ABSTRACT
Ketolides, which represent the newest macrolide antibiotics, are generally perceived to be noninducers of inducible erm genes. In the study described in this paper we investigated the effects of several macrolide and ketolide compounds on the expression of the inducible erm(C) gene by Escherichia coli cells. Exposure to 14-member-ring macrolide drugs and to azithromycin led to a rapid and pronounced increase in the extent of dimethylation of Erm(C) target residue A2058 in 23S rRNA. When cells were incubated with subinhibitory concentrations of ketolides, the extent of A2058 dimethylation was also increased, albeit to a lower level and with kinetics slower than those observed with macrolides. The induction of erm(C) expression by ketolides was further confirmed by using a reporter construct which allows the colorimetric detection of induction in a disc diffusion assay. Most of the ketolides tested, including the clinically relevant compounds telithromycin and cethromycin, were able to induce the reporter expression, even though the induction occurred within a more narrow range of concentrations compared to the concentration range at which induction was achieved with the inducing macrolide antibiotics. No induction of the reporter expression was observed with 16-member-ring macrolide antibiotics or with a control drug, chloramphenicol. The deletion of three codons of the erm(C) leader peptide eliminated macrolide-dependent induction but left ketolide-dependent induction unchanged. We conclude that ketolides are generally capable of inducing erm genes. The narrow range of ketolide inducing concentrations, coupled with the slow rate of induction and the lower steady-state level of ribosome methylation, may mask this effect in MIC assays.
Subject(s)
Enzyme Induction , Escherichia coli/enzymology , Ketolides/pharmacology , Methyltransferases/biosynthesis , Amino Acid Sequence , Anti-Bacterial Agents/chemistry , Anti-Bacterial Agents/pharmacology , Drug Resistance, Bacterial/genetics , Escherichia coli/drug effects , Escherichia coli/genetics , Gene Expression Regulation, Bacterial , Ketolides/chemistry , Macrolides/chemistry , Macrolides/pharmacology , Methyltransferases/chemistry , Methyltransferases/genetics , Microbial Sensitivity Tests , Molecular Sequence Data , Protein Conformation , Protein Sorting Signals/genetics , RNA, Messenger/chemistry , RNA, Messenger/genetics , RNA, Messenger/metabolism , Ribosomes/metabolismABSTRACT
Expression of specific short peptides can render cells resistant to macrolide antibiotics. Peptides conferring resistance to structurally different macrolides including oleandomycin, azithromycin, azaerythromycin, josamycin and a ketolide cethromycin were selected from a random pentapeptide expression library. Analysis of the entire collection of the resistance peptides allowed their classification into five distinct groups according to their sequence similarity and the type of resistance they confer. A strong correlation was observed between the structures of macrolide antibiotics and sequences of the peptides conferring resistance. Such a correlation indicates that sequence-specific interactions between the nascent peptide and the macrolide antibiotic and/or the ribosome can occur in the ribosomal exit tunnel.
