Evidence of plasmid-mediated production of aminoglycoside-modifying enzymes not previously described in Acinetobacter

Two blood culture isolates of Acinetobacter calcoaceticus subsp. anitratus (Herellea vaginicola) were recently observed to be unusually resistant to aminoglycosides. Each strain was found to contain aminoglycoside-modifying enzymes which have not been described previously in this species, including 2"-adenylyltransferase, 3"-adenylyltransferase, 3'-phosphotransferase-III, and 3-acetyltransferase. Treatment of one strain (H-S) with novobiocin led to a loss of resistance to multiple antimicrobial agents and a loss of two aminoglycoside-modifying enzymes; agarose gel electrophoresis of lysates of this strain revealed that the loss of resistance markers was associated with the loss of a large-molecular-weight plasmid. Treatment of the second strain (H-D) with novobiocin produced derivative strains with three different resistance patterns. Agarose gel electrophoresis of deoxyribonucleic acid from crude lysates and from cesium chloride-ethidium bromide gradients of this strain showed only a small plasmid (molecular weight, 5 X 10(6)) common to all variants and failed to explain the loss of resistance markers.