JAC Advance Access originally published online on February 7, 2006
Journal of Antimicrobial Chemotherapy 2006 57(4):767-770; doi:10.1093/jac/dkl013
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Measurement of ampicillin, vancomycin, linezolid and gentamicin activity against enterococcal biofilms
1 Department of Microbiology, The General Infirmary at Leeds, Leeds, LS1 3EX, UK; 2 Division of Microbiology, The University of Leeds, Leeds LS1 3EX, UK
Received 30 June 2005; returned 17 August 2005; revised 6 January 2006; accepted 9 January 2006
* Correspondence address. Department of Microbiology, Old Medical School, Leeds General Infirmary, Leeds, LS1 3EX, UK. Tel: +44-113-343-5634; Fax: +44-113-343-5638; E-mail: jonathan.sandoe{at}leedsth.nhs.uk
Background: Enterococci frequently cause biofilm infections but susceptibility of clinical isolates growing in biofilms has not been investigated. The minimum biofilm eradicating concentration (MBEC) has been suggested as a guide to treatment of biofilm infections. We measured an alternative endpoint, the minimum biofilm inhibitory concentration (MBIC) and compared the results with MIC and MBC.
Objectives: To compare the MIC, MBC and MBIC of ampicillin, vancomycin and linezolid against enterococcal biofilms, to assess the impact of additional gentamicin and correlate findings with clinical outcome.
Methods: MIC and MBC were measured using standard techniques. MBICs were measured using a modification of the Calgary biofilm device method. Fifty-eight enterococcal isolates from episodes of intravascular catheter-related bloodstream infection were tested.
Results: Tolerance to ampicillin, vancomycin and linezolid was seen in 93%, 100% and 93% of isolates, respectively. MIC90s of ampicillin, vancomycin and linezolid were all 4 mg/L for Enterococcus faecalis isolates. MBC90s of ampicillin, vancomycin and linezolid for E. faecalis isolates were 1024, >128 and 2048 mg/L, respectively. MBIC90s of ampicillin, vancomycin and linezolid for E. faecalis isolates were 8192, 4096 and 4096 mg/L, respectively. Results for Enterococcus faecium were similar for vancomycin and linezolid but this species was generally more resistant to ampicillin. Adding 10 mg/L gentamicin had a variable effect on MIC, MBC or MBIC, which was not predictable by gentamicin susceptibility on disc testing.
Conclusions: Very high concentrations of ampicillin, vancomycin and linezolid are required to inhibit enterococcal biofilms in vitro. Combining these agents with gentamicin significantly reduced MIC, MBC and MBIC against only a proportion of enterococcal isolates. No correlation between MBIC and outcome was found.
Keywords: Enterococcus , enterococci , susceptibility , intravascular catheter-related bloodstream infections
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