LOSS OF INTRINSIC AMINOGLYCOSIDE RESISTANCE IN ACINETOBACTER-HAEMOLYTICUS AS A RESULT OF 3 DISTINCT TYPES OF ALTERATIONS IN THE AAC(6')-IG GENE, INCLUDING INSERTION OF IS17

The distribution of the aac(6')-Ig gene, encoding aminoglycoside 6'-N- acetyltransferase-Ig [AAC(6')-Ig], was studied in 96 Acinetobacter hae molyticus strains and 12 proteolytic Acinetobacter strains, including Acinetobacter genomospecies 6, 13, and 14 and 3 unnamed species assign ed to this genomic group by DNA-DNA hybridization, This gene was detec ted by DNA-DNA hybridization in all 96 A. haemolyticus strains and by PCR in 95 strains but was not detected in strains of other species, in dicating that it may be used to identify A. haemolyticus. Three A. hae molyticus strains were susceptible to tobramycin and did not produce a n aminoglycoside 6'-N-acetylating activity, although they contained aa c(6')-Ig-related sequences. An analysis of three susceptible A. haemol yticus strains indicated that aminoglycoside resistance was abolished by the following three distinct mechanisms: (i) a point mutation in aa c(6')-Ig that led to a Met56-->Arg substitution, which was shown by an alysis of a revertant to be responsible for the loss of resistance; (i i) a polythymine insertion that altered the reading frame; and (iii) i nsertion of IS17, a new member of the IS903 family. These observations indicated that AAC(6')-Ig is not essential for the viability of A. ha emolyticus, although the aac(6')-Ig gene was detected in all members o f this species.