Search

Objectives: Mucus hypersecretion is a prominent feature in patients with chronic respiratory tract infections such as cystic fibrosis and diffuse panbronchiolitis, and the clinical effectiveness of macrolide antibiotics has been reported in these patients. Because human neutrophil peptide-1 (HNP-1), an antimicrobial peptide in neutrophils, exists in high concentrations in the airway fluid of these patients, we examined the direct effect of HNP-1 on MUC5AC mucin production using NCI-H292 cells. The effects of macrolide antibiotics on the response were also examined. Methods: MUC5AC synthesis was assayed using RT-PCR and ELISA. Phosphorylation of ERK1/2 was determined by western blotting. Results: Stimulation with HNP-1 or lipopolysaccharide (LPS) derived from Pseudomonas aeruginosa increases the production of MUC5AC mRNA and protein, and an additive effect was found upon co-stimulation with both HNP-1 and LPS. Azithromycin and clarithromycin had inhibitory effects on overproduction of MUC5AC induced by HNP-1 or LPS stimulation. Telithromycin also had an inhibitory effect on MUC5AC production induced by LPS, but not on production by HNP-1. Phosphorylation of ERK1/2 was induced by HNP-1 or LPS stimulation, and azithromycin, clarithromycin and telithromycin had inhibitory effects on ERK1/2 phosphorylation induced by LPS, but not by HNP-1. Conclusions: These findings suggest that neutrophil-derived defensins as bacterial components contribute to excessive mucus production in patients with respiratory tract infections, and that macrolide and ketolide antibiotics directly inhibit these actions by interfering with intracellular signal transduction. However, the mechanism of telithromycin inhibition of MUC5AC synthesis may differ from the response induced by azithromycin and clarithromycin.

OBJECTIVES: In vitro susceptibility testing of 31 Bartonella spp. strains including 21 Bartonella henselae isolates was performed for 17 antimicrobial agents (telithromycin, four macrolides, five fluoroquinolones, five aminoglycosides, doxycycline and rifampicin). METHODS: MICs were determined by agar dilution and Etest using chocolate agar containing 5% defibrinated sheep blood as assay medium. Longer incubation periods of 3-5 days in a humid atmosphere with 5% CO(2) were required until bacterial growth became visible and MICs could be read. RESULTS: The ketolide telithromycin was the most active agent exhibiting the lowest MICs. The Bartonella spp. were also highly susceptible to macrolides, particularly clarithromycin, and to doxycycline and rifampicin, with MICs of

OBJECTIVES The ketolide telithromycin represents a new subclass of 14-membered semisynthetic macrolides. Because there is evidence that traditional macrolides such as roxithromycin exert anti-inflammatory activity, we investigated the anti-inflammatory action of telithromycin against lipopolysaccharide (LPS)-induced acute systemic inflammation in mice in comparison with roxithromycin. METHODS CD-1 mice were injected intraperitoneally with LPS (1 mg/kg), and the effects of pretreatment with a single intraperitoneal dose of telithromycin (150 mg/kg) or roxithromycin (50 mg/kg) for 2 h on the expression and formation of tumour necrosis factor alpha (TNFalpha), interleukin-1 beta (IL-1beta), interferon gamma (IFNgamma) and inducible nitric oxide synthase (NOS-II) as well as nitric oxide (NO) were analysed at different time points after LPS-treatment. Cytokine and NOS-II mRNA abundance was examined using real-time RT-PCR. Tissue cytokine levels were determined by enzyme-linked immunosorbent assay kits (ELISA); NO levels were measured by colorimetric assay kits. RESULTS Pretreatment of mice with telithromycin as well as roxithromycin similarly attenuated the LPS-induced expression and formation of TNFalpha, IL-1beta and IFNgamma. Furthermore, the LPS-induced increase of NOS-II mRNA and the formation of NO were clearly diminished. CONCLUSION These results suggest that the ketolide telithromycin has anti-inflammatory properties like conventional macrolides due to inhibition of the production of proinflammatory cytokines, which leads to a decreased formation of NO in LPS-treated mice. Our data indicate that ketolides may have beneficial therapeutic effects independent of their antibacterial activity.

Erythromycin is the drug of choice to treat human campylobacteriosis. Campylobacter isolates exhibit two different phenotypes with regard to erythromycin resistance: high-level resistant strains (HLR) and low-level resistant strains (LLR).To study the mechanisms of resistance of Campylobacter to erythromycin, its 6-O-methyl derivative clarithromycin and the ketolide telithromycin.We observed a cross-resistance against these three molecules but in contrast, no cross-resistance to quinolones. Analyses of LLR showed no mutation on the 23S rDNA and the presence of a drug transport system, which can be inhibited by phenylalanine arginine beta-naphthylamide (PAbetaN), an efflux-pump inhibitor. In contrast, no PAbetaN-sensitive drug transport was identified in HLR but we found mutations in the rDNA, which were responsible for decreased binding of telithromycin to purified ribosomes. We further showed that the CmeB efflux pump already described in Campylobacter is not involved in the PAbetaN-sensitive transport of telithromycin.Mutations in the ribosome confer high-level macrolide/ketolide resistance. Low-level resistance was mediated by an efflux mechanism which is sensitive to PAbetaN. This efflux pump was selective to macrolides/ketolide and was different from the previously described Campylobacter efflux pump.

OBJECTIVES To determine the quantitative differences in telithromycin and azithromycin MIC values against Streptococcus pneumoniae, Haemophilus influenzae and Streptococcus pyogenes obtained using two recommended and commonly used methodologies: CLSI reference standard broth microdilution in ambient air and Etest((R)) concentration gradient in CO(2). METHODS Four hundred clinical isolates (S. pneumoniae, n = 200; H. influenzae, n = 100; S. pyogenes, n = 100) were evaluated in seven independent laboratories. Telithromycin and azithromycin MICs were determined using CLSI broth microdilution panels incubated in ambient air and Etest strips incubated in CO(2). Standard quality control reference strains-S. pneumoniae ATCC 49619 (n = 10) and H. influenzae ATCC 49247 (n = 10)-were also tested. RESULTS Telithromycin and azithromycin Etest MICs in CO(2) were elevated for all organisms when compared with values obtained using broth microdilution in ambient air. Telithromycin geometric mean MIC values increased in CO(2) by 2.05, 1.00 and 1.78 log(2) dilutions for S. pneumoniae, H. influenzae and S. pyogenes, respectively. The corresponding values for azithromycin were 2.54, 1.21 and 3.0 log(2) dilutions, respectively. CONCLUSIONS Telithromycin MICs measured using Etest in CO(2) are consistently elevated compared with those generated by CLSI broth microdilution measured in ambient air. These findings indicate that Etest should not be routinely used for the determination of telithromycin MICs against S. pneumoniae, H. influenzae and S. pyogenes, unless appropriate corrective factors are applied before reporting MICs or applying interpretive susceptibilities. Based on results from this study, Etest MIC breakpoints and quality control ranges are proposed.

