Background: Antimicrobial resistance is increasing among clinical Campylobacter cases and is common among isolates from other sources, specifically retail poultry - a major source of human infection. In this study the antimicrobial susceptibility of isolates from a UK-wide survey of Campylobacter in retail poultry in 2001 and 2004-5 was investigated. The occurrence of phenotypes resistant to tetracycline, quinolones (ciprofloxacin and naladixic acid), erythromycin, chloramphenicol and aminoglycosides was quantified. This was compared with a phylogeny for these isolates based upon Multi Locus Sequence Typing (MLST) to investigate the pattern of antimicrobial resistance acquisition. Results: Antimicrobial resistance was present in all lineage clusters, but statistical testing showed a non-random distribution. Erythromycin resistance was associated with Campylobacter coli. For all antimicrobials tested, resistant isolates were distributed among relatively distant lineages indicative of widespread acquisition. There was also evidence of clustering of resistance phenotypes within lineages; indicative of local expansion of resistant strains. Conclusions: These results are consistent with the widespread acquisition of antimicrobial resistance among chicken associated Campylobacter isolates, either through mutation or horizontal gene transfer, and the expansion of these lineages as a proportion of the population. As Campylobacter are not known to multiply outside of the host and long-term carriage in humans is extremely infrequent in industrialized countries, the most likely location for the proliferation of resistant lineages is in farmed chickens.
Bibliographical noteFunding Information:
The authors would like to thank Florence Opesan, Olivia Coffey and Sophie Rollinson -Food Standards Agency London for providing data, Keith Jolley (University of Oxford) for help in creating the database, Robert Owens, Ella Powell, Kate Martin, Hopi Yip and Radha Patel (Health Protection Agency, Centre for Infections) for microbiological support and data provision, David Lock (LACORS) and Ian Wilson (Northern Ireland Public Health Laboratory) for survey coordination, and staff in a wide range of participating food control laboratories (HPA, National Public Health Service - Wales and the Northern Ireland Public Health Laboratory, Public Analysts). The Food Standards Agency funded genotyping and analysis. SS is funded by a Wellcome Trust Fellowship.