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

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.