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An Escherichia coli strain with a plasmidic amikacin resistance has been selected for which the evidence strongly indicates that resistance is mediated by aminoglycoside phosphotransferase [APH(3')-II]: (i) this resistance was coupled with resistance against kanamycin and neomycin; (ii) partially purified APH(3')-II[APH(3") free] modified amikacin by phosphorylation; (iii) the product of the APH(3')-II mediated reaction (i.e., 3'-O-phosphoryl-amikacin) lost its antibacterial activity; and (iv) the amikacin-modifying APH(3')-II activity increased 5- to 10-fold after adaptation of the cells to higher concentrations of amikacin. The substrate spectrum of this enzyme showed a low activity against amikacin as compared with neomycin. It is argued that the enzyme level rather than its substrate spectrum is important for enzyme-mediated resistance. The increase in enzyme levels was found to be correlated with an increase in copy number of a 110-Megadalton plasmid (pBN66) which coded for the APH(3')-II and the APH(3") activity. The increase in copy number was irreversible, and therefore this phenomenon is ascribed to a mutation of a gene which affects the copy number. In transconjugants, the original low copy number was present, and therefore the mutation must be located on the chromosome and not on the plasmid.

Aminoglycoside phosphotransferase-II-mediated amikacin resistance in Escherichia coli