Molecular analysis of the plasmid-borne aacA/aphD resistance gene region of coagulase-negative staphylococci from chickens

Abstract

Objectives: The aim of this study was to analyse selected coagulase-negative staphylococci from chickens for the genetic basis of plasmid-borne resistance to gentamicin, tobramycin and kanamycin, and also for the presence of Tn4001-like elements. Methods: Three staphylococcal strains, two Staphylococcus warneri, and one Staphylococcus sciuri, were included in this study. The gene aacA/aphD coding for a bifunctional enzyme that mediates resistance to gentamicin, tobramycin and kanamycin was detected by hybridization with a specific probe. Plasmid location of this gene was also confirmed by hybridization and conjugation. The resistance gene and its adjacent regions were cloned and sequenced. Results: Three different types of Tn4001-like elements in which the IS256 elements were largely truncated and replaced by IS257 elements were identified on large conjugative and non-conjugative plasmids of 33-43 kb in the staphylococcal strains from chickens. Seven different types of IS257-analogous insertion sequences were identified. Conclusions: Resistance to gentamicin, tobramycin and kanamycin in three staphylococcal strains from chickens was mediated by plasmid-borne aacA/aphD genes located on structurally modified Tn4001-like elements. In one of the three plasmids studied, the arrangement of the elements in the aacA/aphD resistance gene area closely resembled that on plasmids pSH6, pSK41 and pUW3626 from Staphylococcus aureus of human origin.