Class 1 integrons mediate antibiotic resistance in the fish pathogen Aeromonas salmonicida worldwide

The presence of class 1 integrons was investigated in 38 sulfonamide-resist ant strains of Aeromonas salmonicida subsp. salmonicida, atypical A. salmon icida and Escherichia coli conjugants with R plasmids originating from A. s almonicida. The strains originated from Finland, France, Japan, Norway, Sco tland, Switzerland, and the United States. Additional resistance determinan ts in strains with class 1 integrons were also determined. Of 21 strains co ntaining a class 1 integron, 19 had a single gene cassette, 1 strain had tw o: cassettes, and 1 strain was found to lack an integrated gene cassette. I n the integrons with single cassettes, aadA2 was present in eight strains, dfr16 in five strains, and aadA1 and dfrIIc in three strains each. In the i ntegron with two cassettes, qacG and orfD were present. Tetracycline resist ance was observed in 20 of the integron-positive strains, caused by the det erminants Tet A and Tet E, in which Tet A frequently was associated with Tn 1721. Class 1 integrons seem to be important in mediating antibiotic resist ance also in the marine environment. The gene cassettes reported in this st udy are all described in bacteria associated with humans, and this demonstr ates once more how the common gene pool is shared between organisms belongi ng to different environments.