Background: The PROTEKT US (Prospective Resistant Organism Tracking and Epidemiology for the Ketolide Telithromycin in the United States) surveillance programme was started in 2000, to chart the emergence and spread of antimicrobial resistance among isolates of Streptococcus pneumoniae, Streptococcus pyogenes and Haemophilus influenzae from across the USA. Methods: In 2001-2002 (Year 2 of PROTEKT US) 242 centres from 46 states and the territory of Puerto Rico submitted a total of 10012 S. pneumoniae, 4508 S. pyogenes and 3296 H. influenzae isolates from community-acquired respiratory tract infections (CARTIs). Susceptibility testing was performed and interpreted using NCCLS methodology and criteria. Results: Overall, 35.4% of S. pneumoniae were non-susceptible to penicillin (14.2% intermediate, MIC 0.12-1 mg/L; 21.2% resistant, MIC ≥2mg/L) and 27.9% were resistant to erythromycin (MIC ≥1 mg/L) (0.2% intermediate, MIC 0.5 mg/L). A total of 105 (1.0%) isolates were resistant to levofloxacin (MIC ≥8 mg/L). More than 99.2% of isolates were susceptible to telithromycin (MIC ≤1 mg/L) irrespective of penicillin and/or erythromycin resistance. All S. pyogenes isolates were susceptible to penicillin (MIC ≤0.12 mg/L) and 5.7% were resistant to erythromycin (MIC ≥1 mg/L) (0.3% intermediate, MIC 0.5 mg/L). The MICA of telithromycin for S. pyogenes was 0.03 mg/L. A total of 27.5% of H. influenzae isolates were β-lactamase producers. Overall, 27.8% were resistant (MIC ≥4 mg/L) and 1.1% were intermediate to ampicillin (MIC 2 mg/L). A total of 96.3% of H. influenzae isolates were susceptible to telithromycin (MIC ≤4 mg/L). Conclusions: Antimicrobial resistance continues to be a problem in the USA. The ketolide telithromycin continues to show high activity against common CARTI pathogens, including those resistant to β-lactams and macrolides.

Background: One of the main factors commonly associated with antibacterial resistance among Streptococcus pneumoniae is the age of the patient. The highest rates of resistance have often been reported among isolates from young children. Methods: Data from the PROTEKT US (Prospective Resistant Organism Tracking and Epidemiology for the Ketolide Telithromycin in the United States) surveillance study were examined to determine the level of antibacterial resistance among S. pneumoniae isolates collected in 2001–2002 from different patient age groups in the USA. Results: A total of 10 012 clinical isolates of S. pneumoniae were submitted by 242 centres across the USA and categorized into four patient age groups: infants (0–2 years, n 5 1556), children (3–14 years, n 5 1125), adults (15–64 years, n 5 4058) and elderly adults (>65 years, n 5 3067) (age unknown n 5 206). With the exception of the fluoroquinolones and linezolid, rates of antibacterial resistance were highest among infants and decreased with increasing patient age. Resistance to penicillin ranged from 33.6% among infants to 17.5% among elderly adults, and erythromycin resistance ranged from 41.1% among infants to 24.0% among adults. In contrast, levofloxacin resistance increased with patient age (from 0.1% to 1.6%). The highest rates of susceptibility were noted for telithromycin and linezolid (> _99.6% and >99.8% susceptible isolates, respectively). Conclusions: The PROTEKT US study data confirmed that the highest antibacterial resistance rates were associated with isolates collected from young children (0–2 years). Telithromycin may offer a reliable alternative to first-line drugs in the empirical treatment of community-acquired respiratory tract infections.

Objectives: The efficacy of telithromycin, a new ketolide antibiotic, was investigated in the treatment of acute Chlamydia pneumoniae infection in a mouse model. Methods: C57BL/6J mice were inoculated intranasally, and the effects of three different doses of telithro- mycin (25, 50 and 100 mg/kg) were assessed after 5 and 10 days of treatment. Lungs for culture, PCR, histo- pathology, and blood for serum samples were collected immediately after each treatment period and at 3 weeks post-inoculation. C. pneumoniae-specific antibodies were analysed, and the effect of treatment was assessed by culture, detection of C. pneumoniae DNA and determination of histopathological inflammatory changes in mouse lungs. Results: Culture negativity in the lungs was achieved with the higher doses, 50 and 100 mg/kg, after 10 days of treatment. C. pneumoniae DNA was not totally eradicated with the treatments, but the groups treated with 50 and 100 mg/kg doses for 10 days had the lowest DNA positivity rates (10%) 3 weeks after the inoculation. In lung histopathology, the efficacy of telithromycin on inflammatory changes was also dose-dependent: higher doses were more effective in reducing the inflammatory reaction. Overall, the 25 mg/kg dose had a weaker effect compared with the others. Conclusions: Telithromycin had both time- and dose-dependent effects on the eradication of chlamydia and on reducing infection-induced inflammatory changes in mouse lungs.

OBJECTIVES To investigate the in vitro acquisition of resistance to antibiotics by Bacillus anthracis. METHODS The in vitro activities of 18 antibacterial agents against two strains of B. anthracis, the Sterne strain and the Russian anthrax vaccine strain ST-1, were tested by determining the MICs and by measuring the rates of antibiotic kill at 5x and 10x MIC. RESULTS The fluoroquinolones ciprofloxacin, ofloxacin, levofloxacin and moxifloxacin, the beta-lactams penicillin G and amoxicillin, the macrolide clarithromycin, the ketolide telithromycin, as well as clindamycin, rifampicin and quinupristin/dalfopristin had MICs in the range of 0.03-0.25 mg/L. Minocycline had an MIC of 0.03 mg/L, as did penicillin, against the ST-1 strain. Ciprofloxacin had an MIC of 0.03 mg/L against both strains. Erythromycin, vancomycin and the oxazolidinone linezolid were less active (MIC 0.5-2.5 mg/L). Ceftriaxone was the least active, having an MIC of 8.0 mg/L. Chloramphenicol was inactive (MIC > 256 mg/L). Quinupristin/dalfopristin, rifampicin and moxifloxacin showed the most rapid bacterial killing, achieving a complete eradication of detectable organisms (2 log(10) reduction within 0.5-3 h and 4 log(10) reduction within 0.5-4 h for both strains at concentrations of 5x and 10x the MIC). The beta-lactams and vancomycin demonstrated a 2-4 log(10) reduction within 5-15 h. Ceftriaxone had a similar effect to penicillin and amoxicillin against the ST-1 strain, but a slower effect than these two beta-lactams against the Sterne strain. The macrolides, tetracyclines and linezolid demonstrated a lower kill rate, while chloramphenicol did not kill at all. CONCLUSIONS These data expand on the spectrum of agents recommended for the treatment of anthrax (ciprofloxacin, penicillin G and tetracyclines) and add new options, such as other fluoroquinolones, amoxicillin, rifampicin and quinupristin/dalfopristin, as potential therapeutic agents.

