clinical isolates (Davies 1991). Three families of aminoglycoside-modifying enzymes have been identified. They are aminoglycoside N-acetyltransferases (AACs), aminoglycoside O-phosphotransferases (APHs) and aminoglycoside O-nucleotidyltransferases (ANTs). They render aminoglycoside antibiotics inactive by catalysing the transfer of an acetyl group (from acetyl CoA), a phosphate group or an adenyl group (both from ATP) to the aminoglycoside. Over 50 aminoglycoside-modifying enzymes have been identified (Table 1) and they are named using nomenclature proposed by Shaw (Shaw et al. 1993). First, each enzyme is designated by the reaction they carry out: AAC for acetylation, APH for phosphorylation, and ANT for adenylation. This is followed by the regiospecificity of the group transferred, designated in parentheses. Next follows a Roman numeral, which specifies the unique aminoglycoside substrate profile. A final lower case letter identifies the distinct genes which confer identical resistance phenotypes.

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