Bacterial populations on high-Dk silicone hydrogel contact lenses: effect of length of wear in asymptomatic patients
Willcox, Harmis and Holden, May 2002,

Reviewed by: L. Sorbara O.D. M.Sc. FAAO

Contact lens wear, especially extended wear can cause corneal inflammation and very rarely severe corneal infections.^1-3 Contact lens corneal infections or microbial keratitis has been shown to be related to bacterial contamination of the CL by gram negative organisms.^3-5The lens acts as a vector for organisms from environmental sources and prolongs the retention time of these organisms on the ocular surface. This increase in bacterial load at the corneal surface, together with bacterial toxins or actual bacterial invasion causes tissue damage⁶ or corneal infection due to Pseudomonas aeruginosa.^7-8

Lens contamination with Gram-negative bacteria are rarely isolated from the lenses of asymptomatic wearers following lens wear.⁹ When lens contamination does occur it involves small numbers of organisms (<30 colony-forming units) where coagulase-negative staphylococci are the most common.^9-10 In a study considering lens type, there was no difference between two lens groups (1 and 4) on the frequency or type of lens colonization over time.¹¹ Willcox and Stapleton¹² confirmed that asymptomatic lens use does not appear to modify the lens biota with lens wear experience. They concluded that lens contamination by pathogenic organisms is rare in asymptomatic wearers.

Recently, the use of silicone hydrogel lens materials for 30 night continuous wear has increased. The question as to whether these materials have any different properties with respect to microbial contamination has been investigated by Willcox et al (2002) and by Keay et al (2001). The Keay study looked at a comparison of high Dk silicone hydrogel (lotrafilcon A) lenses, worn for 30 nights and HEMA based hydrogel (etafilcon A) lenses, worn for 6 nights.¹³ Results indicated that there were no differences in the proportion of lenses colonized by Propionibacterium spp. (48% vs 43%) or coagulase-negative staphylococci (47% ve 54%) or any Gram-negative bacteria. Also, there were no differences in the number of sterile samples (28% vs 27%) from each group.

The purpose of Willcox’s study was to answer the question whether 30 days of continuous wear of silicone hydrogel lenses alters the type or population of bacteria in the eye over a two year period.

The study included neophyte patients to extended wear and to the wear of high Dk lenses. They continued throughout the study only if they were free of ocular disease and symptoms over the two year period. Twenty seven patients were fitted with lotrafilcon A fluorosilicone hydrogel lenses and the lenses were worn for a continuous 30 night schedule in both eyes with monthly replacement. The lenses were collected at one, six and twenty-four month periods. All the lenses were removed aseptically from the eye at each visit and placed in sterile vials and taken to the microbiology laboratory within 30 minutes of collection. Microbial growth on various media was enumerated and the number of colony forming units (cfu’s) was calculated. These colonies were then identified using standard laboratory methods. Full anterior segment eye examinations were done at each visit to ensure normal bacterial flora during asymptomatic lens wear.

This study showed that during a two year span of continuous wear with high Dk silicone hydrogel lens, 37% of the samples were sterile. Of those lenses that were contaminated, Willcox’s paper shows us that continuous 30 day wear of silicone hydrogel lenses, when measured at the one, six and twenty-four month periods, do not appear to alter the type and number of bacteria that colonise contact lenses during monthly replacement, in asymptomatic patients.

The most frequently isolated and most numerous microbes that colonised the contact lenses during wear were the coagulase-negative staphylococci (that is Staphylococcus epidermidis) followed by Propionibacterium sp.. Although the levels of Propionibacterium sp increased slightly during the two year wear period, there were no associated clinical signs or symptoms.

Keay’s study (2001), that examined one year of extended wear with both high Dk silicone and HEMA-based hydrogel lenses, found sterile lenses in 27% and 28% respectively, of the total.¹³ The two papers report a difference in the number of sterile lenses likely due to the change in the ability to isolate and grow bacteria over time.

The authors remark that in general the lenses that were contaminated were sparsely colonised during asymptomatic wear.


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