Journal of Antimicrobial Chemotherapy (2001) 48, 47-52
© 2001 The British Society for Antimicrobial Chemotherapy
Susceptibility of a variety of clinical isolates to linezolid: a European inter-country comparison
a Department of Bacteriology, Glasgow Royal Infirmary, 84 Castle Street, Glasgow G4 0SF, UK
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Abstract |
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Using standardized in vitro susceptibility tests, 3382 bacteria recently isolated from skin, blood or respiratory tract infections were analysed for their susceptibility to linezolid, a new oxazolidinone, and a number of comparator antibacterial agents. Isolates originated in France, Italy, Germany, Spain, Sweden, The Netherlands and the UK. Laboratories in each country independently conducted broth microdilution susceptibility tests using NCCLS methods and epsilonometry (Etest). Isolates of Gram-positive cocci tested in each laboratory included methicillin-susceptible and -resistant Staphylococcus aureus, methicillin-susceptible and -resistant Staphylococcus epidermidis, Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus pneumoniae and Enterococcus spp. Isolates of Moraxella catarrhalis and Haemophilus influenzae were also included. Where appropriate, comparator drugs (oxacillin, vancomycin, gentamicin, co-amoxiclav, ciprofloxacin, erythromycin, penicillin G, clindamycin and ampicillin) were also tested. Linezolid demonstrated excellent activity against all of the Gram-positive cocci with MIC50s ranging from 0.5 to 4 mg/L. The drug demonstrated only modest activity against M. catarrhalis and H. influenzae with MIC50s ranging from 4 to 16 mg/L.
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Introduction |
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The oxazolidinones1 comprise a novel synthetic class of antimicrobial agents with potent activity against Gram-positive aerobes, Gram-negative anaerobes and mycobacteria.2 One member of this class, linezolid, acts by inhibiting the initiation phase of protein translation through direct interaction with the bacterial ribosome and the 70S ribosomal RNA initiation complex.3 Owing to this unique mode of action, organisms resistant to other antibacterial agents remain susceptible to linezolid. This suggests that the agent will be of value in treating infections due to Gram-positive organisms such as staphylococci (including methicillin-resistant strains), enterococci (including vancomycin-resistant strains) and pneumococci (including penicillin-resistant strains).
Since its discovery a number of studies have been directed towards measurement of the in vitro2–8 and in vivo9 activity of linezolid. The agent appears to be bacteriostatic toward staphylococci and bactericidal towards streptococci.2 However, linezolid does not show much activity against Gram-negative bacteria or anaerobes, but does show some potential against mycobacteria.2
The primary objective of this study was to generate in vitro susceptibility data for linezolid and comparator agents on a large number of recent bacterial clinical isolates from numerous sites within Europe. The use of common test methods allowed the pooling of data from the multiple sites, in addition to site-to-site comparisons. The secondary objective of this study was to compare linezolid susceptibility of some species obtained using NCCLS broth microdilution methodology and the epsilonometric assay (Etest).
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Materials and methods |
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Organisms
Each laboratory collected recent isolates of the following species: Staphylococcus aureus, methicillin susceptible (MSSA) (20); S. aureus, methicillin resistant (MRSA) (30); Staphylococcus epidermidis, methicillin susceptible (MSSE) (20); S. epidermidis, methicillin resistant (MRSE) (30); Enterococcus species, vancomycin susceptible (20); Enterococcus species, vancomycin resistant (20); Streptococcus pyogenes (20); Streptococcus agalactiae (20); Streptococcus pneumoniae, penicillin susceptible (20); S. pneumoniae, penicillin intermediate (20); S. pneumoniae, penicillin resistant (20); Haemophilus influenzae (20); Moraxella catarrhalis (20). The enterococci were not speciated in this study.
The following quality control strains were available in each laboratory in order to ensure equivalent inter-laboratory standardization: S. aureus ATCC 29213, E. faecalis ATCC 29212 and S. pneumoniae ATCC 49619.
NCCLS broth microdilution method
MICs were determined using the NCCLS standard broth microdilution method10 using Mueller–Hinton broth (Oxoid, Basingstoke, UK). For testing of Streptococcus species and H. influenzae, the standard broth medium was supplemented with lysed horse blood, as described. Microplates were incubated without carbon dioxide. The inoculum suspension was standardized (0.5 McFarland reading to 1 x 108 cfu/mL) and diluted 1:10, and 5 µL were inoculated into each well giving a final concentration of 5 x 104 cfu/well.
