Macrolide and ketolide-based induction of erythromycin resistance methylase type-C in Escherichia coli

Ketolide antibiotics have been labeled as non-inducers of erm -type resistance methylases. In this work we developed a colorimetric erm reporter construct pERMZalpha that was used to identify macrolide and ketolide antibiotics that can induce erm expression. We verified that cladinose-containing macrolides are active in inducing erm expression. We also saw induction of our reporter with various ketolides such as telithromycin and cethromycin. This is in contrast to reports in literature that classify ketolides as non-inducers. The reporter responses were confirmed in the erm system using primer extension analysis. We have concluded that ketolides can induce erm expression, albeit at a slower rate and within a tighter range of drug concentrations than with macrolides such as erythromycin. We also investigated the effect of sequence alterations in the nascent erm leader peptide. We developed two reporter variants with truncated leader sequences. These constructs showed enhanced induction with ketolides while macrolide induction was eliminated. We concluded that minor alterations in the length or sequence of the erm leader can affect the ability of a particular drug to act as an inducer. If this were to happen clinically, a cell that harbored both a traditional version of inducible erm as well as the ketolide-inducible variant would develop resistance to all MLS B drugs when treated with a macrolide or a ketolide drug.; Overall, the impact of this work is three-fold. The reporter system we developed can be used for studying various aspects of the induction of erm expression and for the investigation of the molecular mechanisms of drug-dependent ribosome stalling. Second, we have presented evidence that "non-inducing" drugs like ketolides can in fact induce expression of the inducible erm genes. Therefore, inappropriate antibiotic therapy employing prolonged treatment with low doses of ketolides is expected to result in the development of resistance. Third, we showed that alteration of the erm leader peptide sequence affects the spectrum of antibiotics capable of inducing erm-based resistance. We anticipate that broad clinical use of ketolides will select for erm variants that can be induced by ketolide antibiotics.