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.
OBJECTIVE
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.
McIntosh
Douglas
D
Research and Productivity Council, 921 College Hill Road, Fredericton, NB, Canada E3B 6Z9. douglas.mcintosh@ul.ie
Cunningham
Michelle
M
Ji
Baijing
B
Fekete
Frank A
FA
Parry
Erin M
EM
Clark
Sarah E
SE
Zalinger
Zachary B
ZB
Gilg
Ilana C
IC
Danner
G Russell
GR
Johnson
Keith A
KA
Beattie
Mike
M
Ritchie
Rachael
R
D016980:Aeromonas hydrophila / Q000235:genetics; D048409:Aeromonas salmonicida / Q000187:drug effects* / Q000235:genetics* / Q000302:isolation & purification; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D002170:Canada; D003227:Conjugation, Genetic; D004251:DNA Transposable Elements; D004269:DNA, Bacterial / Q000737:chemistry / Q000235:genetics; D024901:Drug Resistance, Multiple, Bacterial; D020594:Edwardsiella / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics; D023061:Gene Order; D032023:Integrons; D008628:Mercury / Q000633:toxicity*; D008826:Microbial Sensitivity Tests; D009713:Nucleotidyltransferases / Q000235:genetics; D011815:R Factors / Q000302:isolation & purification*; D020317:Salmo salar / Q000382:microbiology; D017422:Sequence Analysis, DNA; D017385:Sequence Homology
D000900:Anti-Bacterial Agents; D004251:DNA Transposable Elements; D004269:DNA, Bacterial; D009713:Nucleotidyltransferases; D008628:Mercury