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New Mechanism of Bacterial Resistance to Aminoglycosides
Abstract & Commentary
By Dean L. Winslow, MD, FACP, Chief, Division of AIDS Medicine, Santa Clara Valley Medical Center; Clinical Professor of Medicine, Stanford University School of Medicine, Section Editor, HIV, is Associate Editor for Infectious Disease Alert.
Synopsis: Methylation of 16S ribosomal RNA (rRNA) has emerged as an important mechanism of resistance to aminoglycosides in pathogens including Enterobacteriaceae and non-fermenters including Pseudomonas aeruginosa and Acinetobacter species. The presence of this group of newly recognized 16S rRNA methylases confers high-level resistance to all currently available aminoglycosides.
Source: Doi Y and Arakawa Y. 16S ribosomal RNA methylation: Emerging resistance mechanism against aminoglycosides. Clin Infect Dis 2007; 45: 88-94.
While use of aminoglycosides to treat gram negative rods has fallen over the last 30 years due to the availability of broad-spectrum beta lactam antibiotics, aminoglycoside antibiotics remain useful for the treatment of some life-threatening infections. Due to increasing prevalence of resistance to beta lactam antibiotics the use of aminoglycosides is occasionally the best option to treat certain infections.
Aminoglycosides exert their bactericidal activity against susceptible pathogens by binding specifically to the aminoacyl site (A-site) of 16S rRNA within the prokaryotic 30S ribosomal subunit and inhibit bacterial protein synthesis. The most common mechanism of resistance to aminoglycosides is enzymatic inactivation of the drug by adenylation, acetylation or phosphorylation (aminoglycoside modifying enzymes). Mutation of the 30S ribosome, defects in cellular permeability (often developing while on therapy), and active efflux are other mechanisms of bacterial resistance to aminoglycosides.
Since aminoglycosides are produced by certain species of actinomycetes, these organisms are intrinsically resistant to the aminoglycosides they produce. In most cases this intrinsic resistance is caused by ribosomal protection through methylation of specific nucleotides within the A-site of 16S rRNA, which hampers binding of the aminoglycoside.
Recently, clinical strains of Pseudomonas aeruginosa and Klebsiella pneumoniae which produced 16S rRNA methylases were reported.1, 2 These organisms displayed high-level resistance to gentamicin, tobramycin and amikacin. Since this first publication, the literature has documented identification of new methylase enzymes in other gram negative rods and their spread to different areas of the world.
The genes responsible for these enzymes are mostly located on transposons within transferable plasmids and are capable of horizontal spread. Some of the organisms carrying these methylases have been shown to coproduce extended-spectrum beta lactamases (ESBLs) or metallo-beta-lactamases contributing to a multi-drug resistant phenotype. Since resistance to multiple aminoglycosides can be due to acquisition of more than one aminoglycoside modifying enzyme, aminoglycoside resistance due to methylase production may not be recognized. The authors of the paper recommend as a screen for methylase-producing organisms that multi-aminoglycoside resistant gram negative rods (as identified by automated susceptibility testing) be subjected to Bauer-Kirby disk susceptibility testing using gentamicin, amikacin and arbekacin disks. (The latter aminoglycoside is not available for therapeutic use in the U.S., but demonstration of in vitro resistance to this drug raises the positive predictive value of this method of testing to ≥ 90%.) The presence of 16SrRNA methylase is suspected when little or no zone of inhibition surrounds any of the aminoglycoside disks. The only definitive confirmation of the presence of these methylases remains PCR. The paper lists the sequences of the specific primers and appropriate thermal cycling conditions.
This is a paper which is of significant clinical relevance. The demonstration of this novel mechanism of antimicrobial resistance is illustration of the propensity of our microbial nemeses to continually stay at least one step ahead of our best attempts to defeat them.