Aminoglycoside resistance among Danish blood culture isolates of coagulase-negative staphylococci

A sample of 137 coagulase-negative staphylococci isolated from blood cultures in Denmark over a 4-month period during 1992-1993 were tested for aminoglycoside resistance and for the presence of aminoglycoside-modifying enzymes. This was done on the basis of minimum inhibitory concentrations (MICs) measured by agar dilution, inhibition zone diameter by disk diffusion, and DNA dot blot analysis. Using the National Committee for Clinical Laboratory Standards (NCCLS) MIC breakpoints, 5%, 46%, 57% and 63% of the strains were resistant to netilmicin, amikacin, gentamicin and tobramycin, respectively. The large majority of resistant staphylococci strains produced the bifunctional AAC(6')-III+APH(2 '') enzyme. The presence of AAC(6')-III+APH(2 '') explains the high level of resistance to gentamicin, kanamycin and tobramycin. In contrast to our results, Staphylococcus haemolyticus strains are usually reported to be more resistant than Staphylococcus epidermidis strains.