Effects of Ionic and Surfactant Agents on the Antimicrobial Activity of Polyhexamethylene Biguanide

Objectives: Polyhexamethylene biguanide (PHMB) exhibits broad-spectrum antimicrobial activity and is included in multipurpose solutions for contact lenses as a disinfectant. Both cationic and hydrophobic features of PHMB are believed to support its association with microbial cell membranes through electrostatic and hydrophobic interactions. We now evaluated the effects of ionic and surfactant agents on the antimicrobial activity of PHMB. Methods: The antimicrobial activity of PHMB (1 ppm) against various bacteria and fungi was measured with the stand-alone procedure (ISO 14729, 2001). The effect of NaCl as an ionic isotonic agent on such an activity was determined in comparison with that of propylene glycol as a nonionic isotonic agent. The effect of the nonionic surfactant Poloxamer 407 (Px407) was similarly examined in the absence or presence of NaCl. Results: The antimicrobial activity of PHMB increased with time, being especially pronounced after 60 min. This activity was inhibited by NaCl in a concentration-dependent manner but was not affected by propylene glycol. Poloxamer 407 (4%) alone slightly increased the activity of PHMB toward Staphylococcus aureus and fungi. Although Px407 prevented the inhibitory effect of NaCl on PHMB activity against bacteria, it enhanced that observed with Candida albicans. Conclusions: The antimicrobial activity of PHMB was inhibited by adjustment of osmolality with the ionic agent NaCl but not by that with the nonionic agent propylene glycol. The surfactant Px407 exhibited complex effects on PHMB activity in the presence of NaCl. These findings indicate that the electrostatic interaction with the cell membrane is a dominant factor in the antimicrobial activity of PHMB.