A novel multidrug resistance phenotype mediated by the Cfr rRNA methyltransferase is observed in Staphylococcus aureus and Escherichia coli. The cfr gene has previously been identified as a phenicol and lincosamide resistance gene on plasmids isolated from Staphylococcus spp. of animal origin and recently shown to encode a methyltransferase that modifies 23S rRNA at A2503. Antimicrobial susceptibility testing shows that S. aureus and E. coli strains expressing the cfr gene exhibit elevated MICs to a number of chemically unrelated drugs. The phenotype is named PhLOPS A for resistance to the following drug classes: Phenicols, Lincosamides, Oxazolidinones, Pleuromutilins, and Streptogramin A antibiotics. Each of these five drug classes contains important antimicrobial agents that are currently used in human and/or veterinary medicine. We find that binding of the PhLOPS A drugs, which bind to overlapping sites at the peptidyl transferase center that abut nucleotide A2503, is perturbed upon Cfr-mediated methylation. Decreased drug binding to Cfr-methylated ribosomes has been confirmed by footprinting analysis. No other rRNA methyltransferase is known to confer resistance to five chemically distinct classes of antimicrobials. In addition, the findings described in this study represent the first report of a gene conferring transferable resistance to pleuromutilins and oxazolidinones.The bacterial ribosome is the site of protein synthesis and the target for many chemically diverse classes of antimicrobial agents. The antimicrobial drugs target important functional centers of the ribosome and most often bind to rRNA. Recently, a new phenicol and clindamycin resistance phenotype was found to be caused by an RNA methyltransferase designated Cfr. A detailed analysis by drug footprinting studies and matrix-assisted laser desorption-ionization time of flight/tandem mass spectrometry showed that Cfr adds an additional methyl group at position A2503 of 23S rRNA (9). Since A2503 is located in close proximity to the overlapping ribosomal binding sites of phenicols and clindamycin, it was concluded that the Cfr-mediated methylation confers resistance to these two classes of antimicrobial agents by interfering with the positioning of the drugs (9).The cfr gene was first discovered in 2000 during a surveillance study for florfenicol resistance among staphylococci from animals. It was initially detected on the 17.1-kb multiresistance plasmid pSCFS1 from a bovine strain of Staphylococcus sciuri (24) and has also been found in bovine strains of Staphylococcus simulans (6). In addition to cfr, the pSCFS1 plasmid carries the rRNA methylase gene erm(33), the aminocyclitol phosphotransferase gene spc, and the ABC transporter gene lsa(B), which confer resistance to macrolide-lincosamide-streptogramin B (MLS B ) antibiotics, spectinomycin, and lincosamides, respectively. The cfr gene was recently detected on the 35.7-kb plasmid, pSCFS3, from a porcine Staphylococcus aureus strain, together with the chloramphenicol/florfenicol exporter gene f...

Many clinically useful antibiotics exert their antimicrobial effects by blocking protein synthesis on the bacterial ribosome. The structure of the ribosome has recently been determined by X-ray crystallography, revealing the molecular details of the antibiotic-binding sites. The crystal data explain many earlier biochemical and genetic observations, including how drugs exercise their inhibitory effects, how some drugs in combination enhance or impede each other's binding, and how alterations to ribosomal components confer resistance. The crystal structures also provide insight as to how existing drugs might be derivatized (or novel drugs created) to improve binding and circumvent resistance.

SummaryThe pleuromutilin antibiotic tiamulin binds to the ribosomal peptidyl transferase centre. Three groups of Brachyspira spp. isolates with reduced tiamulin susceptibility were analysed to define resistance mechanisms to the drug. Mutations were identified in genes encoding ribosomal protein L3 and 23S rRNA at positions proximal to the peptidyl transferase centre. In two groups of laboratory-selected mutants, mutations were found at nucleotide positions 2032, 2055, 2447, 2499, 2504 and 2572 of 23S rRNA ( Escherichia coli numbering) and at amino acid positions 148 and 149 of ribosomal protein L3 ( Brachyspira pilosicoli numbering). In a third group of clinical B. hyodysenteriae isolates, only a single mutation at amino acid 148 of ribosomal protein L3 was detected. Chemical footprinting experiments show a reduced binding of tiamulin to ribosomal subunits from mutants with decreased susceptibility to the drug. This reduction in drug binding is likely the resistance mechanism for these strains. Hence, the identified mutations located near the tiamulin binding site are predicted to be responsible for the resistance phenotype. The positions of the mutated residues relative to the bound drug advocate a model where the mutations affect tiamulin binding indirectly through perturbation of nucleotide U2504.

