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Influence of thymidine on bactericidal activity of iclaprim against vancomycin-intermediate and vancomycin-resistant Staphylococcus aureus strains

Abstract number: O98

Pankuch G., Appelbaum P.

Background: Iclaprim is a bactericidal diaminopyrimidine antibiotic that exhibits potent and bactericidal activity against Gram-positive pathogens, notably MRSA. Previous studies have shown that excess thymidine (geqslant R: gt-or-equal, slantedmg/ml) in the growth medium abolishes the bactericidal effect of the drug in vitro. Importantly, thymidine levels in humans are extremely low (leqslant R: less-than-or-eq, slant0.01 mg/ml). This study used time-kill analysis to determine the effects of adding or removing excess thymidine in growth medium on the bactericidal potential of iclaprim against one vancomycin intermediate (VISA) and one vancomycin resistant (VRSA) S. aureus isolate.

Methods: Media included 1) Oxoid Mueller-Hinton broth, previously proven to have minimal amounts of thymidine; 2) The latter Oxoid broth + 2 mg/ml thymidine; 3) BBL cation adjusted Mueller-Hinton broth + 0.2 units/ml thymidine phosphorylase; 4) BBL cation-adjusted Mueller-Hinton broth + 2 mg/ml thymidine. MICs were by CLSI macrodilution in each of the four above media. For time-kills organism suspensions (5×105–5×106 cfu/ml) were incubated overnight with iclaprim at MIC, 2 x MIC and 4 x MIC in all four media, in a shaking water bath at 35°C. Viability counts were done at 0, 3, 6, 12, and 24 h on BBL trypticase soy agar plates + 5% sheep blood.

Results: Iclaprim MICs for the VRSA and VISA were as follows: 1) Oxoid Mueller-Hinton broth without added thymidine: 0.06 and 0.25 mg/ml, respectively. 2) Cation-adjusted Mueller-Hinton broth with thymidine phosphorylase: 0.12 and 0.5 mg/ml respectively. On BBL medium with no added thymidine or thymidine phosphorylase, iclaprim MICs were 0.125 and 0.25 mg/ml, respectively. Iclaprim MICs were >8.0 mg/ml after 2 mg/ml thymidine was added to either media. Iclaprim at 2 x MIC showed geqslant R: gt-or-equal, slanted3 log10 (99.9%) killing after 12 h in Oxoid Mueller-Hinton broth and BBL cation-adjusted Mueller-Hinton broth with thymidine phosphorylase. By contrast, no killing by iclaprim occurred in either media after adding thymidine (2 mg/ml).

Conclusions: Iclaprim yielded low MICs and was bactericidal against VRSA and VISA strains when tested in broth media without significant levels of thymidine. For accurate determination of the bactericidal activity of iclaprim, media without significant levels of thymidine should be used.

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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