Etest method
The Etest was conducted according to the manufacturer's instructions (AB Biodisk, Solna, Sweden). Testing of all organisms except streptococci and H. influenzae was conducted with Mueller–Hinton agar (Oxoid). Tests with streptococci were conducted using Mueller–Hinton agar supplemented with 5% defibrinated sheep blood. Tests with H. influenzae were conducted using Mueller–Hinton agar supplemented with 1% haemoglobin and 1% Isovitalex.
Antimicrobial agents
The concentration range of antimicrobial agents included in the NCCLS microdilution MIC tests were: for staphylococci and enterococci: linezolid (64–0.06 mg/L), vancomycin, teicoplanin, gentamicin, oxacillin, ciprofloxacin, ampicillin, erythromycin (128–0.12 mg/L); for streptococci, H. influenzae and M. catarrhalis: linezolid (64–0.06 mg/L), vancomycin, penicillin, co-amoxiclav 16–0.06 mg/L, ciprofloxacin, clindamycin and clarithromycin (16–0.06 mg/L). Linezolid MIC ranges of 1–4 mg/L were acceptable for S. aureus ATCC 29213 and E. faecalis ATCC 29212 and 0.5–2 mg/L for S. pneumoniae ATCC 29619.
The antimicrobial agents included in the Etest were: for staphylococci and enterococci, linezolid and vancomycin; for streptococci, linezolid and penicillin G; for H. influenzae and M. catarrhalis, linezolid and co-amoxiclav.
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Results |
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A total of 3382 isolates were examined in this multicentre study (Table I
). In some instances participating laboratories were unable to contribute data on specific organisms since local isolates were not available (e.g. MRSA in The Netherlands, penicillin-resistant S. pneumoniae in Germany and the UK and vancomycin-resistant enterococci in several countries).
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Linezolid showed good activity against S. aureus (both MSSA and MRSA), S. epidermidis (both MSSE and MRSE), enterococci (both vancomycin susceptible and vancomycin resistant), S. pneumoniae (penicillin susceptible, intermediate or resistant), S. agalactiae and S. pyogenes. In these studies it was comparable (MIC50 1–2 and MIC90 2–4 mg/L) to clindamycin and vancomycin (S. aureus, S. epidermidis), to ciprofloxacin, clarithromycin and erythromycin (S. pyogenes, S. agalactiae, S. pneumoniae) to penicillin and co-amoxiclav (S. pneumoniae penicillin susceptible and intermediate; S. agalactiae, S. pyogenes) and better than penicillin, oxacillin, gentamicin (S. aureus, S. epidermidis and S. pneumoniae, penicillin resistant).
For the enterococci, linezolid MIC50s and MIC90s of 1–2 mg/L and 2–4 mg/L, respectively, were obtained for both vancomycin-susceptible and -resistant isolates. Its activity was comparable to vancomycin for vancomycin-susceptible isolates and better activity was shown against vancomycin-resistant isolates as well as teicoplanin-resistant isolates.
For isolates of M. catarrhalis and H. influenzae, linezolid (MIC50 4 and 16 mg/L and MIC90 16 and 32 mg/L, respectively) compared unfavourably with its comparators. Most agents (ciprofloxacin, erythromycin, clindamycin, clarithromycin, penicillin and co-amoxiclav) had lower MIC50s and MIC90s.
There was little inter-laboratory variation with regard to the results obtained for most of the test bacteria except for strains of MSSA in one site in The Netherlands. At this site the linezolid MIC90 was 8 mg/L as compared with the other Dutch centre, which reported an MIC90 of 4 mg/L. Repeat tests in the coordinator's laboratory showed that the Dutch isolates were more susceptible to linezolid (MIC90 2 mg/L) than originally reported by either Dutch laboratory. However, since this study was designed to evaluate the results obtained in different laboratories using the NCCLS method, the original results remained in the database for analysis. The same centre also gave slightly higher linezolid MICs (MIC50 4 mg/L) for vancomycin-susceptible enterococci. It is likely that differences in interpretation of the end point were responsible for the higher MICs at this centre.
