Resistance to a polyquaternium-1 lens care solution and isoelectric points of Pseudomonas aeruginosa strains.

Objectives: The aim of this study was to correlate the cell surface hydrophobicity and charge of various strains of Pseudomonas aeruginosa with their resistance to a polyquaternium-1 lens care solution.

Methods: The 11 P. aeruginosa strains included were isolated from eyes, contact lenses, lens cases and lens care solutions. Cell surface hydrophobicities were determined from water contact angle measurements and surface charges were measured as a function of pH using particulate micro-electrophoresis.

Results: Strains resistant to polyquaternium-1 had an isoelectric point (IEP; pH where the bacterial zeta potential is zero) ranging from 4.0 to 5.5, whereas susceptible strains were more negatively charged than resistant strains and had an IEP between 1.3 and 1.9. Water contact angles ranged from hydrophilic (34 degrees) to hydrophobic (124 degrees), without showing a relation with antimicrobial resistance.

Conclusions: Results suggest that electrostatic repulsion between cationic molecules on the cell surface and quaternary ammonium compounds impedes the antimicrobial entering the cell.