Sammendrag
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 PCRbased 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.
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