Daniel Tillia is a research optometrist at the Brien Holden Vision Institute, a part-time clinical supervisor at the School of Optometry and Vision Science, University of New South Wales, Australia and occasionally works in private practice
One of the major functions of a contact lens care solution is to disinfect contact lenses to make them safe to wear. Solutions contain antimicrobial agents, which kill various pathogens including Gram-negative and Gram-positive bacteria. Before a solution is released onto the market, certain criteria need to be met in terms of their ability to kill these pathogens. However, recent outbreaks of atypical microbial keratitis have sparked a rethink in terms of how solutions are tested before they become commercially-available. Current testing involves demonstrating efficacy against planktonic or single-cells microbes, while efficacy against biofilms is not required. The following is a review of an article that determines the efficacy of contact lens solutions against three types of bacterial biofilm.
Szczotka-Flynn LB, Imamura Y, et al.. Increased resistance of contact lens-related bacterial biofilms to antimicrobial activity of soft contact lens care solutions. Cornea 2009;28: 918-926.
Bacteria, lenses and solutions
Szczotka-Flynn and colleagues used a bacterial biofilm contact lens model to determine whether three different bacterial strains, commonly associated with contact lens keratitis and inflammation, can form biofilm on silicone hydrogel contact lenses. Additionally, they assessed the antimicrobial activities of contact lens care solutions against bacterial cells grown under planktonic or biofilm conditions.
Two Gram-negative (Pseudomonas aeruginosa, Serratia marcescens) and one Gram-positive (Staphylococcus aureus) bacteria were evaluated. These species were selected as they are common causative agents of contact-lens associated inflammation and infection.
Lotrafilcon A contact lenses were chosen as they represent a leading extended wear silicone hydrogel material while the lens care solutions chosen were the most commonly used at the time of the study. The solutions contained either polyhexamethylene biguanide (PHMB) (ReNu MultiPlus [now renu fresh], Bausch + Lomb, USA, Complete MoisturePlus, AMO, USA, AQuify, CIBA Vision, USA), alexidine (ReNu with MoistureLoc, Bausch + Lomb, USA), polyquaternium-1/myristamidopropyl dimethylamine (OPTI-FREE RepleniSH, ALCON, USA) or hydrogen peroxide (ClearCare, CIBA Vision, USA). Complete MoisturePlus and ReNu with MoistureLoc were withdrawn from the market after study completion.
Method of assessing contact lens care solution efficacy
Lotrafilcon A lenses were inoculated with the three bacterial species. Biofilms were allowed to form on contact lenses for up to 24 hours and the number of colony forming units (CFU) on a contact lens for each species was determined at various time-points over 24 hours. To determine solution efficacy, lenses were washed with phosphate buffered saline (PBS) for at least five seconds and then placed in relevant solution for the manufacturer’s recommended duration. After this period, lenses were then rinsed with PBS, and transferred to a tube containing PBS. Lenses were then sonicated and vortexed. Cell suspensions were treated with neutralisation broth and spread over soy agar plates to evaluate viability. Contact lens care solutions were also tested against planktonically grown cells according to International Organization for Standardization (ISO 14729) Stand Alone Procedure guidelines.
Bacterial biofilm on lotrafilcon A lenses
All reference and clinical bacterial strains tested were able to form biofilms on lotrafilcon A contact lenses. Ultrastructural/scanning electron microscopy showed typical biofilm morphologies for the bacterial isolates investigated. Confocal microscopy analysis showed P. aeruginosa biofilms were significantly thicker than biofilms formed by S. marcescens and S. Aureus.
Contact lens solution efficacy: planktonically grown cells
All lens care solutions met the ISO 14729 criteria of 3 log reduction when tested against planktonically grown cells of all three bacterial species to be considered effective against these microbes.
Contact lens solution efficacy: biofilms
- All three PHMB solutions showed little or no effect on the biofilm formed by all three bacteria.
- ReNu with MoistureLock showed a significant 2 log reduction in biofilms formed by P. aeruginosa but little or no reduction for the other bacterial biofilms.
- OPTI-FREE RepleniSH had a significant 7 log reduction in biofilms formed by P. Aeruginosa and S. aureus but was ineffective in biofilms formed by S. marcescens.
- ClearCare showed at least a 4 log reduction in biofilms formed by all three bacteria.
This investigation showed that, in an in vitro model, P. aeruginosa, S. marcescens and S. aureus were able to form biofilms on the surface of lotrafilcon A contact lenses. All tested lens care solutions were effective against planktonic bacterial growth but the multi-purpose solutions showed much lower and variable activity against biofilms formed by these same organisms. Hydrogen peroxide exhibited activity against biofilms formed by all three bacterial species.
Biofilms form during normal contact lens wear and might therefore be an essential mechanism by which organisms cause contact-lens associated adverse events. While testing guidelines for new solutions should include efficacy against biofilm formation, that simple steps such as rubbing and rinsing contact lenses1 and rubbing, rinsing, and air drying contact lens cases2 reduce microbial contamination and might lead to a reduction of bacterial-driven ocular adverse events. As with most aspects of contact lens practice, the key is patient education and compliance.
1. Kilvington S, Lonnen J. A comparison of regimen methods for the removal and inactivation of bacteria, fungi and Acanthamoeba from two types of silicone hydrogel lenses. Cont Lens Anterior Eye 2009;32:73-7. Epub 2009 Feb 6.
2. Wu YT, Zhu H, Willcox M, Stapleton F. The effectiveness of various cleaning regimens and current guidelines in contact lens case biofilm removal. Invest Ophthalmol Vis Sci 2011;52:5287-92. Print 2011 Jul.