Battle Against the Bugs

The ability of microorganisms to form a biofilm on a contact lens and the surface of lens cases has been reported widely¹. Importantly, these biofilms function as reservoirs for pathogenic bacteria which are the causative organisms in microbial keratitis². Reducing the level of specific microorganisms within the contact lens system, including cases and solutions, may decrease the incidence of ocular adverse events related to lens wear.

Levels of microorganisms on contact lenses and cases can be reduced by patient compliance with hygiene and cleaning routines. However, compliance is highly patient dependent and extremely variable among lens wearers. To reduce the dependence on patient compliance, the application of antimicrobial surfaces to lenses and lens cases could be used. In general medicine, antimicrobial surfaces have been applied successfully to devices such as catheters, spinal shunts, heart valves and stents, where they kill microorganisms and prevent biofilm formation. This technology could be the next step in the battle against microorganisms to increase the safety and efficacy of contact lens wear.

The metals selenium and silver, as well as naturally occurring peptides derived from algae or seaweed called ‘fimbrolides’, possess antimicrobial properties with potential application in new contact lens technologies that may help reduce the incidence of adverse events with contact lens wear.

Reducing the level of specific microorganisms within the contact lens system… may decrease the incidence of ocular adverse events related to lens wear.

Selenium Shows Promise

Selenium is an essential part of our diet, playing an important role in the production of antioxidant enzymes and in the promotion of immune function. It is also able to kill microorganisms by forming superoxide radicals that are toxic to bacteria and viruses. In a recent study³, PureVision lenses were covalently coated with selenium and evaluated after two months of wear in rabbit eyes. In a separate in vitro experiment that was part of this study, the investigators challenged control lenses without selenium coating (untreated lenses) and selenium-coated lenses (test lenses) for bacterial adhesion by inoculating the test lenses with a high bacterial load of Pseudomonas aeruginosa – a common pathogen implicated in corneal infections.

The authors found no significant differences in clinical signs between the control and test rabbit corneas. Additionally, selenium-coated lenses showed resistance to colonisation by P. aeruginosa when compared to the untreated lenses in vitro. The authors concluded that these findings present strong evidence that the selenium-coated lenses did not cause any damage to the rabbit corneas after two months of continuous wear without the presence of adverse events. The in vitro study demonstrated that selenium-coated lenses produced a marked inhibition of P. aeruginosa colonisation. These data are encouraging.

Fimbrolide Disrupt Bacteria

Another study evaluated the use of fimbrolide-coated contact lenses. Fimbrolides disrupt the ability of bacteria to signal each other – a communication that is important in producing biofilms of bacteria on surfaces⁴. Fimbrolide-coated lenses reduced the adhesion of all P. aeruginosa, Staphylococcus aureus and Serratia marcescens by at least 67 per cent and the adhesion of Acanthamoeba was reduced by 70 per cent. There were no significant differences in ocular responses to fimbrolide-coated lenses compared with controls in either the one month animal model or overnight human trial⁵.

Silver Slows Adhesion

Another technology being explored is the use of silver for coating lenses and lens cases. Silver slows the adherence and colonisation of microorganisms by inhibiting DNA and RNA replication, disrupting the cell membrane, and interfering with cell respiration. While there has yet to be any reported clinical evaluation of such lenses, silver-coated lenses have been reported to reduce adhesion of P. aeruginosa and S. aureus in vitro⁶.

A series of antimicrobial lens cases using silver ions are now being released onto the market. When the solution interacts with the case, silver ions are slowly released which provide the necessary antibacterial properties. These lens cases showed significantly reduced levels of contamination compared to uncoated cases during in vivo and in vitro studies^7,8,9. The silver impregnated lens cases proved to be effective against Gram-negative bacteria. However, the presence of Gram-positive bacteria in the lens cases was still reported.

More Research Required

The technologies described above could offer patients significant benefits by reducing exposure to harmful microorganisms.

The availability of new strategies to reduce the incidence of adverse events without any undesirable effects on ocular health will be extremely positive for all contact lens wearers. However, some questions remain unanswered. For example, how efficacious are selenium- or silver-coated lenses in reducing or killing a wide variety of microorganisms such as Gram-positive and Gram-negative bacteria and fungi or protozoa that are commonly isolated from different inflammatory and infectious ocular conditions? Also, how biocompatible are any of the coating strategies on current contact lens materials and lens cases?

Moreover, there may be an effect on the normal ocular microbiota which perhaps protects the eye from colonisation by potential pathogens. The ideal antimicrobial strategy should target pathogenic microorganisms, while having little effect on the normal ocular microbiota – not an easy task!

The efficacy of antimicrobial contact lenses in reducing some ocular inflammatory adverse events ideally should be investigated through prospective clinical trials. However, due to the low incidence of microbial keratitis in contact lenses wearers, it is likely that the effect of this technology will need to be assessed by post-marketing surveillance studies.

Many questions remain to be answered in order to win the battle against the bugs. This is a fascinating field and the research community and industry need to work closely together in order to achieve their mutual goals of successful contact lens wear.

Dr. Percy Lazon de la Jara is Head of Clinical Research at the Clinical Research and Trials Centre, Brien Holden Vision Institute.

Images (further explanation): Colonisation/biofilm of Staphylococcus aureus and Pseudomonas aeruginosa on control or selenium coated contact lenses after 24 hours of incubation in vitro. Bacterial cells attached on lens surfaces were stained with florescence. Selenium-coated lenses showed less amount of bacteria on the lens surface, suggesting that selenium-coated lenses may be able to inhibit bacterial colonisation.

References:

1. McLaughlin-Borlace L, Stapleton F, Matheson M & Dart JK. Bacterial biofilm on contact lenses and lens storage cases in wearers with microbial keratitis. Journal of Applied Microbiology 84(5):827-38, 1998.

2. Vermeltfoort PB, Hooymans JM, Busscher HJ & Van der Mei HC. Bacterial transmission from lens storage cases to contact lenses-Effects of lens care solutions and silver impregnation of cases. Journal of Biomedical Materials Research, Part B, Applied Biomaterials, 87(1):237-43, 2008.

3. Mathews SM, Soallholz JE, Grimson MJ, Dubielzig RR, Gray T & Reid TW. Prevention of bacterial colonization of contact lenses with covalently attached selenium and effects on the rabbit cornea. Cornea 2006;25:806-814.

4. Kirisits MJ & Parsek MR. Does Pseudomonas aeruginosa use intercellular signalling to build biofilm communities? Cell Microbiol 2006;8: 1841-9.

5. Zhu H, Kumar A, Ozkan J, Bandara R, Ding A, Perera I, Steinberg P, Kumar N, Lao W, Griesser SS, Britcher L, Griesser HJ & Willcox MD. Fimbrolide-coated antimicrobial lenses: their in vitro and in vivo effects. Optom Vis Sci 2008 85:292-300.

6. S.Nissen & FH Furkert. Antimikrobielle Wirksamkeit einer Silberbeschichtung von Hydrogellinsen. Ophthalmologe 2000: 97:640-643.

7. Amos C & George M. Clinical and laboratory testing of a silver impregnated lens case. Contact Lens & Anterior Eye 29 (2006) 247-255

8. Mowrey-McKee M, George M, Ajello M &Tolliver C. Comparative Disinfection Efficacy of Currently Marketed One-Bottle Peroxide Solutions and Silver-Containing Lens Cases. BCLA 2008.

9. Lakkis C & Lakkola S. Antibacterial lens case efficacy during silicone hydrogel daily wear. ARVO 2009.