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

J Antimicrob Chemother. 2008 Jun;61(6):1221-8. doi: 10.1093/jac/dkn123. Epub 2008 Mar 28.


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 bla(CMY-2) beta-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 beta-lactamase sequence in a member of the Aeromonadaceae.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Aeromonas hydrophila / genetics
  • Aeromonas salmonicida / drug effects*
  • Aeromonas salmonicida / genetics*
  • Aeromonas salmonicida / isolation & purification
  • Animals
  • Anti-Bacterial Agents / pharmacology*
  • Canada
  • Conjugation, Genetic*
  • DNA Transposable Elements
  • DNA, Bacterial / chemistry
  • DNA, Bacterial / genetics
  • Drug Resistance, Multiple, Bacterial*
  • Edwardsiella / genetics
  • Escherichia coli / genetics
  • Gene Order
  • Integrons
  • Mercury / toxicity*
  • Microbial Sensitivity Tests
  • Nucleotidyltransferases / genetics
  • R Factors / isolation & purification*
  • Salmo salar / microbiology
  • Sequence Analysis, DNA
  • Sequence Homology


  • Anti-Bacterial Agents
  • DNA Transposable Elements
  • DNA, Bacterial
  • Nucleotidyltransferases
  • Mercury