Tetracycline and streptomycin resistance genes, transposons, and plasmids in Salmonella enterica isolates from animals in Italy

Fifty-eight multidrug-resistant Salmonella enterica strains of 20 serotypes, isolated from animal sources in Italy, were analyzed for tet(A) and strA-strB, conferring tetracycline and streptomycin resistance, respectively. The strA and strB genes were highly prevalent in Salmonella strains of our collection, being detected in 84% of the streptomycin-resistant strains. In many strains, the strA and strB genes were linked to a particular Tn5393-derivative transposon characterized by the presence of the insertion sequence IS1133, previously identified only in the plant pathogen Erwinia amylovora. Sixty-eight percent of the tetracycline-resistant strains were tet(A) positive, indicating that this gene is widely diffused in Salmonella strains circulating in animals in Italy. Most of the tet(A) genes were localized within a deleted Tn1721 transposon variant. Two prevalent repN and repI1 resistance plasmids were identified in Salmonella isolates of our collection.