In situ measurement of linezolid and vancomycin concentrations in intravascular catheter-associated biofilm

We describe a new method for the measurement of antimicrobial concentration s in the biofilm associated with the endoluminal surface of intravascular c atheters. We quantified endoluminal planktonic bacteria in haemodialysis ca theters using the acridine orange method on catheter blood. After catheter removal, separate lumens were perfused in vitro with either vancomycin or l inezolid to simulate in vivo antibiotic infusion. Biofilm was recovered usi ng endoluminal brushes, weighed and assayed by fluoroimmunoassay for vancom ycin and by bioassay for linezolid. Viable bacteria were counted by serial dilution and agar plating. Biofirm had measurable amounts of vancomycin in 11/11 catheter lumens post-infusion (0.3-18.2 mg biofilm per lumen, mean 6. 8 mg; vancomycin concentration 0.2-89 mg/g biofilm, median 19 mg/g). By com parison, linezolid was detected in 4/11 catheter lumens post-infusion (0.5- 18.1 mg biofilm per lumen, mean 5.9 mg; linezolid concentration 0.9-6.1 mg/ g biofilm, median 1.5 mg/g). Percentage reductions in biofilm-stssociated b acterial counts post-antibiotic were 84-100%, median 95% (vancomycin) and 0 -98%, median 91% (linezolid). We found a significant difference (P = 0.05; Wilcoxon rank sum test) in vancomycin concentrations in coagulase-negative staphylococcal biofilm (median 17.0 mg/g, mean 27.9 mg/g) compared with gly copeptide levels found in biofilm associated with other microorganisms (med ian 5.5 mg/g, mean 6.9 mg/g). Biofilm concentrations of vancomycin are gene rally higher than linezolid after antibiotic infusion, which can be explain ed partly by glycopeptide binding to glycocalyx. Neither antibiotic achieve d consistent 100% kill of biofilm bacteria after single infusions, even whe n a very high concentration was present. The endoluminal brush technique ca n be used to measure antibiotic concentration in intravascular catheter-ass ociated biofilm in situ. This approach can be exploited to measure biofilm antibiotic concentrations in vivo, without the need for catheter removal.