MicroBlock: the new antimicrobial lens case
Although currently marketed contact lens care products are extremely effective against bacteria and fungi, contamination of contact lens storage cases may still occur.
Previous studies have shown that microbial contamination of lens cases occurs in 20-80 per cent of contact lens wearers.1 The most common types of microbial contaminants isolated from lens cases (and approximate incidences) are, in decreasing prevalence, bacteria (78 per cent), fungi (24 per cent), and protozoa (20 per cent).2
The most frequently cited source of lens case contamination is the lens wearer's fingers, due to handling of the lens case and contact lenses.3 Contamination of care products such as disinfecting solutions and saline, as well as other fluids that come into contact with the fingers and case, such as tap water, may also contribute to lens case contamination. Due to the efficacy of current care products, contamination of disinfecting solutions occurs more rarely than that of saline or tap water.4
The degree of lens case contamination appears to be dependent upon several factors. These may include climate, personal hygiene (failure to clean lenses and storage cases routinely, infrequent hand washing, reuse of case solution for several cleaning periods), the addition of fresh fluids (including preservative-free saline and tap water) to residual fluid in the lens case, and variable storage time of the lenses between disinfection and insertion. The type of lens disinfectant used and the chemical composition of the contact lenses being stored also influence the level of contamination.5-8
The majority of microbes isolated from cases are normal skin flora, especially Staphylococcus species, and many exist in mixed cultures.2 Other frequently isolated microbes, such as the Bacillus species, are ubiquitous in the environment6,7,9 and are probably introduced by handling of lens care components.7 Most fungi isolated are typically found in association with bacteria.2
Potential ocular pathogens, such as the bacteria Pseudomonas aeruginosa and Serratia marcescens, are occasionally identified. Pseudomonas aeruginosa is the most common causative agent of microbial keratitis, a rare but progressively destructive disease of the cornea. Organisms isolated from contact lens-associated microbial keratitis are often identical to those isolated from the patient's lens case.2 Problems such as dryness symptoms, itching and redness have been reported by patients with contaminated lens cases.10 Thus, lens case contamination may contribute to contact lens drop-out.
Several infecting micro-organisms have an inherent propensity to attach to surfaces, including contact lenses and lens cases. Adhesion to contact lenses increases when the lenses carry deposits, such as protein.7 Acanthamoeba accounts for approximately 8 per cent of total microbes isolated from contaminated lens cases.2 Amoebae may adhere directly to the surface of lenses and lens cases. Bacteria adhered to these surfaces act as a food source for Acanthamoeba growth and replication. Contact lenses, therefore, may serve as a vector for transmitting organisms from contaminated cases to the cornea.
Many microbial species are capable of forming a biofilm on contact lenses and lens cases that contribute to the pathogenesis of keratitis.6-11 Biofilms consist of a consortium of micro-organisms (somewhat less than one-third of the total film) that may embed themselves in a 'slime' or glycocalyx matrix which they secrete and which makes up a majority of the film.12 Planktonic bacteria that enter the lens case may adhere to the surface and rapidly colonise the case as sessile microbes when deprived of nutrients.13
Cells growing in a biofilm are typically more resistant to disinfection than planktonic (free-swimming) micro-organisms because the slime makes it harder for disinfectants to penetrate effectively to cells in the biofilm's interior.6,12 Cells on the outer surface of the biofilm may serve as protectors for interior cells.
Development of an antimicrobial lens case
In an attempt to reduce the degree of contamination and, potentially, biofilm build-up on lens cases, a polypropylene lens case containing the inorganic antimicrobial agent silver was developed and evaluated. Silver has been used as an antimicrobial agent for centuries because of its low toxicity and ability to target multiple micro-organism sites, a characteristic that greatly impedes the development of bacterial resistance.14
Silver acts upon direct contact with micro-organisms. The exact antimicrobial mode of action is not known, but several possible mechanisms include membrane disruption, inhibition of respiration, inactivation and alteration of enzyme conformation, and formation of complexes with DNA bases, which prevents cell replication.14
In the antimicrobial lens case, silver ions are enclosed in a carrier and are released only when moisture comes into contact with the case. Silver ions are released in exchange for other environmental ions, such as sodium, until equilibrium is reached. Conservation of charge is therefore maintained.
The resulting product, MicroBlock, is a lens case with the long-lasting ability to kill micro-organisms upon contact. Because the silver is injection-moulded into the plastic, the efficacy of the lens case is not affected by surface abrasion.
Tests for antimicrobial efficacy and toxicity
Several tests have been performed to assess the efficacy and cytotoxicity of the lens case.
In the first test, the bowls of polypropylene lens cases containing the antimicrobial agent and standard-production lens cases with no antimicrobial (used as controls) were directly challenged with 1.0x103 CFU/ml (colony forming units per millilitre) suspensions of bacteria and fungi in phosphate buffered saline. Strains tested included a clinical isolate from a microbial keratitis patient and ATCC (American type culture collection) lens disinfection solution challenge strains. A low inoculum was employed to mimic more closely real-world exposure levels and to best observe direct contact action of the antimicrobial additive. Samples were assessed in triplicate after 24 hours.
In a second test, all lens cases (test and controls) were evaluated by exposure to 1.0x105 CFU/ml sterile buffer suspensions of Pseudomonas aeruginosa, Staphylococcus aureus, Serratia marcescens, and Escherichia coli in a flask for 24 hours on a wrist-action shaker at 37