Effect of Cholesterol Deposition on Bacterial Adhesion to Contact Lenses

Abstract

Purpose: : Contact lens surfaces can attract tear film components during wear. Protein deposition on lens surfaces have been previously shown to modulate bacterial adhesion but there have been few studies on the effects of adsorbed lipids. This study was designed to determine whether surface bound cholesterol could alter the number of bacteria adhering to a lens.

Methods: : Contact lenses were collected from subjects wearing balafilcon A or senofilcon A lenses on a daily wear schedule using either a PHMB or Polyquad based disinfection system (30 subjects for each solution). Lenses were then extracted in chloroform:methanol (1:1, v/v) and cholesterol separated by thin-layer chromatography. Using a standard curve of cholesterol, the amount of cholesterol in extracts was estimated. Unworn lenses were soaked in solutions of cholesterol at the concentrations ranging from 2.5 to 20 µg/mL to achieve the same cholesterol loadings as were found ex vivo. Pseudomonas aeruginosa 6294 or Staphylococcus aureus 31, at 1x10⁷ CFU/mL in phosphate buffered saline, were added to the soaked lenses and incubated for 24h. Numbers of bacteria adherent to lenses were recovered and counted by plating method.

Results: : 3.2 ± 0.2 µg/lens cholesterol was removed from ex vivo balafilcon A lenses irrespective of solution used. For senofilcon A lenses with the PHMB solution, 2.4 ± 0.2 µg/lens cholesterol was extracted; whereas with Polyquad solution 1.0 ± 0.1 µg/lens cholesterol was extracted (p<0.05). The amount of cholesterol in vitro adsorption onto balafilcon A (3.2 ± 0.2 µg/lens) was similar to that of senofilcon A lenses (3.3 ± 0.2 µg/lens) under the same cycling conditions (i.e. in 10 µg/mL colersterol for 24h). In vitro adsorption of cholesterol to both lens types was dose dependent. The adhesion of S. aureus to cholesterol soaked lenses was slightly but not significantly reduced compared to adhesion to un-soaked lenses (4.5 ± 0.2 vs 4.6 ± 0.3 log CFU/lens for balafilcon A; 4.4 ± 0.2 vs 4.7± 0.4 for senofilcon A) (p>0.05). For P. aeruginosa, there was no difference between adhesion on cholesterol-soaked or unsoaked lenses (6.4 ± 0.1 vs 6.4 ± 0.2 log CFU/lens for balafilcon A; 6.6 ± 0.05 vs 6.5 ± 0.06 for senofilcon A) (p>0.05).

Conclusions: : Whilst cholesterol has been shown to adsorb to contact lenses during wear, this lipid does not appear to modulate bacterial adhesion to a lens surface. This contrasts with previous published data that has shown significant effects of tear film proteins on bacterial adhesion to lenses.

Keywords: contact lens • cornea: tears/tear film/dry eye • bacterial disease