Aminoglycoside resistance mechanisms in Enterobacteriaceae and Pseudomonas spp. from two Danish hospitals: correlation with type of aminoglycoside used.

Sixty-two aminoglycoside-resistant Gram-negative enteric bacteria were isolated over a 3-year period from two hospitals (Bispebjerg and Esbjerg) among a total of almost 270,000 isolates. These hospitals were selected because of their different aminoglycoside policies during the years investigated. At Bispebjerg Hospital the principal aminoglycoside used was tobramycin, while gentamicin was the first choice at Esbjerg Hospital. Escherichia coli was the most frequently found aminoglycoside-resistant species. Among the 61 aminoglycoside-resistant strains studied, resistance was due to aminoglycoside-modifying enzymes in all except two Xanthomonas maltophilia strains. The ANT(2") enzyme occurred significantly more often at Esbjerg Hospital (p = 0.001), while enzymes of the AAC(3) or AAC(6') moieties were more common, but not significantly so, at Bispebjerg Hospital. The phenotypic pattern of aminoglycoside resistance, as determined by disc diffusion, correlated 100% with the ANT(2") and AAC(3)-V (the two most common enzymes among the isolates) genotype of the organisms as established using DNA probes. Median minimum inhibitory concentrations (MICs) (mg/l) for clinically utilized aminoglycosides were: amikacin (1.6), gentamicin (25.0), kanamycin (50.0), netilmicin (1.6-25.0) and tobramycin (12.5-50.0). Isolates from Bispebjerg Hospital revealed significantly higher MICs for netilmicin and tobramycin (p < 0.01) as compared to isolates from Esbjerg Hospital.