Nomenclature of Plasmid-Mediated 16S rRNA Methylases Responsible for

Production of 16S rRNA methylase has recently drawn at-tention as a novel aminoglycoside resistance mechanism in pathogenic gram-negative bacteria (1). It confers very-high-level resistance to all aminoglycosides that are currently avail-able for parenteral formulation. Six distinct genes, rmtA, rmtB, rmtC, rmtD, armA, and npmA, encoding their respective en-zymes have been identified in clinical and veterinary strains from various geographic areas, including East Asia, Europe, and the Americas, since 2003 (1, 10). NpmA is the only enzyme among them that methylates residue A1408, whereas the oth-ers methylate residue G1405, both within the aminoacyl site (A site) of the 16S rRNA (7, 10). All six genes are confirmed to be or are likely to locate on plasmids (3, 4, 10, 11, 12, 14). Recent findings also indicate that some of these genes are capable of crossing the barrier between glucose-fermenting and nonfer-menting species. For instance, armA has been identified in both members of the family Enterobacteriaceae and in Acineto-bacter baumannii (5, 13), and rmtD has been identified in Klebsiella pneumoniae and Pseudomonas aeruginosa (unpub-lished data). We will likely see an increasing number of reports about this resistance mechanism, including identification of genes encoding new 16S rRNA methylases. Historically, the nomenclature of genes and enzymes for many resistance mechanisms has become complicated and nonsystematic (6). An extreme example is that of aminoglyco-side acetyltransferases, where new gene names are arbitrarily assigned from one of the two coexisting nomenclature systems (9). The situation is somewhat better with -lactamases and macrolide resistance genes, due to a registry and guidelines, respectivel