Post-antibiotic effects (PAE) and post-antibiotic sub-MIC effects (PA-SME) of the ketolide telithromycin (HMR 3647) were determined for 10 Gram-positive cocci with various macrolide resistance mechanisms, including inducible and constitutive ribosomal methylase and macrolide efflux resistance genes. Strains tested included four Streptococcus pneumoniae, three Streptococcus pyogenes and three Staphylococcus aureus. Telithromycin MICs were 0.008-0.015 mg/L for pneumococci, 0.015-4.0 mg/L for S. pyogenes and 0.03 mg/L for staphylococci. PAE were determined after exposure of strains at 10 x MIC for 1 h. PA-SME were determined in the presence of 0.12x, 0.25x and 0.5x MIC of the agent after the initial 1 h exposure period. The PAE of telithromycin varied from 0.3 to 3.8 h; the PA-SME varied from 0.8 to 4.6 h, with maximal PA-SME varying from 1.5 to 4.6 h. PAE tended to be shortest for S. pyogenes (0.4-2.7 h) and S. aureus (0.3-2.4 h), compared to 1.5-3.8 h for S. pneumoniae. The duration of the PA-SME was similar for the three species tested. The low MICs for many strains as well as the PAE and PA-SME demonstrated in this study for telithromycin show promise for increasing the dosing interval of this ketolide, but will need verification by pharmacokinetic/pharmacodynamic and clinical studies.

Our aim was to study the antimicrobial susceptibilities and macrolide resistance mechanisms of viridans group streptococci isolated from the normal flora.In vitro susceptibilities of 16 antimicrobials were studied for 161 viridans streptococci (on average 5.8 isolates per person) from the normal flora of 28 elderly persons. Resistance mechanisms of erythromycin-resistant isolates were studied by the double disc test and PCR.In all, 16.8% of the isolates were non-susceptible (MICor =0.25 mg/L) to penicillin, but none showed high-level resistance (MICor =4 mg/L). Resistance to erythromycin, tetracycline, quinupristin/dalfopristin, levofloxacin and moxifloxacin was found in 22.4, 27.3, 13.0, 1.9 and 1.9% of the isolates, respectively. Combined resistance to erythromycin and tetracycline was found in 13.0% of the isolates. Erythromycin-resistant isolates were isolated from 57% of the study persons. Of the erythromycin-resistant isolates 80.6% were of the M phenotype and 19.4% were of the macrolide-lincosamide-streptogramin B (MLSB) phenotype (one isolate with constitutive and six with inducible expression). Isolates with the M phenotype were the least susceptible to telithromycin, a new ketolide. The mef(A) gene was found in the isolates with the M phenotype and the erm(B) gene in the isolates with the MLSB phenotype.The distribution of phenotypes among the viridans streptococci resembles that found in Streptococcus pyogenes, with predominance of the M phenotype. However, the coding gene for the MLSB phenotype, erm(B), is the same in viridans streptococci as in Streptococcus pneumoniae. Viridans group streptococci carrying different resistance traits provide a pool of resistant bacteria that may transfer resistance determinants to more pathogenic organisms.

Telithromycin (HMR 3647) is the first ketolide introduced into clinical practice. Ketolides are semisynthetic derivates of erythromycin A that carry novel biological properties on the erythronolide A ring. This new class of antimicrobials was designed to overcome current resistance mechanisms against erythromycin A within Gram-positive cocci. Ketolides do not induce macrolide-lincosamide-streptogramin B (MLS(B)) resistance and are active against erythromycin resistance methylase gene (erm)-carrying Gram-positive cocci. This review summarizes published data on telithromycin and intends to define the challenge that a new antimicrobial brings to medical practice.

Objectives: (i) To determine the inhibitory and bactericidal activities of ABT-773, a novel ketolide, against sensitive and erythromycin-resistant pneumococci; (ii) to subdivide erythromycin-resistant pneumococci into resistance phenotypes, more extensive than the conventional M and MLS B groups, by assessing susceptibilities to, and interactions between, erythromycin (14-membered macrolide), clindamycin (lincosamide), rokitamycin (16-membered macrolide), ABT-773 (ketolide), quinupristin (streptogramin B) and dalfopristin (streptogramin A). Methods: MICs and MBCs of ABT-773 were determined for 165 strains of pneumococci (113 resistant to erythromycin). Extended phenotypes for the erythromycin-resistant strains were described in terms of intrinsic susceptibility to, and induction of resistance by, the antibiotics listed above. Results: Erythromycin-resistant strains could be divided into 10 extended phenotypes (designated II-XI), two of which (II and IX) predominated. ABT-773 at 0.12 mg/L inhibited 109 strains (median 0.03 mg/L). MICs for the other four strains (of phenotypes X and XI) were 0.25-1 mg/L. MICs were only slighter higher when measured on agar in CO 2 than by the NCCLS method (in broth in air). MBCs were usually ≤2 x MIC, but for 10 strains (eight of phenotype X, one each of types IX and XI) MBCs were >1 mg/L, and three of the latter (all type X) were tolerant. Clones of reduced susceptibility (MICs 1-8 mg/L, increased by up to 32-fold) could be isolated from some strains of phenotypes VII, IX and X, but not from those of type II (efflux mechanism) or from erythromycin-sensitive strains. Conclusions: ABT-773 was active against all 113 erythromycin-resistant pneumococci tested, which belonged to 10 phenotypes. Extended phenotyping of pneumococci revealed interesting and potentially useful subdivisions of the classical phenotypes.

