CHARACTERIZATION OF THE AAC(6')-IB GENE ENCODING AN AMINOGLYCOSIDE 6'-N-ACETYLTRANSFERASE IN PSEUDOMONAS-AERUGINOSA BM2656

Pseudomonas aeruginosa BM2656 was resistant to tobramycin and suscepti ble to gentamicin and amikacin by disk diffusion testing. This unusual resistance was not transferable by conjugation to Escherichia coli or P. aeruginosa PAO38, and plasmid DNA was not detected in this strain. A 0.9-kb fragment harboring the tobramycin resistance gene was cloned from BM2656 into pUC18, generating pAT129. Analysis for aminoglycosid e-modifying activity in extracts of BM2656 and E. coli harboring pAT12 9 indicated that tobramycin resistance was due to synthesis of an amin oglycoside 6'-N-acetyltransferase type I [AAC(6')-I] enzyme which modi fied amikacin and tobramycin. Although amikacin was acetylated, the ba ctericidal synergism of this aminoglycoside with ceftazidime against B M2656 was minimally affected. The sequence of the DNA fragment was det ermined. It contained an aac(6')-Ib-like gene and was located downstre am from a conserved region related to Tn21. The translated sequence of this aac(6')-Ib gene possessed 99.2% identity with the putative produ cts of the aac(6')-Ib gene cassettes from Serratia marcescens and Kleb siella pneumoniae and 69% identity with the putative aacA(6')-II gene product from P. aeruginosa. We conclude that an aac(6')-Ib gene has sp read to the chromosome of P. aeruginosa, probably by transposition.