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Background: The already high and increasing occurrence of extended-spectrum beta-lactamases (ESBL) producing Escherichia coli in European broiler populations is of concern due to the fact that third and fourth generation cephalosporins are deemed critically important in human medicine. In Sweden 34% of the broilers carry ESBL/ pAmpC producing E. coli in their gut, despite the absence of a known selection pressure such as antimicrobial usages. The aim of the current study was to characterise a selection of E. coli strains carrying the blaCTX-M-1, to determine if the spread was due to a specific clone. Findings: Ten isolates carrying blaCTX-M-1 from Swedish broilers belonged to eight different multi-locus sequence types with three isolates belonging to ST155. The ST155 isolates were identical as assessed by PFGE. The blaCTX-M-1 was in all isolates carried on a plasmid of replicon type incI, which also transferred resistance to tetracycline and sulfamethoxazole. Conclusion: The occurrence of ESBL-producing E. coli in the Swedish broilers is not due to the emergence of a single clone, but rather the spread of a specific incI plasmid carrying blaCTX-M-1. [ABSTRACT FROM AUTHOR]