RmtD 16S RNA Methylase in Epidemiologically Unrelated SPM-1-Producing Pseudomonas aeruginosa Isolates from Brazil

We read with interest the article by Doi et al. (2) suggesting that the coproduction of the metallo-β-lactamase SPM-1 and the RmtD 16S RNA methylase is highly prevalent among imipenem-resistant Pseudomonas aeruginosa isolates from Brazil. SPM-1 is widespread among Brazilian carbapenem-resistant P. aeruginosa isolates, and isolates carrying blaSPM-1 have been observed in distinct regions of this country (3). The majority of SPM-1-producing isolates belong to a unique molecular clone identified by pulsed-field gel electrophoresis and automated ribotyping (3). The findings of Doi et al. (2) led us to test a very broad collection of epidemiologically unrelated blaSPM-1-carrying P. aeruginosa strains recovered from diverse Brazilian regions for the presence of rmtD. We evaluated 26 epidemiologically unrelated SPM-1-producing P. aeruginosa clinical isolates collected from different patients during 2002 in 10 Brazilian cities located in nine states (Table ​(Table1).1). These isolates belonged to 16 different ribogroups, and six isolates belonged to the major ribotype spread throughout the country (ribotype 72-3). Screening for rmtD was performed by PCR using the primers rmtD-F (5′-GAG CGA ACT GAA GGA AAA AC-3′) and rmtD-R (5′-CAG CAC GTA AAA CAG CTC-3′), which produce a 730-bp product. Amplicons were sequenced on both strands by using the ABIPrism 377 system (Applied Biosystems, Foster City, CA). The nucleotide sequences were analyzed using the Lasergene software package (DNASTAR, Madison, WI), and the sequences obtained were compared to sequences available over the Internet (http://www.ncbi.nlm.nih.gov/blast/Blast.cgi). TABLE 1. Characteristics of the 26 epidemiologically unrelated SPM-1-producing P. aeruginosa clinical isolates from different regions in Brazil Among the 26 isolates, rmtD was detected in only 4 (15.4%) strains. This rate is much lower than the results obtained previously (51.0%), implying that the production of RmtD is not prevalent among SPM-1-producing isolates with distinct genetic backgrounds and/or from different geographic areas in Brazil. blaSPM-1 is different from other metallo-β-lactamase genes that are carried in class 1 integrons also harboring aminoglycoside resistance genes as part of other gene cassettes (5) in that blaSPM-1 is embedded in a distinct genetic element (4) where the correlation of β-lactam and aminoglycoside resistance is not as evident. However, many SPM-1-producing isolates are resistant to other antimicrobial agents, including aminoglycosides. Recently, Carvalho et al. (1) reported that all 13 SPM-1-producing P. aeruginosa isolates found in another Brazilian state carried a class 1 integron, named In163, harboring two aminoglycoside resistance genes, aacA4 and aadA7, showing that aminoglycoside resistance among SPM-1-producing isolates can be also encoded by distinct resistance mechanisms other than the RmtD 16S RNA methylase gene.