Sixteen (1.5%) of the 1,043 clinical macrolide-resistant Streptococcus pneumoniae isolates collected and analyzed in the 1999-2000 PROTEKT (Prospective Resistant Organism Tracking and Epidemiology for the Ketolide Telithromycin) study have resistance mechanisms other than rRNA methylation or efflux. We have determined the macrolide resistance mechanisms in all 16 isolates by sequencing the L4 and L22 riboprotein genes, plus relevant segments of the four genes for 23S rRNA, and the expression of mutant rRNAs was analyzed by primer extension. Isolates from Canada (n ‫؍‬ 4), Japan (n ‫؍‬ 3), and Australia (n ‫؍‬ 1) were found to have an A2059G mutation in all four 23S rRNA alleles. The Japanese isolates additionally had a G95D mutation in riboprotein L22; all of these originated from the same collection center and were clonal. Three of the Canadian isolates were also clonal; the rest were not genetically related. Four German isolates had A2059G in one, two, and three 23S rRNA alleles and A2058G in two 23S rRNA alleles, respectively. An isolate from the United States had C2611G in three 23S rRNA alleles, one isolate from Poland had A2058G in three 23S rRNA alleles, one isolate from Turkey had A2058G in four 23S rRNA alleles, and one isolate from Canada had A2059G in two 23S rRNA alleles. Erythromycin and clindamycin resistance gradually increased with the number of A2059G alleles, whereas going from one to two mutant alleles caused sharp rises in the azithromycin, roxithromycin, and rokitamycin MICs. Comparisons of mutation dosage with rRNA expression indicates that not all alleles are equally expressed. Despite their high levels of macrolide resistance, all 16 isolates remained susceptible to the ketolide telithromycin (MICs, 0.015 to 0.25 g/ml).Macrolide, lincosamide, and streptogramin B (MLS B ) resistance in Streptococcus pneumoniae occurs either by modification of the drug-binding site or by active efflux of the drug. Target modification is usually the result of dimethylation of the adenine residue at position 2058 on the 23S rRNA by a methylase enzyme (30). In S. pneumoniae, Erm(B) [encoded by the erm(B) gene] is the enzyme mostly responsible (7, 30), although, more rarely, a methylase encoded by the erm(A) subclass erm(TR) gene is implicated (7,23).In vitro studies have demonstrated that target modification can also be achieved via mutations in domains II and V of 23S rRNA and in the genes encoding riboproteins L4 and L22 and can confer macrolide, lincosamide, streptogramin, and ketolide resistance (2, 24). Although previous reports are rare, such mutations have been found in MLS B -resistant clinical isolates (3,14,25). However, until now there have been no studies on the prevalence and epidemiology of these types of mutations in clinical isolates on a worldwide scale.PROTEKT (Prospective Resistant Organism Tracking and Epidemiology for the Ketolide Telithromycin) is a longitudinal, global, multicenter surveillance study of respiratory tract pathogens. We screened all macrolide-resistant S. pneumoniae isolat...

SummaryTiamulin and valnemulin target the peptidyl transferase centre (PTC) on the bacterial ribosome. They are used in veterinary medicine to treat infections caused by a variety of bacterial pathogens, including the intestinal spirochetes Brachyspira spp. Mutations in ribosomal protein L3 and 23S rRNA have previously been associated with tiamulin resistance in Brachyspira spp. isolates, but as multiple mutations were isolated together, the roles of the individual mutations are unclear. In this work, individual 23S rRNA mutations associated with pleuromutilin resistance at positions 2055, 2447, 2504 and 2572 (Escherichia coli numbering) are introduced into a Mycobacterium smegmatis strain with a single rRNA operon. The single mutations each confer a significant and similar degree of valnemulin resistance and those at 2447 and 2504 also confer cross-resistance to other antibiotics that bind to the PTC in M. smegmatis. Antibiotic footprinting experiments on mutant ribosomes show that the introduced mutations cause structural perturbations at the PTC and reduced binding of pleuromutilin antibiotics. This work underscores the fact that mutations at nucleotides distant from the pleuromutilin binding site can confer the same level of valnemulin resistance as those at nucleotides abutting the bound drug, and suggests that the former function indirectly by altering local structure and flexibility at the drug binding pocket.