JAC Advance Access originally published online on August 27, 2008
Journal of Antimicrobial Chemotherapy 2008 62(5):1158-1160; doi:10.1093/jac/dkn344
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Research letters |
In vitro activity of tigecycline against Gram-positive cocci: a multicentre study in Greece
1 Department of Microbiology, University Hospital of Larissa, Mezourlo, 41110 Larissa, Greece 2 Department of Microbiology, School of Medicine, Rio, 26500 Patras, Greece 3 Department of Molecular Microbiology, Institute of BioMedical Research and Science, 41222 Larissa, Greece 4 Department of Microbiology, University Hospital of Alexandroupolis, Dragana, 68100 Alexandroupolis, Greece 5 Department of Microbiology, University Hospital of Heraklion, Stavrakia, 71110 Heraklion, Crete, Greece 6 Department of Microbiology, University Hospital ‘AHEPA’, 54636 Thessaloniki, Greece 7 Department of Microbiology, General Hospital of Lamia, 35100 Lamia, Greece 8 Department of Microbiology, General Hospital ‘Asclepeion’, Voula, 16673 Athens, Greece 9 Department of Microbiology, General Hospital ‘Georgios Genimatas’, 11527 Athens, Greece 10 Department of Microbiology, ‘Attikon’ University Hospital, Athens, 12462 Athens, Greece 11 Department of Microbiology, General Hospital of Volos, 38222 Volos, Greece
* Corresponding author. Tel: +30-2410-682517; Fax: +30-2410-682535; E-mail: petinaki{at}med.uth.gr or petinaki{at}hotmail.com
Keywords: resistance , staphylococci , enterococci , streptococci , pneumococci
Tigecycline, a new glycylcycline antibiotic with broad-spectrum activity against aerobic and anaerobic Gram-positive and Gram-negative bacteria, appears to be a therapeutic option for serious infections caused by multidrug-resistant organisms.1 The purpose of this study was firstly to evaluate the in vitro activity of this drug against Gram-positive cocci in Greek hospitals and secondly to define a baseline for monitoring possible future emergence of resistance to tigecycline in our clinical settings.
From January 2006 to December 2007, a total of 10 420 Gram-positive cocci were tested for their susceptibility to tigecycline. The numbers of isolates of the various genera and species tested are shown in Table 1. The isolates were recovered from clinically significant specimens (blood, pus, pleural fluid etc.) in 10 Greek hospitals, located in different areas of the country (Northern, Central and Southern Greece). Each participant institution was requested to collect a minimum of 700 Gram-positive cocci, equally distributed during the study period, that were sent to the Department of Microbiology of the University Hospital of Larissa for susceptibility testing. Isolates were identified at each participating laboratory using routine methodology and were sent in transport swabs (Culturette; Becton-Dickinson Microbiology Systems, Sparks, MD, USA) to the coordinating laboratory at the University Hospital of Larissa. Upon receipt, isolates were subcultured onto 5% sheep blood agar to ensure purity, while identification was confirmed with Slidex Staph, Slidex Strepto, Slidex Pneumo, ID 32 STAPH, ID 32 STREP (bioMérieux, Marcy l'Étoile, France). The clonality of the isolates was tested by PFGE, after digestion of chromosomal DNA by SmaI, while the interpretation of the results was based on the criteria of Tenover et al.2
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The MICs of tigecycline were first determined by an agar-dilution method; the wells were prepared in-house using fresh Mueller–Hinton agar (Difco Laboratories, Detroit, MI, USA), containing the following serial 2-fold dilutions from 0.008 to 2 mg/L.3 For testing streptococci, 5% sheep blood was also added. In addition, all MICs were re-determined by using Etest strips (AB Biodisk, Solna, Sweden). Based on the US-FDA criteria, the isolates were categorized as susceptible and resistant; a tigecycline-susceptible breakpoint of
0.5 mg/L was used for Staphylococcus aureus, including methicillin-resistant strains, whereas 0.25 mg/L was used for the interpretation of vancomycin-susceptible Enterobacter faecalis and streptococci other than Streptococcus pneumoniae. In the absence of agreed US-FDA breakpoints for coagulase-negative staphylococci (CoNS), the breakpoints for staphylococci were used, whereas for S. pneumoniae, viridans streptococci and the various enterococcal species, including vancomycin-resistant strains, the criteria for E. faecalis were used. Susceptibility testing for selected comparator agents (ampicillin, cefoxitin, erythromycin, linezolid, minocycline, oxacillin, penicillin, quinupristin/dalfopristin, teicoplanin, tetracycline and vancomycin) was done by the disc diffusion method, according to the CLSI guidelines.3 The following quality control organisms were concurrently tested: E. faecalis ATCC 29212, S. aureus ATCC 29213 and S. pneumoniae ATCC 49619. According to the results obtained by the agar-dilution method, 99.97% of the isolates tested were considered susceptible to tigecycline. No discrepancies in the characterization of the isolates were observed between the agar-dilution method and Etest, indicating that Etest is a reliable method and could be easily applied in the clinical microbiological laboratory. During the 2 year study period, no differences in the tigecycline susceptibility results between hospitals were observed.
