Cephalosporin-resistant escherichia coli in the Norwegian broiler production pyramid : genetic characterization and determination of risk factors

Sammendrag

Antimicrobial resistance is defined as one of our times’ main public health challenges. The prevalence of Escherichia coli displaying resistance against extended-spectrum cephalosporins, a class of antimicrobials that are critically important for treatment of human infections, has increased in both human and veterinary medicine during the last decade. In Norway, the use of antimicrobial agents for food producing animals is low, and in the broiler production almost non-existent. Therefore, it was surprising to find a relatively high occurrence of cephalosporin-resistant E. coli in broilers following implementation of a selective detection method of such resistance as part of the Norwegian monitoring programme for antimicrobial resistance in bacteria from food, feed and animals (NORMVET) from 2011. The aim of the present study was to increase knowledge on cephalosporinresistant E. coli in the Norwegian broiler production in order to provide scientifically based advice to the authorities and the broiler industry regarding preventive measures related to introduction, persistence and spread of cephalosporin-resistant E. coli. Data collected in 2011 and 2012 were compiled and analysed. An epidemiological study was performed in order to determine risk factors related to the occurrence of cephalosporinresistant E. coli in Norwegian broiler flocks. Furthermore, thorough genetic characterization of cephalosporin-resistant isolates was performed in order to determine whether successful clones and/or resistance plasmids were present in the broiler production. Further, transfer experiments using relevant recipient strains and two donor strains harbouring well characterized plasmids with cephalosporin-encoding genes were carried out under different conditions. All cephalosporin-resistant E. coli included in the present study displayed an AmpCphenotype, and the vast majority carried the blaCMY-2 gene. AmpC-producing E. coli have been detected at all levels of the Norwegian broiler production pyramid, and are present in all areas of Norway with broiler production. The risk for occurrence of cephalosporinresistant E. coli was associated with the status of the previous flock in the broiler house, number of parent flocks supplying the broiler flock with day-old chickens, routines for disinfection of floor between production cycles, and transport personnel entering the room where the broilers are raised. The results indicated that a high level of biosecurity, including implementation of thorough cleaning and disinfection routines, will aid in minimizing the odds of cephalosporin-resistant E. coli occurring in broiler flocks. Characterization of isolates revealed that cephalosporin-resistance was generally mediated by blaCMY-2 on IncI1 or IncK plasmids. The majority of the plasmids were self-transmissible. Furthermore, a large cluster of closely related isolates grouping into multilocus sequence type (ST)38, and a cluster grouping into ST1158 were identified. These isolates harboured blaCMY-2 on IncK plasmids. Interestingly, E. coli ST38 and IncK and IncI1 plasmids highly similar to plasmids found in Norway have been reported from other European countries, indicating that some successful clones and plasmids are circulating in the European broiler production. In addition, plasmid stability systems were identified in both IncK (pNVI1292/IncK) and IncI1 plasmids (pNVI2798/IncI1). The presence of such systems might partly explain the persistence of cephalosporin-resistant E. coli in the Norwegian broiler production despite the lack of antimicrobial selection pressure. Transfer experiments revealed the ability of IncK and IncI1 plasmids to self-transfer to other hosts within biofilm. Furthermore, selftransfer of pNVI1292/IncK into Serratia spp. was observed. Bacteria in biofilm have an increased ability to survive cleaning and disinfection. Environmental bacteria can also have intrinsic properties making them more tolerant to some disinfectants. Therefore, the results might indicate that biofilms and environmental bacteria can act as reservoirs for cephalosporin-resistant E. coli and plasmids encoding cephalosporin-resistance in the broiler production. Overall, the results presented in this thesis provide important knowledge regarding the molecular epidemiology of cephalosporin-resistant E. coli in the Norwegian broiler production. Furthermore, the identification of risk factors for occurrence of cephalosporinresistant E. coli in broiler flocks can contribute to implementation of preventive measures aimed at minimization of persistence of these bacteria.

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NVA : hdl.handle.net/11250/2422803