In this study, the distribution of macrolide resistance mechanisms was determined for isolates of Streptococcus pneumoniae and Streptococcus pyogenes obtained from the PROTEKT 1999–2000 study (a global, longitudinal study of the antibacterial susceptibility of bacterial pathogens associated with community-acquired lower respiratory tract infections). The global macrolide resistance mechanism distribution results for 1043 macrolide-resistant S. pneumoniae isolates collected from 25 countries were as follows: 35.3% mef (A) , 56.2% erm (B) , 6.8% both mef (A) and erm (B) , 0.2% erm (A) subclass erm (TR) and 1.5% negative for mechanisms tested. Mechanisms of macrolide resistance were found to vary widely between countries and different geographical regions with mef (A) predominating in North America and erm(B) in Europe. Approximation of genotype from macrolide MIC without molecular determination of the mechanism of resistance resulted in an error of 10.2% (106 isolates). Overall, for 143 macrolide-resistant S. pyogenes isolates, 46.1% of the isolates tested were mef (A), 30.8% were erm (B), 23.1% were erm (A) subclass erm (TR) and no isolates were negative for all the genetic markers tested. Again, the distribution varied widely between countries and geographical regions. This study provides valuable baseline data for the continued monitoring of the evolution of macrolide resistance development in these important respiratory tract pathogens. The ketolide telithromycin retained excellent anti-pneumococcal activity irrespective of macrolide resistance mechanism, having a MIC 90 of 0.25, 0.5 and 0.5 mg/L against mef (A), erm (B) and mef (A)+ erm (B) macrolide-resistant S. pneumoniae , respectively. It also exhibited potent activity against S. pyogenes that had become resistant to macrolides via either mef (A), (MIC 90 0.5 mg/L) or erm (TR), (MIC 90 0.03 mg/L).

The prevalence of resistance to a range of antimicrobials was determined for isolates of Streptococcus pneumoniae examined in the PROTEKT (Prospective Resistant Organism Tracking and Epidemiology for the Ketolide Telithromycin) surveillance study (1999–2000) using NCCLS testing methods and interpretative criteria. Of 3362 pneumococcal isolates collected from 69 centres in 25 countries, 22.1% overall were resistant to penicillin G, with the highest rates of resistance found among isolates from Asia (53.4%), France (46.2%) and Spain (42.1%). Erythromycin A resistance occurred in 31.1% of isolates overall with the highest rates found in Asia (79.6%), France (57.6%), Hungary (55.6%) and Italy (42.9%). Marked geographical differences in the prevalence of both penicillin G (the Netherlands 0%; South Korea 71.5%) and erythromycin A (Sweden 4.7%; South Korea 87.6%) resistance were observed. Asia was characterized by the highest prevalence of resistance, overall, with only eight of 19 antimicrobials (co-amoxiclav, linezolid, vancomycin, teicoplanin, quinupristin/dalfopristin, levofloxacin, moxifloxacin and telithromycin) retaining high activity against isolates of S. pneumoniae from this region. Notable rates of resistance to clarithromycin, azithromycin, co-trimoxazole and tetracycline were observed in the majority of countries submitting isolates of S. pneumoniae to the PROTEKT surveillance study. Fluoroquinolone resistance was low (1%), overall, although 14.3% of 70 isolates from Hong Kong were resistant to levofloxacin and moxifloxacin, all but one of these isolates belonging to a single clone of the 23F serotype. Although, at present, apparently limited to pockets of clonal spread, continued vigilance with regard to the evolution of fluoroquinolone resistance is indicated. Telithromycin (MIC 90 0.12 mg/L; 99.9% of isolates susceptible) and lin- ezolid (MIC 90 2 mg/L; 100% of isolates susceptible) were the two most active oral agents tested, both compounds retaining activity against isolates of fluoroquinolone-resistant S. pneumoniae . The results of the PROTEKT surveillance study 1999–2000 emphasize the widespread evolution of resistance to a variety of antimicrobials amongst isolates of S. pneumoniae and demonstrate the potential of telithromycin as a therapeutic option for the treatment of community-acquired respiratory tract infections caused by this organism.

The aim of this study was to investigate whether a Staphylococcus aureus strain that carried an inducibly expressed erm(A) gene might exhibit resistance to the non-inducers telithromycin, ABT-773, clindamycin, quinupristin, dalfopristin or the combination quinupristin-dalfopristin after incubation in the presence of inhibitory concentrations of any of these compounds. Whenever resistant mutants were obtained, these were investigated for the molecular basis of the altered resistance phenotype. Resistant mutants were not selected with dalfopristin or quinupristin-dalfopristin, but were obtained with the other four agents. Irrespective of which drug was used for selection, all mutants were cross-resistant to clindamycin, quinupristin, telithromycin and ABT-773, and exhibited structural alterations in the erm(A) translational attenuator. The structural alterations observed included deletions of 14, 83, 121, 131, 147 or 157 bp, three different tandem duplications of 23, 25 or 26 bp, two different types of point mutation, as well as the insertion of IS256. All these alterations either completely prevented the formation of mRNA secondary structures in the erm(A) regulatory region or favoured the formation of those mRNA secondary structures that allowed translation of the erm(A) transcripts. Deletions, which were observed in almost two-thirds of the mutants, might be explained by illegitimate recombination between different parts of the erm(A) regulatory region.

The anti-staphylococcal activity of ABT-773, a new member of the ketolides, was examined and compared with those of erythromycin and clarithromycin using the agar dilution method. A total of 336 well-defined clinical isolates of staphylococci, including a large number of clonally different methicillin-resistant strains with defined macrolide-lincosamide-streptogramin B (MLS(B)) resistance phenotypes were tested. The novel ketolide exhibited broad-spectrum anti-staphylococcal activity against both methicillin-susceptible and methicillin-resistant strains. Apart from strains with constitutive expression of MLS(B) resistance, all staphylococcal strains tested were inhibited by < or = 0.063 mg/L of ABT-773, irrespective of the resistance phenotype for methicillin.

Antimicrobial resistance amongst common respiratory pathogens has increased worldwide at an alarming rate and now threatens the clinical usefulness of a number of antibacterial agents. A major concern is the selection of resistance in the community, which tends to parallel the (often inappropriate) overuse of such agents. Such problems highlight the need for new antibacterial agents that retain activity against bacterial strains resistant to existing agents, and have a low potential to select for resistance or induce cross-resistance. Telithromycin is the first of a new family of antibacterials—the ketolides—and has been designed specifically for the treatment of community-acquired respiratory tract infections (RTIs). Numerous in vitro studies confirm the potent activity of telithromycin against pathogens commonly implicated in community-acquired RTIs, irrespective of their β-lactam, macrolide or fluoroquinolone susceptibility. Against pneumococci, for example, MICs were ≤ 1 mg/L irrespective of penicillin susceptibility, with ≥ 98% of macrolide-resistant strains inhibited at ≤ 0.5 mg/L, regardless of the underlying mechanism of resistance (including erm, mef and ribosomal L4 mutations). Against Haemophilus influenzae and Moraxella catarrhalis, including β-lactamase-positive strains, telithromycin is at least as potent as azithromycin. In addition, telithromycin has a very low potential for selection of resistant isolates or induction of cross-resistance. Importantly, and unlike existing macrolides, telithromycin does not induce MLSB resistance, a finding explained by the presence of the innovative 3-keto group in its chemical structure. Telithromycin therefore represents an important addition to the therapeutic armamentarium in an era of increasing antimicrobial resistance, with an expected low likelihood of the development of resistance in clinical use.