Table 1 summarizes the MICs at which 50% and 90% of the isolates were inhibited (MIC50 and MIC90, respectively) and the MIC distribution. No differences in the MICs were observed between the susceptible isolates and those with characterized resistance determinants (vancomycin and linezolid resistance for enterococci, glycopeptide-intermediate resistance for staphylococci and penicillin resistance for pneumococci). As shown in Table 1, tigecycline showed excellent activity against S. pneumoniae, Streptococcus agalactiae, Streptococcus pyogenes and viridans group streptococci. In addition, all S. aureus and coagulase-negative isolates were found to be susceptible to tigecycline, except 10, clonally unrelated, methicillin-resistant CoNS (6 Staphylococcus epidermidis and 4 Staphylococcus haemolyticus) that had MICs of 1 mg/L. Among enterococci, 10 E. faecalis and 12 Enterobacter faecium strains had MICs >0.25 mg/L. In more detail, among E. faecalis, three and seven strains had MICs of 0.512 and 1 mg/L, respectively, whereas among E. faecium, eight and four strains had MICs of 0.512 and 1 mg/L, respectively; no clonal relatedness was observed. Repetition of MICs by both the agar-dilution method and Etest revealed that these strains had reduced susceptibility to tigecycline. The mechanism of resistance is under investigation. The tigecycline non-susceptible staphylococci and enterococci of our collection also exhibited resistance to minocycline; 
8.5% of the isolates of our collection were minocycline-resistant.
Since 2007, tigecycline has been occasionally used in Greek intensive care units. Previous studies conducted in various countries have demonstrated that the agent is active against Gram-positive bacteria;4,5 our data also verify that tigecycline is highly active against staphylococci, enterococci, pneumococci and streptococci. However, the appearance of some staphylococci and enterococci with decreased susceptibility to tigecycline must be an alarm for a future emergence of tigecycline-resistant Gram-positive bacteria in our country.6
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Funding |
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 Top Funding Transparency declarationsReferences |
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This study was funded by an educational/research grant from the Institute of Biomedical Research and Technology awarded to E. P.
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Transparency declarations |
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TopFundingTransparency declarationsReferences |
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None to declare.
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Acknowledgements |
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We wish to thank M. Karanika for excellent technical support.
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References |
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TopFundingTransparency declarationsReferences |
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1 Livermore DM. Tigecycline: what is it, and where should it be used? J Antimicrob Chemother (2005) 56:611–4.
2 Tenover FC, Arbeit RD, Goering RV, et al. Interpreting chromosomal DNA restriction patterns produced by pulsed-field gel electrophoresis: criteria for bacterial strain typing. J Clin Microbiol (1995) 33:2233–9.[Web of Science][Medline]
3 Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing: Fifteenth Informational Supplement M100-S15 (2005) Wayne, PA, USA: CLSI.
4
Milatovic D, Schmitz FJ, Verhoef J, et al. Activities of the glycylcycline tigecycline (GAR-936) against 1924 recent European clinical bacterial isolates. Antimicrob Agents Chemother (2003) 47:400–4.
5 Hoban DJ, Bouchillon SK, Johnson BM, et al. In vitro activity of tigecycline against 6792 Gram-negative and Gram-positive clinical isolates from the global Tigecycline Evaluation and Surveillance Trial (TEST Program, 2004). Diagn Microbiol Infect Dis (2005) 52:215–27.[CrossRef][Web of Science][Medline]
6
Werner G, Gfrörer S, Fleige C, et al. Tigecycline-resistant Enterococcus faecalis strain isolated from a German intensive care unit patient. J Antimicrob Chemother (2008) 61:1182–3.
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