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

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 McIntosh  1 , Michelle Cunningham, Baijing Ji, Frank A Fekete, Erin M Parry, Sarah E Clark, Zachary B Zalinger, Ilana C Gilg, G Russell Danner, Keith A Johnson, Mike Beattie, Rachael Ritchie
Affiliations

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 McIntosh et al. J Antimicrob Chemother. 2008 Jun.

Abstract

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.

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Figures

Figure 1
Figure 1
Plasmid profiles generated from reference strains and a sample of Atlantic Canadian isolates. M, 0.5–12 kb DNA ladder; lane 1, 718 (pRAS1); lane 2, 7067 (pRAS3); lane 3, ATCC 33658; lane 4, FFA16; lane 5, FFA17; lane 6, FFA18; lane 7, FFA19; and lane 8, FFA20.
Figure 2
Figure 2
Schematic representation of the antibiotic and mercury resistance cassettes identified in MDR A. salmonicida subsp. salmonicida. Sequences encoding antibiotic resistance determinants are shaded black. (a) Class 1 integron (3812 bp) carrying an aadA7 sequence. (b) A 7483 bp region with the predicted genes floR, tetA/R, strA/B and sulII—this region showed 99.9% sequence homology to nucleotides 25 530–33 012 of the plasmid pSN254 (GenBank accession no. CP000604). (c) A 3431 bp region predicted to include blaCMY-2, blc and sugE genes and showing 100% nucleotide sequence homology to nucleotides 415–3845 of a transposon-like element (GenBank accession no. AY253913). (d) Gene organization of a 4266 bp mer operon (merRTPABDE) showing 100% sequence homology to nucleotides 143 996–148 261 of the plasmid pSN254.
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