title: Measurement of ampicillin, vancomycin, linezolid and gentamicin activity against enterococcal biofilms.
abstract: 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.
  
  
  OBJECTIVE
  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. MIC(90)s of ampicillin, vancomycin and linezolid were all 4 mg/L for Enterococcus faecalis isolates. MBC(90)s of ampicillin, vancomycin and linezolid for E. faecalis isolates were 1024, >128 and 2048 mg/L, respectively. MBIC(90)s 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.
authors.affiliation: Department of Microbiology, The General Infirmary at Leeds, UK. jonathan.sandoc@leedsth.nhs.uk
authors.forename: Jonathan A T
authors.initials: JA
authors.lastname: Sandoe
authors.forename: Joanne
authors.initials: J
authors.lastname: Wysome
authors.forename: Andrew P
authors.initials: AP
authors.lastname: West
authors.forename: John
authors.initials: J
authors.lastname: Heritage
authors.forename: Mark H
authors.initials: MH
authors.lastname: Wilcox
mesh_terms: D000081:Acetamides / Q000494:pharmacology; D000667:Ampicillin / Q000494:pharmacology; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D016470:Bacteremia / Q000382:microbiology; D018441:Biofilms / Q000187:drug effects* / Q000254:growth & development; D002405:Catheterization, Central Venous / Q000009:adverse effects; D013293:Enterococcus faecalis / Q000187:drug effects* / Q000254:growth & development; D016984:Enterococcus faecium / Q000187:drug effects* / Q000254:growth & development; D005839:Gentamicins / Q000494:pharmacology; D016908:Gram-Positive Bacterial Infections / Q000382:microbiology; D006801:Humans; D000069349:Linezolid; D008826:Microbial Sensitivity Tests / Q000379:methods; D023303:Oxazolidinones / Q000494:pharmacology; D014640:Vancomycin / Q000494:pharmacology
chemical_list: D000081:Acetamides; D000900:Anti-Bacterial Agents; D005839:Gentamicins; D023303:Oxazolidinones; D014640:Vancomycin; D000667:Ampicillin; D000069349:Linezolid