Community-acquired respiratory tract infections (RTIs) are among the most prevalent infectious diseases in the developed world. They cause considerable morbidity, resulting in a major impact on public health both clinically and socioeconomically. The bacterial pathogens most commonly associated with community-acquired RTIs are Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis, with Streptococcus pyogenes predominating in pharyngitis. Over the past years, each of these pathogens has developed mechanisms to evade susceptibility to antibacterials, leading to an alarming global increase in antibacterial resistance among these pathogens. There is great concern that currently available antibacterials are insufficient to treat community-acquired RTIs and there is an urgent requirement for new agents with activity against all strains of common community-acquired RTI pathogens. Telithromycin (HMR 3647) belongs to a new family of antibacterials, the ketolides, and has been specifically designed for the treatment of community-acquired RTIs. This review covers the potent in vitro activity of telithromycin against the most common community-acquired RTI pathogens compared with other currently available antimicrobial agents.

Ketolides are the most recent generation of antimicrobials derived from the 14-membered ring macrolide, erythromycin A. The main structural feature that differentiates ketolides from erythromycin is the keto group, which replaces the l-cladinose moiety at position 3 of the macrolactone ring. The keto group bestows greater acid stability on the drugs, and enables them to bind to their ribosomal target without causing expression of MLSB resistance in inducible strains. Several ketolides are described here, including ABT 773 and telithromycin (HMR 3647), both of which possess a carbamate at C11/C12 of the macrolactone ring. In telithromycin, which is the first ketolide to be approved for clinical use, the carbamate is linked to an alkyl-aryl extension, which is responsible for the increased potency of this compound relative to macrolides. This review examines how the structural differences between macrolides and the new ketolides are related to their antimicrobial activities in inhibiting protein synthesis and blocking the assembly of new ribosomal subunits.

An experimental Streptococcus pneumoniae pneumonia model in rabbits was used to assess the efficacy of amoxycillin, erythromycin and a new ketolide, telithromycin (HMR 3647). The MICs of amoxycillin, erythromycin and HMR 3647 for the three clinical S. pneumoniae strains used were, respectively, (mg/L): 0.01, 16 and 0.02 (strain 195); 2, 0.25 and 0.02 (strain 16089); 8,64 and 0.02 (strain 11724). Antibiotic therapy reproduced human serum pharmacokinetics (amoxycillin 1 g iv tds or erythromycin 500 mg qds or HMR 3647 800 mg bd). Forty-eight hours of therapy with HMR 3647 and amoxycillin resulted in significant bacterial clearance in the lungs and spleen of rabbits infected by S. pneumoniae strain 195 and strain 16089 (at least 3 log(10) cfu/g decrease, P0.001). Erythromycin was active against only the erythromycinsusceptible strain (3 log(10) cfu/g decrease at 48 h, P0.001). None of the antibiotics showed significant efficacy with strain 11724. All agents produced significant bacterial clearance when time above MBC was33%, and microbiological failure when it was25%, whereas MIC was not correlated with microbiological outcome with HMR 3647. Our findings suggest that pharmacodynamic data integrating MBC may be predictive of microbiological success or failure with greater accuracy than with MIC. HMR 3647 produced significant bacterial clearance in both penicillin- and erythromycin-resistant pneumonia, but was less effective against the highly erythromycin-resistant S. pneumoniae strain.

The in vitro activity of ABT773, a ketolide antimicrobial agent, was investigated and compared with those of seven other antibiotics. Type strains and 733 Gram-positive, Gram-negative and anaerobic isolates of clinical origin and four CHLAMYDIA: isolates were used. The activity of ABT773 was very similar to that of telithromycin, the other ketolide tested. The MIC(90) was < or = 0.5 mg/L for all bacteria examined except methicillin-resistant Staphylococcus aureus, Enterococcus faecalis, Enterococcus faecium, Haemophilus influenzae and BACTEROIDES: spp. The antichlamydial activity of ABT773 was greater than that of telithromycin, erythromycin and ciprofloxacin. Neither an increase in the size of the inoculm nor the addition of human serum had any marked affect on the in vitro activity of ABT773.

Phenotypes of resistance to the macrolide-lincosamide-ketolide-streptogramin (MLKS) group of antibiotics have been determined in 540 clinical isolates of staphylococci (210 Staphylococcus aureus and 330 coagulase-negative species). Results of disc diffusion tests using erythromycin A, oleandomycin, rokitamycin, clindamycin, telithromycin, quinupristin and dalfopristin delineated four main groups corresponding to those defined classically using erythromycin and clindamycin only, but with sub-divisions. Resistance to erythromycin was more common in coagulase-negative strains (56%) than in S. aureus (16%); telithromycin, clindamycin, quinupristin-dalfopristin and rokitamycin were active against >97% of S. aureus strains and >88% of the coagulase-negative strains. The commonest resistance phenotype was 'inducible MLS(B)' (12% in S. aureus, 31% in coagulase-negative strains); this group could be divided in terms of the different inducing abilities of erythromycin and oleandomycin. 'Constitutive MLS(B)' and 'MS' phenotypes were more often found in coagulase-negative strains (11 and 13%, respectively) than in S. aureus (2 and 1%). Novel phenotypes were found during the isolation of constitutively resistant mutants from inducible strains, and of resistant mutants from 'MS' strains. This extended phenotyping scheme has revealed further complexities and evolutionary possibilities in patterns of resistance to this group of antibiotics.

