In-vitro effects of antimicrobial agents on Pseudomonas aeruginosa biofilm formation

The in-vitro effects of ten antimicrobial agents on the biofilm formation of Pseudomonas aeruginosa were investigated. The production of alginic acid by mucoid P. aeruginosa cells cultured in agar media with sub-MICs of antimicrobial agents was quantified by high-performance liquid chromatography. Alginic acid production was inhibited by 1/4 MIC of minocycline (P < 0.002) and tobramycin (P < 0.02), and by 1/256-1/1/64 MIC of macrolides (erythromycin, clarithromycin, roxithromycin, and rokitamycin) and clindamycin (P < 0.02), compared with drug-free controls. Piperacillin, ceftazidime, and ofloxacin did not inhibit alginic acid production. The production of exopolysaccharide by non-mucoid P. aeruginosa cells grown on silicone plates in sub-MICs of antimicrobial agents was determined by quantitative tryptophan assay. Exopolysaccharide production was inhibited by 1/16 MIC of macrolides and clindamycin, but not by other antimicrobial agents. Electron microscopy showed that biofilm formation by mucoid and non-mucoid type P. aeruginosa strains was inhibited by sub-MICs of erythromycin and correlated with the in-vitro production of alginic acid and exopolysaccharide. These results suggest that sub-MICs of macrolides and clindamycin suppress biofilm formation by P. aeruginosa and that intractable chronic respiratory tract infections due to P. aeruginosa might be prevented.