Effect of clarithromycin on Pseudomonas aeruginosa biofilms

Using an experimental in vitro culture system, we investigated the effect o f clarithromycin on biofilm formation by a leucine-requiring Pseudomonas ae ruginosa mutant strain (HU1). Biofilm formation on celldesks in a chemicall y defined medium was assessed by viable cell count as well as by measuremen t of glycocalyx production and scanning electron-microscopic observation. C ells proliferated exponentially until day 3 and remained stationary afterwa rds. The amount of glycocalyx, simultaneously semiquantified, showed a line ar increase from day 1 to day 12. Scanning electron microscopy revealed fir m biofilms on day 5. Three different concentrations of Clarithromycin (CAM) (minimum inhibitory concentration MIC 64 mu g/ml) were added continuously at the early and late phases of biofilm formation, and the antibiofilm effe ct of CAM was evaluated by the changes in cell count and glycocalyx product ion. CAM was effective on biofilms at 100 mu g/ml but neither at 1 nor at 1 0 mu g/ml. It is suggested that glycocalyx production started following bac terial multiplication and continued even after the cells had entered the st ationary phase to form mature biofilms. No antibiofilm effect of CAM was ob served at sub-MIC. Copyright (C) 2000 S. Karger AG, Basel.