Aminoglycoside resistance genes aph(2′′)-Ib and aac(6′)-Im detected together in strains of both Escherichia coli and Enterococcus faecium

Escherichia coli SCH92111602 expresses an aminoglycoside resistance profile similar to that conferred by the aac(6 ')-Ie-aph(2 ")-Ia gene found in gram-positive cocci and was found to contain the aminoglycoside resistance genes aph(2 ")-Ib and aac(6 ')-Im (only 44 nucleotides apart). aph(2 ")-1b had been reported previously in Enterococcus faecium SF11770. aac(6 ')-Im had not been detected previously in enterococci and was found to be present also 44 nucleotides downstream from aph(2 ")-Ib in E. faecium SF11770. aph(2 ")-1b and aac(6 ')-Im are separate open reading frames, each with its own putative ribosome binding site, whereas aac(6 ')-Ie-aph(2 ")-Ia appears to be a fusion of two genes with just one start and one stop codon. The deduced AAC(6 ')-Im protein exhibits 56% identity and 80% similarity to the AAC(6 ')-Ie domain of the bifunctional enzyme AAC(6 ')-APH(2 "). Our results document the existence of a member of the aph(2 ") family of genes in gram-negative bacteria and provide evidence suggesting the horizontal transfer of aph (2 ")-Ib and aac(6 ')-Im as a unit between gram-positive and gram-negative bacteria.