Incidence of the novel rmtF and rmtG methyltransferases in carbapenem-resistant Enterobacteriaceae from a hospital in India

Copyright © 2015 Filgona et al. This is an open-access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dear Editor, Aminoglycosides play a critically important role in antimicrobial therapy, mostly against the rapidly evolving β-lactamase-producing Enterobacteriaceae, often for their co-drug effect with β-lactams. However, the evolution of acquired 16S rRNA methyltransferases, which confer high-level and broad-spectrum aminoglycoside resistance, and their frequent co-occurrence with carbapenemases, constitute a major threat to this class of antibiotic [1]. Reported cases of aminoglycoside methyltransferase within carbapenemase-producing bacteria were relatively rare until the extensive worldwide spread of New Delhi metallo-beta-lactamase 1 (NDM-1) [2]. Recently, novel variants of methyltransferases have emerged, although their true prevalence and distribution are yet to be ascertained. In the present study, the prevalence of methyltransferase activity was determined in clinical isolates of carbapenem-resistant Enterobacteriaceae (CRE) from our center. Furthermore, incidence of the novel rmtF and rmtG methyltransferase genes within these isolates was reported. From 512 previously characterized (n = 761) clinical isolates of multidrug-resistant Enterobacteriaceae (MDRE) [3], carbapenem-non-susceptible isolates (CNSI) (defined as isolates intermediate or resistant to any of the carbapenems, namely ertapenem, imipenem, meropenem, and doripenem) were determined by the Kirby-Bauer disk susceptibility test. Further susceptibility tests to tigecycline, colistin, carbapenems, and third-generation cephalosporins; ceftazidime, ceftriaxone, and cefotaxime by minimum inhibitory concentration (MIC) breakpoints determined by the Clinical and Laboratory Standards Institute (CLSI)-referenced agar dilution method were performed on the CNSI and CRE, defined as isolates not susceptible to any of the carbapenems and resistant to all the third-generation cephalosporins [4]. Food and Drug Administration (FDA) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines for susceptibility were used for tigecycline and colistin, respectively [5]. E. coli ATCC 25922 was used as a control. The disk diffusion test revealed 62.1% (318/512) MDRE to be CNSI. Resistance rate by MIC breakpoint determination was 47. (Table 1). The most active agent of the carbapenems was doripenem, and ertapenem was the least active agent. Although tigecycline and colistin retained the best activity against the isolates, declining activity of tigecycline was evidenced by its poor performance on K. pneumoniae (42.5% resistance) (Table 1). Comparatively, in vitro activity of carbapenems, tigecycline, and colistin on the isolates significantly differed from each other (p = 0.00). While the Wilcoxon post-hoc test revealed significant difference between tigecycline and colistin (p = 0.04), …