Effects of tonicity-adjusting and surfactant agents on the antimicrobial activity of alexidine

Alexidine is a bis-biguanide disinfectant with two cationic active sites and hydrophobic ethylhexyl end groups, both of which are believed to support its association with microbial cell membranes through electrostatic and hydrophobic interactions. We evaluated the effects of tonicity-adjusting and surfactant agents on the antimicrobial activity of alexidine to assess its suitability as a disinfectant in multipurpose solutions for contact lenses.

The antimicrobial activity of alexidine (4.5 ppm) against various bacteria and fungi was measured with the stand-alone procedure (ISO 14729, 2001). The effect of NaCl as an ionic tonicity-adjusting agent on such activity was determined in comparison with that of propylene glycol as a nonionic tonicity-adjusting agent. The effect of the nonionic surfactant Poloxamer 407 (Px407) was similarly examined in the absence or presence of NaCl.

Alexidine showed robust antimicrobial activity, with no organisms surviving after 1 hr. Antifungal activity was inhibited by NaCl in a concentration-dependent manner, whereas neither antibacterial nor antifungal activity was affected by propylene glycol. The activity of alexidine was not affected by Px407 (4%) alone but was attenuated by the combination of NaCl and Px407 with all microorganisms tested.

The antifungal activity of alexidine was inhibited by adjustment of osmolality with the ionic agent NaCl but not by that with the nonionic agent propylene glycol. The surfactant Px407 reduced antimicrobial activity only in the presence of NaCl. These findings indicate that electrostatic and hydrophobic interactions with the microbial cell membrane are a key factor in the antimicrobial activity of alexidine.