Detailed structure of integrons and transposons carried by large conjugative plasmids responsible for multidrug resistance in diverse genomic types of Salmonella enterica serovar Brandenburg.
OBJECTIVE
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, bla(TEM) 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.
Martínez
Noelia
N
Departamento de Biología Funcional (Area de Microbiología) and Instituto Universitario de Biotecnología de Asturias, Universidad de Oviedo, C/ Julián Clavería 6, 33006 Oviedo, Spain.
Mendoza
M Carmen
MC
Rodríguez
Irene
I
Soto
Sara
S
Bances
Margarita
M
Rodicio
M Rosario
MR
D000900:Anti-Bacterial Agents / Q000494:pharmacology; D003001:Cloning, Molecular; D003227:Conjugation, Genetic / Q000235:genetics*; D004251:DNA Transposable Elements / Q000187:drug effects*; D004269:DNA, Bacterial / Q000032:analysis; D024901:Drug Resistance, Multiple, Bacterial / Q000235:genetics*; D016521:Electrophoresis, Gel, Pulsed-Field; D006801:Humans; D032023:Integrons / Q000235:genetics*; D008826:Microbial Sensitivity Tests; D008969:Molecular Sequence Data; D010957:Plasmids; D016133:Polymerase Chain Reaction / Q000379:methods; D019105:Random Amplified Polymorphic DNA Technique; D012480:Salmonella Infections / Q000453:epidemiology / Q000382:microbiology; D019779:Salmonella enterica / Q000145:classification* / Q000187:drug effects / Q000235:genetics / Q000302:isolation & purification; D017422:Sequence Analysis, DNA; D012703:Serotyping