the emergence of the genotype was caused by clonal expansion or horizontal dissemination of blaCTX–M–1-carrying plasmids. All included isolates were subjected to whole genome sequencing. Plasmid transferability was determined by conjugation, and plasmid replicons in the transconjugants were described using PCR-based replicon typing. Plasmid sizes were determined using S1 nuclease digestion. Plasmids in a subset of strains were reconstructed and compared to plasmids from broiler production in other European countries. The isolates belonged to nine different sequence types (STs), with the largest group being ST57 (n = 12). The vast majority of blaCTX–M–1-carrying plasmids were conjugative. All transconjugants were positive for the IncI1-Iγ replicon, and several also harbored the IncFIB replicon. Highly similar plasmids were present in different E. coli STs. Additionally, high similarity to previously published plasmids was detected. A reconstructed plasmid from an ST57 isolate harbored both IncI1-Iγ and IncFIB replicons and was considered to be co-integrated. The presence of one large plasmid was confirmed by S1 nuclease digestion. Our results show that dissemination of blaCTX–M–1 in Norwegian broiler production is due to both clonal expansion and horizontal transfer of plasmids carrying blaCTX–M–1. The blaCTX–M–1/IncI1-Iγ plasmids grouped into two main lineages, namely clonal complex (CC)-3 and CC-7. The genetic diversity at both strain and plasmid level indicates multiple introductions to Norway. We also show that the blaCTX–M–1 plasmids circulating in Norwegian broiler production are highly similar to plasmids previously described in other countries.
Highlights
Since the first description of an Escherichia coli displaying extended-spectrum cephalosporin (ESC) resistance in broilers in 2000–2001 (Briñas et al, 2003), numerous reports have underlined the global occurrence of these bacteria in the broiler production (Carattoli, 2008; Smet et al, 2008; Dierikx et al, 2010; Doi et al, 2010; Ewers et al, 2012; Hiroi et al, 2012b)
Two ST57 isolates with identical cgMLST profiles and a third isolate differing by only one allele were detected from farm C
Our results point toward a scenario where both clonal dissemination and horizontal transfer of plasmids have contributed to the dissemination of blaCTX−M−1 in Norway
Summary
Since the first description of an Escherichia coli displaying extended-spectrum cephalosporin (ESC) resistance in broilers in 2000–2001 (Briñas et al, 2003), numerous reports have underlined the global occurrence of these bacteria in the broiler production (Carattoli, 2008; Smet et al, 2008; Dierikx et al, 2010; Doi et al, 2010; Ewers et al, 2012; Hiroi et al, 2012b). The isolate originated from healthy broilers and harbored the blaTEM−20 gene on an IncI1 plasmid (NORM/NORM-VET, 2007; Sunde et al, 2009). In 2011, a selective method for detection of ESC-resistant E. coli was implemented in the Norwegian monitoring programme for antimicrobial resistance in bacteria from food, feed and animals (NORM-VET) (NORM/NORMVET, 2012). All ESC-resistant E. coli isolated from broiler production in the NORM-VET program have displayed an AmpC phenotype due to presence of the blaCMY−2 gene (NORM/NORM-VET, 2012, 2013, 2015, 2017). An extensive sampling of broiler- and parent flocks during 2016 revealed an additional presence of E. coli producing extendedspectrum beta-lactamases (ESBLs), with blaCTX−M−1 being the most frequently detected ESBL-encoding gene (Mo et al, 2019 and unpublished data). Several studies have shown that the blaCTX−M−1 gene commonly occurs on conjugative IncI1 plasmids (Zurfluh et al, 2014; Smith et al, 2015; Irrgang et al, 2018; Touzain et al, 2018)
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