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JAC Advance Access originally published online on March 28, 2008
Journal of Antimicrobial Chemotherapy 2008 61(6):1221-1228; doi:10.1093/jac/dkn123
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© The Author 2008. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org

Original research

Transferable, multiple antibiotic and mercury resistance in Atlantic Canadian isolates of Aeromonas salmonicida subsp. salmonicida is associated with carriage of an IncA/C plasmid similar to the Salmonella enterica plasmid pSN254

Douglas McIntosh1,*, Michelle Cunningham1, Baijing Ji1, Frank A. Fekete2, Erin M. Parry2, Sarah E. Clark2, Zachary B. Zalinger2, Ilana C. Gilg2, G. Russell Danner2,3, Keith A. Johnson4, Mike Beattie5 and Rachael Ritchie1

1 Research and Productivity Council, 921 College Hill Road, Fredericton, NB, Canada E3B 6Z9 2 Department of Biology, Colby College, Waterville, ME 04901, USA 3 Maine Department of Inland Fisheries and Wildlife, Augusta, ME 04330, USA 4 Department of Biology, Bradley University, Peoria, IL 61625, USA 5 New Brunswick Department of Agriculture and Aquaculture, 107 Mount Pleasant Road, St George, NB, CanadaE5C 3S9

Received 20 December 2007; returned 1 February 2008; revised 22 February 2008; accepted 26 February 2008

* Corresponding author. Present address: Life Sciences Department, University of Limerick, Limerick, Ireland. Tel: +1-353-61-202903; E-mail: douglas.mcintosh{at}ul.ie

Objectives: The aim of this study was to examine the molecular basis for multiple antibiotic and mercury resistance in Canadian isolates of Aeromonas salmonicida subsp. salmonicida.

Methods: Phenotypic and genotypic methods were employed to identify plasmid-associated antibiotic and mercury resistance genes and to determine the organization of those genes in multidrug-resistant (MDR) A. salmonicida isolates.

Results: The MDR phenotype was transferable via conjugation using Escherichia coli, Aeromonas hydrophila and Edwardseilla tarda as recipients. Antibiotic and mercury resistance genes were carried by a conjugative IncA/C plasmid. Three distinct antibiotic resistance cassettes were characterized; first a class I integron containing an aadA7 gene encoding for an aminoglycoside-3'-adenyltransferase, the second cassette showed 99.9% nucleotide sequence homology to a cassette previously identified in the Salmonella enterica IncA/C plasmid pSN254, containing floR, tetA, sulII and strA/strB sequences. The third cassette showed 100% nucleotide sequence similarity to a transposon-like element, containing a blaCMY-2 β-lactamase in association with sugE and blc sequences. This element is known to be widely distributed among clinical and food-borne Salmonella and other Enterobacteriaceae throughout Asia and the United States. Mercury resistance was linked to the presence of a mer operon that showed 100% nucleotide sequence homology to the mer operon carried by plasmid pSN254.

Conclusions: Each MDR A. salmonicida isolate carried the same plasmid, which was related to plasmid pSN254. This is the first report of plasmid-mediated florfenicol-resistant A. salmonicida in North America. In addition, it is the first report of a plasmid-associated AmpC β-lactamase sequence in a member of the Aeromonadaceae.

Keywords: AmpC β-lactamase , aquaculture , florfenicol , co-selection


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