Exploring microwave irradiation as a method to disinfect contact lens cases

Abstract

Purpose

Biofilm formation caused by infrequent contact lens case replacement and the ineffectiveness of multi-purpose solutions (MPS) on biofilm removal is associated with high rates of bacterial keratitis infections. This study demonstrated biofilm elimination from the contact lens case by microwave irradiation.

Methods

Staphylococcus aureus biofilms indicative of 3–9 months of contact wear were cultured in contact lens cases and visualized with crystal violet (CV) staining. Biofilms in contact cases were then exposed to four treatment regimens: No treatment (n = 8), 45 s microwave irradiation (n = 8), tap water (n = 6), and MPS (n = 9). Bacterial survival was assessed by colony forming unit (CFU) assay using streak dilutions.

Results

Visualization of the biofilms through CV staining revealed that biofilms coalesce between ribs of the contact case. In 5/8 cases no CFU were cultivated from the case after treatment with microwave irradiation. In tap water and MPS the first dilution averaged 6 ± 2 and 31 ± 13 CFUs per plate, respectively, while microwave irradiation averaged < 1 CFU per plate. In Dilution 2, the average reduced to 0.7 ± 0.7 and 6 ± 5 CFUs per plate for tap water and MPS, respectively, while microwave irradiation had 0 CFUs in Dilution 2.

Conclusion

Biofilms that coalesce between the ribs of the contact case pose a threat because this area is difficult to thoroughly scrub and could act as a basis for infection through fouling of contact lenses. Of the four treatment regimens, microwave irradiation displayed the most consistent and highest rate of bacterial eradication. Tap water was less effective compared to microwave irradiation, and poses other harmful side effects, but greatly reduced CFU count compared to no treatment. MPS displayed the poorest bacterial eradication of the treatments. Thus, microwave irradiation is worth further investigation as a viable in-home disinfecting option.

Introduction

The popularization of soft contact lenses has corresponded to a dramatic increase in infectious keratitis, in some cases by 95% [1]. Keratitis is estimated to cost the US healthcare system $175 million annually [2]. There is a strong association between patient lens case contamination and keratitis [3], [4], [5]. Yet, many contact wearers regularly fail to follow their contact lens care guidelines. Bui et al. found that education alone does not sufficiently improve behavior regarding proper lens and case care [6]. Over 40% of adult users do not replace their case within the recommended 3 months [7], [8], [9], [10]. It is common for contact lens wearers to order 6 months to a year supply of lenses and this may be the only time in which they consider replacing their case, instead of the recommended 3 months. If not replaced regularly, bacteria can coalesce within the case forming biofilms known to play a significant role in infection [11], [12], [13], [14], which is illustrated in Fig. 1. Also, because bacteria inside the case are not easily visible, contact lens wearers may not be motivated to purchase or replace their case. It has been well established that multi-purpose contact solutions are not potent enough to disinfect the contact lens case once biofilms have formed [11], [15], [16], [17]. Possibly, because the physical attributes of biofilm that confer resistance to antibiotics may play a similar role in reducing the effectiveness of MPS [18], [19], [20]. Further, there is concern that MPS is not strong enough to address real world applications where there exists synergistic multi-species biofilms, and more invasive bacterial genotypes than those tested in International Organization for Standardization (ISO) guidelines [21], [22], [23], [24]. The majority of keratitis infections are bacteria based, particularly from the Staphylococcus aureus and Pseudomonas aeruginosa species [8], [12], [25]. Of these two species, S. aureus is more associated with recurrent infection and a higher rate of corneal transplantation [26]. Published work has already highlighted ISO testing standards for not encompassing realistic conditions for MPS biofilm eradication [21], [22], [27], [28]. Inadequate disinfection by MPS and improper patient compliance with cleanliness standards means that an additional at-home technique would be beneficial to reduce biofilm formation in contact cases.

The microwave is a common household appliance and the majority of contact wearers will have regular access to one. Previous work has proven microwave irradiation is effective against Candida albicans on dentures and Acanthamoeba in the contact lens case [29], [30]. This work seeks to further investigate the effects of microwave irradiation on S. aureus survival in comparison to three prevailing methods: no treatment, water-based treatment, and MPS treatment. At-home microwave irradiation may provide a route to disinfection of contact cases more favorable to patients than complete case replacement, as well as a safer alternative to the use of tap water which has been shown to greatly increase a wearers risk for infection [31].