The pharmacokinetics in plasma and saliva of a new ketolide, telithromycin (HMR 3647), and the effect on the normal oropharyngeal and intestinal microflora were studied in healthy volunteers and compared with those of clarithromycin. Ten subjects received 800 mg telithromycin perorally once daily and 10 other subjects received 500 mg clarithromycin bid for 10 days. Blood, saliva and faecal specimens were collected at defined intervals before, during and after administration for pharmacokinetic and microbiological analyses. In subjects receiving telithromycin, the mean C:(max), AUC and C:(24) (24 h) in saliva exceeded the values obtained from plasma, while saliva and serum pharmacokinetic parameters were in the same range for the clarithromycin group. The quantitative ecological disturbances in the normal microflora during administration of telithromycin were moderate and comparable to those associated with clarithromycin administration. No overgrowth of yeasts or Clostridium difficile occurred. Emergence of resistant strains was seen in both treatment groups. Administration of both telithromycin and clarithromycin was associated with significant increases in MICs for intestinal Bacteroides isolates, which persisted 2 weeks after discontinuation of treatment. In addition, a significant emergence of highly clarithromycin-resistant alpha-haemolytic streptococci, intestinal enterococci and Enterobacteriaceae was detected at day 10 in the clarithromycin group. In conclusion, administration of telithromycin resulted in high drug levels in saliva, which indicates a good therapeutic profile for throat infections. Telithromycin seems to have a more favourable ecological profile compared with clarithromycin in terms of resistance development in the normal microflora.

cin, as the MIC90 of cethromycin was 0.12 mg/L (4-fold lower) compared with an MIC90 of telithromycin of 0.5 mg/L (Table 1). Among penicillin-non-susceptible (MIC 4 mg/L) isolates, the MIC90 of ceftaroline was 0.25 mg/L and the MIC90 of ceftobiprole was 1.0 mg/L, whereas that of ceftriaxone was 2 mg/L (Table 1). Similarly, the MIC90 of cethromycin was 0.12 mg/L, which was 4-fold lower than that of telithromycin. Among erythromycin-non-susceptible (MIC 0.5 mg/L) isolates, the MIC90 of cethromycin was 0.12 mg/L compared with an MIC90 of telithromycin of 0.5 mg/L (Table 1). Similarly, the results showed that ceftaroline, ceftobiprole and cethromycin were more active against emerging MDR serotype 19A than currently available drugs. In summary, among b-lactams tested, ceftaroline was the most potent against MDR S. pneumoniae isolates. Similarly, the ketolide cethromycin was more active than telithromycin.

The in-vitro activity of HMR 3647, a novel ketolide, was investigated in comparison with those of erythromycin A, roxithromycin, clarithromycin (14-membered ring macrolides), amoxycillin-clavulanate and ciprofloxacin against 719 recent clinical Gram-positive, Gram-negative and anaerobic isolates and type cultures. HMR 3647 generally demonstrated greater activity than the other compounds with MIC 90 s of ≤0.5 mg/L, except for Staphylococcus epidermidis (MIC 90 > 128 mg/L), Haemophilus influenzae (MIC 90 = 2 mg/L), Enterococcus faecalis (MIC 90 = 2 mg/L), Enterococcus faecium (MIC 90 = 1 mg/L) and the anaerobes, Bacteroides fragilis (MIC 90 = 2 mg/L) and Clostridium difficile (MIC 90 = 1 mg/L). In general, an increase in the size of the inoculum from 10 4 to 10 6 cfu on selected strains had little effect on the MlCs of HMR 3647. Additionally, the in-vitro activity of HMR 3647 was not affected by the presence of either 20 or 70% (v/v) human serum. The antichlamydial activity of HMR 3647 was generally greater than that of commonly used antichlamydial antimicrobials.

The pharmacodynamic properties of a novel ketolide (a new class of macrolide), HMR 3647, were investigated by studying time-kill kinetics and postantibiotic effect (PAE). The time-kill kinetics were studied at two inocula against three strains each of Staphylococcus aureus, Haemophilus influenzae, Moraxella catarrhalis, Streptococcus pneumoniae, Streptococcus pyogenes, Enterococcus faecalis, Enterococcus faecium and Bacteroides fragilis. The PAEs of HMR 3647 were also investigated on these organisms at concentrations equivalent to 1, 4 and 10 x MIC. The time-kill kinetic data demonstrated that HMR 3647 is predominantly bacteriostatic and only slowly bactericidal at higher concentrations. HMR 3647 exhibited a significant PAE with all strains studied, ranging from 1.2 h to 8.2 h at 10 x MIC. The bacteriostatic activity and significant PAE demonstrated by HMR 3647 are similar to those previously obtained with other macrolides.

Objectives To investigate the in vitro activity of cethromycin, a new ketolide, against Chlamydia pneumoniae. Methods The in vitro activity of cethromycin against 20 isolates of C. pneumoniae was compared with the activities of telithromycin, erythromycin A, azithromycin and clarithromycin against those isolates. Results The MIC at which 90% of the isolates were inhibited and the minimal chlamydiacidal concentration at which 90% of the isolates were killed by cethromycin were both 0.016 mg/L (range 0.016-0.031 mg/L). Cethromycin was the most active antibiotic tested in this study. Conclusions Our results appear to indicate that cethromycin is an effective antibiotic that should play some role in the treatment of respiratory tract infections caused by C. pneumoniae.

The in vitro activity of telithromycin, a new ketolide, was compared with those of roxithromycin, azithromycin, clarithromycin and erythromycin A against 20 strains of Chlamydia pneumoniae. The MICs and minimal chlamydiacidal concentrations of telithromycin for the 20 C. pneumoniae strains both ranged between 0.031 and 0.25 mg/L. Telithromycin was twice as active as roxithromycin, azithromycin and erythromycin A, but less active than clarithromycin. These results appear to indicate that telithromycin is an effective antibiotic that should play some role in the treatment of respiratory tract infections caused by C. pneumoniae.

