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Evaluation of linezolid, vancomycin, gentamicin, and ciprofloxacin activity against Staphylococcus aureus biofilms using XTT and Resazurin

Abstract number: R2263

Fernández-Hidalgo N., Martín M.T., López P.M., Gomis X., Gavaldà J., Almirante B., Pahissa A.

Objective:S. aureus (SA) biofilms (BF) show reduced susceptibility to antimicrobials. The activity of linezolid (LNZ), vancomycin (VAN), gentamicin (GEN), and ciprofloxacin (CIP) against SA BF was evaluated using the tetrazolium salt XTT, and the redox indicator resazurin (RZ).

Methods: 10 meticillin and quinolone resistant (MRSA) and 7 meticillin susceptible (MSSA) SA strains isolated from patients with catheter-related bacteraemia were studied. BF were obtained on flat bottom microtiter plates inoculated with 106 cfu/mL in Trypcase Soy Broth (TSB). Growth control (GC) and sterility control (SC) were included. Plates were incubated (24 h, 37°C), and then washed and filled with Mueller-Hinton broth containing 2-fold dilutions of LNZ, VAN, CIP, and GEN (2048–0.125 mg/L). After incubation (37°C, 24 h) plates were washed, and TSB containing 0.2 mg/L XTT + 0.1 microM menadione or RZ 0.0015% as appropriate was added. After 2.45 h of incubation at 37°C, the absorbance of XTT (492 nm) or the fluorescence of RZ (530/590 nm excitation/emission) was read. SC values were subtracted to those obtained from the BF wells. The % of metabolic activity (MA) of BF exposed to antibiotics respect to their respective GC was calculated. Concentrations (mg/L) that inhibited >75 and >90% (CI75, CI90) of MA were considered; ANOVA was used to compare log10 of CI75 and 90. Studies were run in triplicate.

Results: XTT and RZ rendered similar results. For MSSA, the geometric mean concentration that resulted in CI90 was 4.9 mg/L for VAN, 9 mg/L for GEN, 2.3 mg/L for CIP and 50 mg/L for LNZ. For MRSA, the geometric mean that achieved the CI90 was of 8 mg/L for VAN, 157.6 for GEN and 1024 for LNZ. Only CIP resulted in 100% of MA at 2048 mg/L. For MSSA the log10 CI90 for VAN, GEN and CIP were significant lower with respect to that of LNZ (p < 0.05). SA. For MRSA, the log10 CI90 of VAN was significant lower than that for GEN (p < 0.01) and both VAN and GEN showed significant lower log10 CI90 respect to LNZ (p < 0.021).

Conclusions: XTT and RZ performed equally for detection of the activity of LNZ, VAN, GEN, and CIP activity against SA BF. For both categories of SA LNZ showed significant higher log10 CI90 with respect to the other drugs. 100% inhibition of MA was only seen in BF of CIP-susceptible SA exposed to high concentrations of CIP.

Session Details

Date: 19/04/2008
Time: 00:00-00:00
Session name: 18th European Congress of Clinical Microbiology and Infectious Diseases
Subject:
Location: Barcelona, Spain
Presentation type:
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