Overall the NCCLS broth microdilution assay10 gave MIC90s of linezolid of 2–4 mg/L for all groups of staphylococci (total of 1328 isolates), which was comparable to those of vancomycin and teicoplanin. Linezolid was equally active against methicillin-susceptible and -resistant strains. A significant level of resistance to gentamicin, ciprofloxacin, erythromycin and ampicillin was demonstrated (Table I
). MIC90s of linezolid were 2–4 mg/L for the enterococci (total of 439 isolates), including both vancomycinsusceptible and -resistant isolates. The activity of linezolid was comparable to that of vancomycin for vancomycinsusceptible isolates (MIC90 2 mg/L) and it retained activity against the vancomycin-resistant strains. A significant level of enterococcal resistance was seen for all of the other agents tested. MIC90s of linezolid were 1.0 mg/L for all S. pneumoniae isolates tested (total of 551 isolates), including penicillin-susceptible, -intermediate and -resistant strains. The activity of linezolid was similar to that of vancomycin for all susceptibility groups (Table I). With the exception of ciprofloxacin, the MIC90s for the comparator agents increased significantly for the penicillin-intermediate and -resistant strains. The MIC90 of linezolid for 281 isolates of S. agalactiae (2 mg/L) was comparable to that of vancomycin, and significantly less than that of some of the other comparator agents. The linezolid MIC90 for 268 isolates of S. pyogenes was 1 mg/L, which was slightly less active than vancomycin. Overall, linezolid demonstrated MIC90s of 
2 mg/L for all of the streptococci tested. Against M. catarrhalis (258 isolates) and H. influenzae (257 isolates), linezolid was less active with MIC90s of 16 and 32 mg/L, respectively. Several of the comparator agents demonstrated excellent activity against these Gram-negative organisms.
Comparison of mean MIC determined by NCCLS microdilution method with mean MIC determined by investigational Etest
The mean MICs of linezolid and certain comparator agents for each organism group at each site determined by these different methods was calculated and is presented in Tables II and III. The vancomycin mean microdilution MIC agreed closely with the mean Etest MIC for staphylococci and enterococci, except for vancomycin-resistant enterococci; in this instance the values were so discrepant (59.69 mg/L versus 192.84 mg/L) that interpretation was difficult. Excellent agreement was also achieved for penicillin with the streptococci, and co-amoxiclav for H. influenzae and M. catarrhalis (Table III). Mean linezolid Etest values were lower than the mean microdilution MICs for all organism groups. In general, the mean Etest MIC was approximately one two-fold dilution lower than the mean microdilution MIC.
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Discussion |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Linezolid demonstrated broad activity against all of the Gram-positive bacteria tested, including staphylococci, enterococci and streptococci. Linezolid had modest activity against the Gram-negative pathogens M. catarrhalis and H. influenzae. Regional differences in the susceptibilities of the Gram-positive species were not detected. Assessment of the agreement between linezolid microdilution MICs determined by NCCLS methods and MICs determined by Etest indicated that the mean Etest value was approximately two-fold lower than the mean microdilution value. Although not investigated further, it is possible that this effect is due to the bacteriostatic rather than bactericidal activity of linezolid;2 no differences were seen with recognized bactericidal comparators vancomycin and co-amoxiclav.
The present multicentre study compares favourably with others in North America2,4–6,11,12 and elsewhere.7,8 Overall there are clear indications that linezolid displays good activity against a range of Gram-positive bacteria (MIC50 range 0.5–4 mg/L).
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Acknowledgments |
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This paper was presented in part at the Tenth European Congress on Microbiology and Infectious Diseases, Stockholm, Sweden, May 2000, Abstract WeP100.
The participating investigators were: A. Bouvet, Hôpital Hotel Dieu, Paris, France; I. Braveny, Technische Universitaet Muenchen, Munich, Germany; R. I. A. Diepersloot, Diakonessen Hospital, Utrecht, The Netherlands; R. Finch, Nottingham City Hospital, Nottingham, UK; G. French, St Thomas' Hospital, London, UK; B. Halle, Universittätsklinikum Charité Institut fur Medizinische Mikrobiologie, Berlin, Germany; J. Kluytmans, Ignatius Hospital Breda, Breda, The Netherlands; J. Linares, Hospital de Bellvitge, Barcelona, Spain; Q. Noury, Université Bordeaux, Bordeaux, France; M. Rylander, Karolinska Hospital, Stockholm, Sweden; G. Schito, Universita degli Studi di Genova, Genova, Italy; F. Soriano, Fundacíon Jimenez Diaz, Madrid, Spain; and G. Tempera, Universita di Catania, Catania, Italy.
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Notes |
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* Tel: +44-141-211-4654; Fax: +44-141-552-1524; E-mail: c.g.gemmell{at}clinmed.gla.ac.uk
The participating investigators are listed in the Acknowledgements.
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References |
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3 . Shinabarger, D. L., Marotti, K. R., Murray, R. W., Lin, A. H., Melchior, E. P., Swane, S. M. et al. (1997). Mechanism of action of oxazolidinones: effects of linezolid and eperezolid on translation reactions. Antimicrobial Agents and Chemotherapy 41, 2132–6.[Abstract]
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Received 12 July 2000; returned 1 March 2001; accepted 19 March 2001
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