JAC Advance Access originally published online on September 7, 2007
Journal of Antimicrobial Chemotherapy 2007 60(6):1227-1234; doi:10.1093/jac/dkm336
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Detailed structure of integrons and transposons carried by large conjugative plasmids responsible for multidrug resistance in diverse genomic types of Salmonella enterica serovar Brandenburg
1 Departamento de Biología Funcional (Área de Microbiología) and Instituto Universitario de Biotecnología de Asturias (IUBA), Universidad de Oviedo, C/ Julián Clavería 6, 33006 Oviedo, Spain; 2 Laboratorio de Salud Pública, Consejería de Sanidad del Principado de Asturias, 33001 Oviedo, Spain
Received 5 March 2007; returned 17 April 2007; revised 3 August 2007; accepted 8 August 2007
* Corresponding author. Tel: +34-985103562; Fax: +34-985103148; E-mail: rrodicio{at}fq.uniovi.es
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 Tn21-related transposons associated with Tn9; (ii) a Tn1721-derivative containing tet(A); (iii) a defective Tn10 that carried tet(B), and was linked to an integron; and (iv) large conjugative plasmids carrying a class 1 integron-Tn21-Tn9-like structure, together with the Tn1721- or the Tn10-related element. Two-way-RAPD and XbaI-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.
Keywords: conjugative plasmids , transposons , 1600 bp/dfrA1-aadA1 integrons , RAPD , PFGE