Laboratoire de Microbiologie, CHU de la côte de Nacre, Service de Microbiologie, Avenue de la côte de Nacre, 14033 Caen Cedex, France
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Abstract |
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Introduction |
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Linezolid belongs to a new class of antimicrobial agents, the oxazolidinones, which are active against a variety of Gram-positive pathogenic bacteria, including methicillin-resistant isolates of Staphylococcus aureus and Staphylococcus epidermidis, vancomycin-resistant isolates of Enterococcus spp. and penicillin-resistant pneumococci.13 Early studies have demonstrated that this new agent exerts bacteriostatic effect by inhibiting protein synthesis.4 Linezolid binds to the 50S ribosomal subunit in a similar way to chloramphenicol, macrolides, lincosamides and streptogramins. The linezolid binding site is close to that of chloramphenicol and lincomycin, and linezolid competes with these two antimicrobials.5 However, in contrast to the latter two classes, oxazolidinones clearly have a distinct mechanism of action, as they do not have any effect on the peptidyl transferase activity but inhibit the formation of the initiation complex in bacterial translation systems.6
Many studies have shown that linezolid is active against multidrug-resistant organisms. However, mechanisms of resistance to protein synthesis inhibitors, such as aminoglycosides, chloramphenicol, macrolides and related antimicrobials or tetracyclines, have not been characterized at the genetic level in the isolates studied. Therefore, possible cross-resistance between linezolid and other protein synthesis inhibitors has not been systematically studied.
Three types of resistance mechanisms to antimicrobials that inhibit protein synthesis have been identified in clinical isolates of Gram-positive bacteria. For each type, several resistance genes have been described. The major ones are listed in Table I.
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A second type of resistance is the drug inactivation conveyed by various enzymes including acetylases (aminoglycosides, chloramphenicol, streptogramins A), hydrolases (streptogramins B), nucleotidyltransferases (aminoglycosides, lincosamides) and phosphotransferases (aminoglycosides).
The third mechanism affects the rate of transport of streptogramins A, tetracyclines or macrolides across the cell membrane by active efflux.
In this study, we have evaluated the activity of linezolid against various Gram-positive bacteria exhibiting the major mechanisms of resistance against inhibitors of protein synthesis characterized in clinical isolates.
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Materials and methods |
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For a majority of resistance gene classes, isogenic pairs of bacteria, differing only by the presence or absence of a known resistance determinant (transconjugants, transformants, transductants or cured derivatives) were used. When pairs were not available, a control isolate belonging to the same species but devoid of the resistance gene was used. Mutants constitutively resistant to macrolides were obtained from inducibly resistant isolates after selection on agar plates containing inhibitory concentrations of clindamycin, a non-inducer macrolide. Before testing the activity of linezolid, the presence of resistance genes in the isolate studied was checked by a specific PCR reaction.
Antimicrobial susceptibility
MICs of linezolid were determined by the agar dilution method according to NCCLS standards with Mueller Hinton medium (bioMérieux, La Balme-les-Grottes, France).28 An inoculum of 104 cfu/spot was used. For streptococci, MuellerHinton medium was supplemented with 5% horse blood. The plates were incubated aerobically for 24 h at 37°C.
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Results and discussion |
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Recent in vitro studies have shown that resistance to linezolid can appear by point mutation in the 23S rRNA domain V in S. aureus and Enterococcus faecalis probably at the binding site of this antimicrobial.40 Moreover, the same mutation was found in two clinical Enterococcus faecium isolates resistant to linezolid, which have emerged during linezolid therapy within the Linezolid Compassionate Use Program.41
Despite a possible but still very rare resistance by target mutation, this molecule could represent an advance in antimicrobial therapy, particularly for the problematic resistant organisms that are becoming widespread.
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Acknowledgments |
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Notes |
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References |
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Received 9 August 1999; returned 7 December 1999; revised 7 January 2000; accepted 28 January 2000