dfrA25, a novel trimethoprim resistance gene from Salmonella Agona isolated from a human urine sample in Brazil

Objectives: To describe a novel trimethoprim resistance gene, designated dfrA25, which was detected as a gene cassette within a class 1 integron in Salmonella Agona. Methods: The gene was cloned into Escherichia coli MT102 and resistance to 10 different antimicrobial drugs was measured. A phylogenetic tree was constructed based on representative trimethoprim-resistance-mediating DfrA proteins retrieved from GenBank. Filter-mating experiments and Southern blots of plasmid preparations were performed with the donor and selected transconjugants. Results and conclusions: dfrA25 encodes a dihydrofolate reductase of 157 amino acids with closest identity (85%) to dfrA5 dihydrofolate reductase. dfrA25 was located on a transferable plasmid (similar to 150 kb) that also harboured the tetracycline resistance gene tet(A).