Detailed structure of integrons and transposons carried by large conjugative plasmids responsible for multidrug resistance in diverse genomic types of Salmonella enterica serovar Brandenburg

Abstract

Objectives: To evaluate the incidence, molecular basis and distribution among genomic types of antimicrobial drug resistance in Salmonella enterica (S.) serovar Brandenburg isolates recorded in the Principality of Asturias, Spain. Methods: Thirty-seven S. Brandenburg isolates were tested for susceptibility to antimicrobial agents and typed by random amplified polymorphic DNA (RAPD) and pulsed-field gel electrophoresis (PFGE). PCR amplifications, together with DNA cloning and sequencing, were used to identify resistance genes, integrons and transposons and to establish the structure and physical associations between them. Conjugation experiments were applied to establish the location of the identified elements. Results: Twenty-one isolates were resistant to one or more unrelated drugs. Resistances to streptomycin, tetracycline, kanamycin, chloramphenicol, ampicillin and trimethoprim-sulfamethoxazole, encoded by aadA1, tet (A) or tet (B), aphA1, catA1, blaTEM and dfrA1 - sul1 - sul3, respectively, were most frequently observed. Multidrug resistance (32.4%) was mainly mediated by mobile genetic elements. These included: (i) class 1 integrons (with dfrA1 - aadA1 gene cassettes in their variable region), which were part of Tn 21 -related transposons associated with Tn 9 ; (ii) a Tn 1721 -derivative containing tet (A); (iii) a defective Tn 10 that carried tet (B), and was linked to an integron; and (iv) large conjugative plasmids carrying a class 1 integron-Tn 21 -Tn 9 -like structure, together with the Tn 1721 - or the Tn 10 -related element. Two-way-RAPD and Xba I-PFGE discriminated the isolates into 15 and 12 profiles, respectively. Conclusions: Complex genetic elements have apparently been responsible for the recruitment, assembly and dispersion of resistance genes among the highly diverse genomic types of S. Brandenburg, identified as causal agents of human salmonellosis in the Principality of Asturias, over recent years. © The Author 2007. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy. All rights reserved.