DNA Sequence Analysis of Regions SurroundingblaCMY-2from MultipleSalmonellaPlasmid Backbones

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Abstract
The emergence in the United States of resistance to expanded-spectrum cephalosporin (e.g., ceftriaxone) within the salmonellae has been associated primarily with three large (>100-kb) plasmids (designated types A, B, and C) and one 10.1-kb plasmid (type D) that carry theblaCMY-2gene. In the present study, the distribution of these four knownblaCMY-2-carrying plasmids among 35 ceftriaxone-resistantSalmonellaisolates obtained from 1998 to 2001 was examined. Twenty-three of these isolates wereSalmonella entericaserotype Newport, 10 wereSalmonella entericaserotype Typhimurium, 1 wasSalmonella entericaserotype Agona, and 1 wasSalmonella entericaserotype Reading. All 23 serotype Newport isolates carried a type C plasmid, and 5, 4, and 1 serovar Typhimurium isolate carried type B, A, and C plasmids, respectively. Both the serotype Agona and serotype Reading isolates carried type A plasmids. None of the isolates carried a type D plasmid. Hybridization data suggested that plasmid types A and C were highly related replicons. DNA sequencing revealed that the region surroundingblaCMY-2was highly conserved in all three plasmid types analyzed (types B, C, and D) and was related to a region surroundingblaCMY-5from theKlebsiella oxytocaplasmid pTKH11. These findings are consistent with a model in whichblaCMY-2has been disseminated primarily through plasmid transfer, and not by mobilization of the gene itself, to multipleSalmonellachromosomal backbones.

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