Characterization of a bifunctional aminoglycoside-modifying enzyme with novel substrate specificity and its gene from a clinical isolate of methicillin-resistant Staphylococcus aureus with high arbekacin resistance.

A clinical isolate (designated PRC104) of methicillin-resistant Staphylococcus aureus was discovered with a novel aminoglycoside resistance profile, including unusually high resistance (MIC 128 microg/ml) to arbekacin (an effective anti-MRSA drug in Japan). We characterized the activity and gene of its bifunctional aminoglycoside-modifying enzyme, AAC(6')/APH(2"), in comparison with those of a regular one that has been known as the critical resistance basis to both gentamicin and arbekacin in methicillin-resistant Staphylococcus aureus. The aac(6')/aph(2") gene of strain PRC104 contained a single base alteration at a novel site (G1126A) resulting in one amino acid substitution (S376N) in the phosphorylation catalytic motif. The phosphorylation activity of the PRC104 enzyme was enhanced for arbekacin and reduced for gentamicin. Both strain PRC104 and S. aureus RN4220 containing the cloned gene were identical in terms of the substrate specificity of the enzyme as well as the aminoglycoside resistance profile, although both mRNA and aminoglycoside resistance levels were markedly high in strain PRC104. Therefore, the cloned aac(6')/aph(2") gene may represent the molecular basis for the novel aminoglycoside modification capability as well as novel aminoglycoside resistance profile of S. aureus PRC104.