Kanamycin resistance in eight strains of Klebsiella pneumoniae isolated during an outbreak of infection in a neonatal intensive care unit was found to be transferable and mediated by neomycin phosphotransferase. After gentamicin was used to control infections caused by kanamycin-resistant organisms, a strain resistant to gentamicin emerged. Gentamicin resistance in this ninth strain was not transferable and was accompanied by resistance to tobramycin, amikacin, and streptomycin. Enzymatic modifications of aminoglycosides other than neomycin and kanamycin could not be demonstrated either by filter binding assays or by electrophoresis of a radioactive aminoglycoside substrate. The strain with broad aminoglycoside resistance contained six plasmid deoxyribonucleic acid bands, none of which appeared to be different in molecular weight from plasmid deoxyribonucleic acid bands in strains isolated before the institution of gentamicin therapy. The broadening of resistance was accompanied by reduced uptake of radioactively labeled streptomycin and gentamicin. The relationship between aminoglycoside resistance and reduced drug transport in the absence of any enzymatic modification is discussed
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