Induction of erm(C) expression by noninducing antibiotics

A study on erm(B)-mediated MLS resistance in Streptococcus pyogenes clinical isolates

The constitutive or inducible macrolide-lincosamide-streptogramin (MLS) phenotype of 30 erm(B)-positive Streptococcus pyogenes isolates was determined by different methods and under various growth conditions and correlated to the sequence of the 5'-untranslated regions of erm(B). The MLS phenotype of one-third of the isolates could not be classified. In liquid medium, some of these isolates responded to induction only during the logarithmic phase of growth, while others expressed clindamycin resistance even under noninducing conditions. By increasing the growth rate, we observed a shift from a constitutive towards an inducible pattern of resistance. All data were confirmed by analysis of the 23S rRNA methylation level. The erm(B)-5'-untranslated region was 99% similar in sequence. In erm(B)-positive S. pyogenes, the MLS phenotype is strongly influenced by culture conditions and control of its expression does not depend exclusively on the sequence of the erm(B)-5'-untranslated region.

Induction of efflux-mediated macrolide resistance in Streptococcus pneumoniae

The antimicrobial efflux system encoded by the operon mef(E)-mel on the mobile genetic element MEGA in Streptococcus pneumoniae and other Gram-positive bacteria is inducible by macrolide antibiotics and antimicrobial peptides. Induction may affect the clinical response to the use of macrolides. We developed mef(E) reporter constructs and a disk diffusion induction and resistance assay to determine the kinetics and basis of mef(E)-mel induction. Induction occurred rapidly, with a >15-fold increase in transcription within 1 h of exposure to subinhibitory concentrations of erythromycin. A spectrum of environmental conditions, including competence and nonmacrolide antibiotics with distinct cellular targets, did not induce mef(E). Using 16 different structurally defined macrolides, induction was correlated with the amino sugar attached to C-5 of the macrolide lactone ring, not with the size (e.g., 14-, 15- or 16-member) of the ring or with the presence of the neutral sugar cladinose at C-3. Macrolides with a monosaccharide attached to C-5, known to block exit of the nascent peptide from the ribosome after the incorporation of up to eight amino acids, induced mef(E) expression. Macrolides with a C-5 disaccharide, which extends the macrolide into the ribosomal exit tunnel, disrupting peptidyl transferase activity, did not induce it. The induction of mef(E) did not require macrolide efflux, but the affinity of macrolides for the ribosome determined the availability for efflux and pneumococcal susceptibility. The induction of mef(E)-mel expression by inducing macrolides appears to be based on specific interactions of the macrolide C-5 saccharide with the ribosome that alleviate transcriptional attenuation of mef(E)-mel.

The key function of a conserved and modified rRNA residue in the ribosomal response to the nascent peptide

The ribosome is able to monitor the structure of the nascent peptide and can stall in response to specific peptide sequences. Such programmed stalling is used for the regulation of gene expression. The molecular mechanisms of the nascent-peptide recognition and ribosome stalling are unknown. We identified the conserved and posttranscriptionally modified 23S rRNA nucleotide m(2)A2503 located at the entrance of the ribosome exit tunnel as a key component of the ribosomal response mechanism. A2503 mutations abolish nascent-peptide-dependent stalling at the leader cistrons of several inducible antibiotic resistance genes and at the secM regulatory gene. Remarkably, lack of the C2 methylation of A2503 significantly function induction of expression of the ermC gene, indicating that the functional role of posttranscriptional modification is to fine-tune ribosome-nascent peptide interactions. Structural and biochemical evidence suggest that m(2)A2503 may act in concert with the previously identified nascent-peptide sensor, A2062, in the ribosome exit tunnel to relay the stalling signal to the peptidyl transferase centre.

Dissecting the ribosomal inhibition mechanism of a new ketolide carrying an alkyl-aryl group at C-13 of its lactone ring

Ketolides are effective not only against macrolide-sensitive bacteria but also against some macrolide-resistant strains. Here we present data regarding a new ketolide with an alkyl-aryl side chain at C-13 of its lactone ring. It behaves as a strong inhibitor of protein synthesis in a model coupled transcription/translation system, although it does not affect the accuracy of translation. In addition, detailed kinetic analysis shows that it slowly forms a very tight, slowly reversible complex with prokaryotic ribosomes, a property that could be correlated with its superior activity compared with erythromycin against Escherichia coli both in vivo and in vitro.

A novel gene, erm(41), confers inducible macrolide resistance to clinical isolates of Mycobacterium abscessus but is absent from Mycobacterium chelonae

Mycobacterium abscessus infections tend to respond poorly to macrolide-based chemotherapy, even though the organisms appear to be susceptible to clarithromycin. Circumstantial evidence suggested that at least some M. abscessus isolates might be inducibly resistant to macrolides. Thus, the purpose of this study was to investigate the macrolide phenotype of M. abscessus clinical isolates. Inducible resistance to clarithromycin (MIC > 32 microg/ml) was found for 7 of 10 clinical isolates of M. abscessus previously considered susceptible; the remaining 3 isolates were deemed to be susceptible (MIC <or= 0.5 microg/ml). Inducible resistance was conferred by a novel erm gene, erm(41), which was present in all 10 isolates and in an isolate of Mycobacterium bolletii (M. abscessus type II). However, the erm(41) alleles were nonfunctional in the three susceptible M. abscessus isolates. No evidence of erm(41) was found in Mycobacterium chelonae, and an isolate of Mycobacterium massiliense appeared to be an erm(41) deletion mutant. Expression of erm(41) in M. abscessus conferred resistance to clarithromycin and erythromycin and the ketolide HMR3004. However, this species was found to be intrinsically resistant, independent of erm(41), to clindamycin, quinupristin (streptogramin B), and telithromycin. The ability to confer resistance to clindamycin and telithromycin, but not quinupristin, was demonstrated by expressing erm(41) in Maycobacterium smegmatis. Exposure of M. abscessus to the macrolide-lincosamide-streptogramin B-ketolide agents increased the levels of erm(41) mRNA 23- to 250-fold within 24 h. The inducible macrolide resistance phenotype of some M. abscessus isolates may explain the lack of efficacy of macrolide-based chemotherapy against this organism.