Sir, Macrolides and azalides frequently fail in the treatment of Lyme borreliosis and the recent Infectious Diseases Society of America (IDSA) guidelines therefore discourage their routine application in the chemotherapy of Lyme disease. Nevertheless, pregnancy and -lactam allergy remain important indications for the administration of macrolides in acute cases, when -lactams and tetracyclines cannot be administered owing to the possibility of side effects. For these indications, new therapeutic alternatives with increased effectiveness against Borrelia burgdorferi are needed. Recently, the 3-keto, 6-methoxy, 11,12-carbamate clarithromycin derivatives, known as the ketolides, were shown to have potent antibacterial activity against a number of Gram-positive and Gram-negative microorganisms, including Chlamydia pneumoniae, Legionella spp. and, in part, macrolide-resistant bacteria. Accordingly, this new class of antimicrobials also merits consideration for the stagedependent therapy of Lyme disease. ABT-773 is a new ketolide agent; it is acid-stable and, as such, can be administered orally. The present pilot study was designed to determine the in vitro activity of ABT-773 against 17 isolates of the B. burgdorferi sensu lato (s.l.) complex. The origins of the borrelial isolates used in this study are summarized in the Table. For MIC and MBC determination in all cases except B. burgdorferi strain B31, low passage isolates (10–20 passages) were employed. Genospecies were identified by restriction fragment length polymorphism (RFLP) analysis after MluI digestion of DNA and by application of plasmid analysis. For control purposes, Staphylococcus aureus ATCC 29213 and Enterococcus faecalis ATCC 29212 were tested. The 96-well microtitre trays containing concentrations of ABT-773 (Abbott, Chicago, IL, USA) ranging from 0.002 to 2 mg/L were prepared in cooperation with MerlinDiagnostika GmbH (Bornheim-Hersel, Germany). In order that these data can be related to our recent publications on the in vitro susceptibility of Borreliae, MICs and MBCs were determined under the same standardized conditions as described previously: for MIC determination, a colorimetric assay and conventional subculture experiments in combination with dark-field microscopy were performed. To determine the drug concentrations that provided complete killing of the inoculum, after 72 h aliquots were taken in triplicate from all vials lacking detectable growth, diluted (1:1000) below the MIC, and subcultured for an additional 3 weeks in liquid medium. Moreover, the time–kill curve technique was representatively applied to isolate PSth at 2.5 10 cells/mL in duplicate using ABT-773 at a test range of 0.002 mg/L (MIC) to 0.125 mg/L (60 MIC) for 120 h. Significant antibiotic–medium interactions were excluded because for the ATCC reference strains the MIC ranges of ABT-773 read in triplicate after 24, 72 and 120 h of pre-incubation of the antibiotic–medium test preparation were within the tentative MIC ranges supplied by the manufacturer. The individual MIC and MBC values of ABT-773 for the 17 borrelial strains are summarized in the Table. After 72 h, the colorimetric MIC determination demonstrated 100% agreement with the results of conventional cell counts. The novel ketolide ABT-773 proved to be uniformly effective against Borreliae displaying an MIC90 of 0.002 mg/L. By using a 100% rather than a 99.9% killing criterion after 72 h, which constitutes a very stringent standard for any antibiotic against B. burgdorferi, the MBC50 and the MBC90 for ABT-773 were found to be 0.25 and 0.75 mg/L, respectively (MBC range 0.002–1 mg/L), thus indicating a slow killing of Borreliae. In a second approach in which motile organisms were considered viable, our time–kill experiments with isolate PSth revealed that after 96–120 h, drug concentrations of ABT-773 required for a 3 log10 unit (99.9%) reduction of the final inoculum decreased to 0.008 mg/L, which is 5 log2 unit dilutions below the MBC obtained after 72 h by subculture experiments. Proposed tentative interpretative criteria for susceptibility testing of ABT-773 against fastidious organisms have determined an MIC of 0.5 mg/L for susceptible and

Sir, Historically, methicillin-resistant Staphylococcus aureus (MRSA) has been regarded as a nosocomial pathogen responsible for severe toxin-mediated disease or invasive pyogenic infections. In recent years, however, community-associated MRSA (CA-MRSA) has been reported from around the world. Although the first CA-MRSA strains were susceptible to most antimicrobials, antimicrobial resistance has increased. One CA-MRSA clone, identified by the CDC as pulsotype USA300, has been implicated in outbreaks within the USA and is resistant to many currently marketed antimicrobial agents. This clone carries both the SCCmec IV element and the Panton–Valentine leukocidin (PVL) locus. The aim of this study was to compare the activity of a novel ketolide, cethromycin (previously ABT-773), with potentially improved pharmacodynamic and therapeutic benefits, three other macrolides (azithromycin, clarithromycin and erythromycin) and one lincosamide (clindamycin) against the 170 MRSA USA300 isolates that are in the Center for Biological Defense (CBD) collection. Seventeen MRSA isolates were procured from two hospital laboratories in Seattle, WA, USA. These were isolated from cultures of blood, skin lesions and tissue abscesses of patients seen in their clinics or emergency department in 2003 and 2004. Of the remaining 153 MRSA, 121 isolates were obtained from cultures of lesions, abscesses and nasopharyngeal specimens from outpatients seen at west central Florida clinical laboratories and doctor’s offices, whereas 32 MRSA were isolated from lesions, abscesses, nasopharyngeal and blood specimens acquired from patients seen at hospitals in the Tampa Bay area from 2004 to 2006. These isolates were previously characterized as USA300 by PFGE using Network on Antimicrobial Resistance in Staphylococcus aureus (NARSA) standards for comparison. Cethromycin (obtained from Advanced Life Sciences, Woodridge, IL, USA) was prepared and diluted with calciumadjusted Mueller–Hinton broth (Remel, Lenexa, KS, USA) using 96-well microtitre plates following standard protocols. The MIC and MBC were determined using orthodox procedures. Clarithromycin, erythromycin and clindamycin were tested with Sensititre by TREK Diagnostic Systems (Cleveland, OH, USA), whereas azithromycin, erythromycin and clindamycin were tested using Etest (AB Biodisk, North America Inc., Piscataway, NJ, USA), according to manufacturer’s instructions. All MIC interpretations followed CLSI guidelines. Clindamycin resistance was examined further with the D-test. Controls were S. aureus ATCC 29213, S. aureus ATCC BAA977 and S. aureus ATCC BAA976. Cethromycin was effective against all 170 isolates (Table 1). The cethromycin MIC range was 0.002–0.125 mg/L, whereas the MIC50 and MIC90 were both 0.002 mg/L. The cethromycin MBC range was 0.002–1.0 mg/L, with the MBC50 of 0.002 mg/L and the MBC90 of 0.008 mg/L. Of the 170 CA-MRSA, 169 (99.4%) were intermediate or resistant to at least one macrolide. In contrast, 164 (96.5%) isolates were susceptible to clindamycin. Only six (3.5%) isolates demonstrated either constitutive resistance [4 (2.4%)] or inducible resistance [2 (1.2%)] to clindamycin. This is unlike the USA300 isolates in Boston where 57% of the isolates were clindamycin-resistant. Against 10 erythromycin-susceptible isolates, the cethromycin MIC and MBC ranges were 0.002–0.008 mg/L, compared with the MIC range of 0.002–0.25 mg/L and MBC range of 0.002–1.0 mg/L obtained with the erythromycin-resistant isolates. However, MIC50 and MBC50 values, as well as MIC90 and MBC90 values, did not change regardless of erythromycin susceptibility. There were six isolates resistant to both erythromycin and clindamycin, against which cethromycin MIC and MBC ranges were 0.002–0.06 mg/L. This population’s susceptibility to cethromycin is especially important in the light of the fact that most isolates were resistant to one or more macrolides. This collection of MRSA USA300 isolates was overwhelmingly macrolide-resistant and clindamycinsusceptible. The study of Chavez-Bueno et al. demonstrated a similar pattern in CA-MRSA isolates among children in Texas where the inducible resistance to clindamycin was slowly replaced by clindamycin susceptibility (non-inducible resistance). Most of the CA-MRSA isolates in the Texas study carried the erm(C) gene, with others carried erm(A) or msr(A) with or without erm(C). This phenotypic change suggests an alteration in the proportion of the macrolide resistance determinants in the population. Tenover et al. reported the msr(A) gene in isolates comprising a specific clone (USA300-0114). Accordingly, we hypothesized that most of the CBD collection of MRSA USA300 isolates (only one being USA300-0114) also carried a different determinant, putatively the msr(A)/msr(B) gene encoding an efflux protein that normally renders a bacterial cell resistant only to macrolides and streptogramin B compounds (MS phenotype) instead of the MLSB resistance genes [i.e. erm(A) and erm(C)]. Using the oligonucleotides described by Sutcliffe et al. for PCR primers and a well-characterized MRSA strain NARSA 384 (USA300-0114) as positive control, we performed a PCR assay for the msr(A) gene on DNA extracted from all 170 isolates. After the amplicons were electrophoresed on a 1% agarose gel, a positive PCR reaction yielded a DNA band of 399 bp. All negative and questionable results were performed three times. The assay produced a positive reaction in 160 (94.1%) isolates, whereas only 10 (5.9%) of the 170 MRSA yielded negative PCR reactions. These Research letters

