Dissemination of nosocomial multiple-aminoglycoside-resistant Staphylococcus aureus caused by horizontal transfer of the resistance determinant (aacA/aphD) and clonal spread of resistant strains.

BACKGROUND

The multiple-aminoglycoside-resistant gene aacA/aphD exists as a transposable genetic element (Tn4001) in gram-positive cocci. Here we describe our retrospective investigation of the mechanism responsible for the dissemination of Tn4001 among staphylococci present in clinical isolates collected in our university hospital. At its peak, about 80% of the total isolates of methicillin-resistant Staphylococcus aureus showed multiple-aminoglycoside resistance, and all harbored aacA/aphD.

METHODS

Clonal relatedness was analyzed by pulsed field gel electrophoresis after SmaI endonuclease digestion of the genomic DNA from isolates collected in 1991 and 1997. To detect Tn4001 on the chromosome and conjugative plasmids, specific sequences from aacA/aphD and the insertion sequence IS256, whose inverted sequence flanks aacA/aphD, were amplified by polymerase chain reaction.

RESULTS

Pulsed field gel electrophoresis of genomic DNA, plasmid analysis, and polymerase chain reaction detection of the resistance determinant all indicated the presence of disseminated clones that had survived among hospitalized patients through acquisition of conjugative plasmids harboring aacA/aphD. Furthermore, aacA/aphD also disseminated among nosocomial strains other than S aureus as a consequence of the self-transferability of Tn4001.

CONCLUSIONS

The nosocomial prevalence of multiple-aminoglycoside-resistant staphylococci is the result of both horizontal and interspecific transfer of aacA/aphD and the clonal spread and survival of resistant strains.