Incidence of an aminoglycoside 6'-N-acetyltransferase, ACC (6')-1b, in amikacin-resistant clinical isolates of gram-negative bacilli, as determined by DNA-DNA hybridisation and immunoblotting

Abstract

Seventy amikacin-resistant clinical isolates of gram-negative bacteria belonging to nine genera were examined by immunoblotting and by DNA-DNA hybridisation for the presence of ACC(6')1b enzyme, previously called AAC(6')-4, or its encoding gene aacA1b. The organisms mostly had resistance profiles compatible with AAC(6') production and were from South and North America, the Far East and Europe. Polyclonal (rabbit) anti-AAC(6')-1b antisera and an intragenic aacA1b (aacA4) probe derived from the multiresistance plasmid pAZ007 were used. The aacA1b gene was found to be widespread. Positive hybridisation, and immunologically cross-reactive proteins, were observed in 44% of the isolates examined. They were present most frequently (greater than or equal to 70%) in isolates of Klebsiella, Escherichia and Enterobacter spp., but less often (less than or equal to 25%) in Serratia, Citrobacter, Acinetobacter and Pseudomonas spp. The strains that reacted with the probe produced enzymes that varied in their apparent mol. wts between c. 24,000 and 26,000. The existence of multiple electrophoretic forms of amikacin-acetylating enzymes of the ACC(6')-1b type may be useful in epidemiological surveys of AAC(6')-mediated amikacin resistance.