Sir, The antibiotic resistance determinants Msr(A), MsrC and Msr(D) are all members of a group of ‘incomplete’ or Class 2 ABC transporters, characterized by the presence of two fused nucleotidebinding domains but lacking any identifiable transmembrane domains, and are typically involved in cellular processes other than transport. The msr(A) gene is carried on large staphylococcal plasmids, and has been shown to confer high-level inducible resistance to 14and 15-membered ring macrolides and type B streptogramins (MSB antibiotics) in Staphylococcus aureus in the absence of any additional plasmid-encoded determinants. A single copy of msrC is found on the chromosome of Enterococcus faecium. Insertional inactivation of msrC in this species resulted in increased susceptibility to MSB antibiotics. 3 We recently reported the cloning of msrC into S. aureus, where it was found to confer a greatly enhanced level of protection against MSB antibiotics compared with that in E. faecium. Both determinants confer high-level MSB-resistance in S. aureus, whether cloned on a high copy number vector or integrated in single copy into the chromosome. The homologousmsr(D) determinant, found on large chromosomal genetic elements in streptococci, was recently shown to confer low-level resistance to 14-membered ring macrolides and ketolides in Streptococcus pneumoniae. Msr(D) shares a high level of amino acid sequence similarity with Msr(A) and MsrC (75% and 78%, respectively). We report here the cloning of msr(D) into S. aureus RN4220 on a multicopy shuttle vector. The msr(D) gene and its promoter were amplified from Streptococcus pyogenes ATCC BAA-946, cloned into the PCR cloning vector pDrive (Qiagen Ltd) and subcloned into the KpnI site of the staphylococcal vector pSK265. The cloning vectors pDrive and pSK265 carry ampicillinand chloramphenicol-resistance determinants, respectively. All molecular cloning was carried out in Escherichia coli JM109. Transformants carrying this shuttle vector construct, designated pMsrD, were selected on LB agar containing 50 mg/L ampicillin and 5 mg/L chloramphenicol. S. aureus RN4220 was transformed with pMsrD by electroporation, with selection of transformants on agar containing 5 mg/L chloramphenicol. S. aureus RN4220 transformed with pMsrD was found to have enhanced resistance to erythromycin, a 14-membered ring macrolide (MIC 4–8mg/L), and telithromycin, a ketolide antibiotic (MIC 0.125–0.25 mg/L), representing 16-fold and 4-fold increases in MICs, respectively (Table 1). The MIC of telithromycin was increased further to 1 mg/L upon induction with 1 mg/L erythromycin (Table 1). However, pMsrD transformants remained susceptible to 16-membered ring macrolides, lincosamides and to streptogramins A and B, even following induction with erythromycin. Daly et al. recently reported that msr(D) similarly confers resistance to macrolides and inducible resistance to ketolides in S. pneumoniae. Determination of telithromycin MICs gave rise to the unexpected finding thatmsr(A) andmsrC also confer inducible resistance to this ketolide antibiotic in S. aureus (Table 1). MICs of telithromycin were increased by up to 128-fold following induction with 5 mg/L of erythromycin for plasmid-borne msr(A) and msrC in S. aureus RN4220. To the best of our knowledge, this is the first report of ketolide resistance conferred by msr(A) or msrC in any species. The msr(A) gene is preceded by a control region containing a number of inverted repeat sequences and an open reading frame encoding a short ‘leader peptide’ (LP). Deletion of this control region resulted in constitutive resistance to MSB antibiotics in S. aureus. We found that MICs of telithromycin for S. aureus RN4220 carrying msr(A) which was deleted for its control region (2 mg/L) were not affected by the addition of erythromycin, indicating that the msr(A) control region may also be required for inducible resistance to telithromycin. A similar control region has been identified upstream of the msrC gene in E. faecium, and is likely to be responsible for the inducible nature of resistance conferred by this determinant. We located an open reading frame preceded by a Shine–Dalgarno sequence 110 nt upstream of the msr(D) coding sequence that could potentially encode a short LP (Met-Tyr-Leu-Ile-Phe-Met). A series of inverted repeat sequences were identified in this putative control region, but none of these was found to encompass the ribosome binding site preceding msr(D). Moreover, consensus –10 and –35 promoter sequences were identified within the open reading frame of the proposed LP. No promoter sequences were found upstream of the LP. The nucleotide sequence upstream of msr(D) did not closely resemble the control regions upstreamofmsr(A) andmsrC, which are thought to be involved in the regulation of gene expression by translational attenuation. It is therefore unlikely that msr(D) expression is regulated by the samemechanism. Althoughmsr(D) was expressed from its own promoter in S. aureus in this study, it may be noted that other researchers have demonstrated the co-transcription of mef(A) and msr(D). In this case, the LP sequence upstream of msr(D) would also be co-transcribed, and its expression could potentially regulate translation of the msr(D) transcript. It remains unclear how such ‘incomplete’ ABC transporters provide the bacterial cell with protection against antibiotics. It was originally proposed that active efflux of the drugs could be orchestrated by ‘hijacking’ and altering the specificity of