Distinct mode of interaction of a novel ketolide antibiotic that displays enhanced antimicrobial activity

Ketolides represent the latest generation of macrolide antibiotics, displaying improved activities against some erythromycin-resistant strains, while maintaining their activity against erythromycin-susceptible ones. In this study, we present a new ketolide, K-1325, that carries an alkyl-aryl side chain at C-13 of the lactone ring. According to our genetic and biochemical studies, K-1325 binds within the nascent polypeptide exit tunnel, at a site previously described as the primary attachment site of all macrolide antibiotics. Compared with telithromycin, K-1325 displays enhanced antimicrobial activity against wild-type Escherichia coli strains, as well as against strains bearing the U2609C mutation in 23S rRNA. Chemical protection experiments showed that the alkyl-aryl side chain of K-1325 interacts specifically with helix 35 of 23S rRNA, a fact leading to an increased affinity of U2609C mutant ribosomes for the drug and rationalizing the enhanced effectiveness of this new ketolide.

Time-resolved binding of azithromycin to Escherichia coli ribosomes

Azithromycin is a semisynthetic derivative of erythromycin that inhibits bacterial protein synthesis by binding within the peptide exit tunnel of the 50S ribosomal subunit. Nevertheless, there is still debate over what localization is primarily responsible for azithromycin binding and as to how many molecules of the drug actually bind per ribosome. In the present study, kinetic methods and footprinting analysis are coupled together to provide time-resolved details of the azithromycin binding process. It is shown that azithromycin binds to Escherichia coli ribosomes in a two-step process: The first-step involves recognition of azithromycin by the ribosomal machinery and places the drug in a low-affinity site located in the upper part of the exit tunnel. The second step corresponds to the slow formation of a final complex that is both much tighter and more potent in hindering the progression of the nascent peptide through the exit tunnel. Substitution of uracil by cytosine at nucleoside 2609 of 23S rRNA, a base implicated in the high-affinity site, facilitates the shift of azithromycin to this site. In contrast, mutation U754A hardly affects the binding process. Binding of azithromycin to both sites is hindered by high concentrations of Mg(2+) ions. Unlike Mg(2+) ions, polyamines do not significantly affect drug binding to the low-affinity site but attenuate the formation of the final complex. The low- and high-affinity sites of azithromycin binding are mutually exclusive, which means that one molecule of the drug binds per E. coli ribosome at a time. In contrast, kinetic and binding data indicate that in Deinococcus radiodurans, two molecules of azithromycin bind cooperatively to the ribosome. This finding confirms previous crystallographic results and supports the notion that species-specific structural differences may primarily account for the apparent discrepancies between the antibiotic binding modes obtained for different organisms.

Erythromycin- and chloramphenicol-induced ribosomal assembly defects are secondary effects of protein synthesis inhibition

Several protein synthesis inhibitors are known to inhibit ribosome assembly. This may be a consequence of direct binding of the antibiotic to ribosome precursor particles, or it could result indirectly from loss of coordination in the production of ribosomal components due to the inhibition of protein synthesis. Here we demonstrate that erythromycin and chloramphenicol, inhibitors of the large ribosomal subunit, affect the assembly of both the large and small subunits. Expression of a small erythromycin resistance peptide acting in cis on mature ribosomes relieves the erythromycin-mediated assembly defect for both subunits. Erythromycin treatment of bacteria expressing a mixture of erythromycin-sensitive and -resistant ribosomes produced comparable effects on subunit assembly. These results argue in favor of the view that erythromycin and chloramphenicol affect the assembly of the large ribosomal subunit indirectly.

Transcriptional and translational control of the mlr operon, which confers resistance to seven classes of protein synthesis inhibitors

The methyltransferase genes erm(B) and cfr are adjacent to each other in the chromosome of methicillin-resistant Staphylococcus aureus strain CM05. Analyses of the transcriptional organization of the erm(B) and cfr genes in the chromosome of strain CM05 showed that the two genes are organized into an operon, designated mlr (for modification of the large ribosomal subunit), which is controlled by the erm(B) promoter. Analysis of the translation control and the inducibility of the erm(B) and cfr genes in the mlr operon showed that despite the presence of putative regulatory short open reading frames, both genes are expressed constitutively. The combined action of the two methyltransferases encoded in the mlr operon results in modification of two specific residues in 23S rRNA, A2058 and A2503, and renders cells resistant to all clinically useful antibiotics that target the large ribosomal subunit. Furthermore, simultaneous modification of both rRNA sites synergistically enhances resistance to 16-member-ring macrolides.