The use of microwaves for sterilization has been reported in various fields of medicine including the disinfection of soft contact lenses, dental instruments, dentures, milk, and urinary catheters for intermittent self-catheterization (Centers for Disease Control and Prevention [CDC], 2016). Depending on the challenge organism, radio-frequency waves produced by a “home-type” microwave oven (2,450MHz) fully inactivate bacterial cultures, mycobacteria, viruses, and G. stearothermophilus spores within 60 seconds to five minutes (CDC, 2016).

Microwave disinfection of soft contact lenses has been explored (Harris et al, 1990). Contact lens cases filled with sterile non-preserved saline were contaminated with one of six of the U.S. Food and Drug Administration’s test challenge microorganisms at a concentration of approximately 103 colony-forming units per milliliter (CFU/ml). Twenty cases were placed in a 10cm diameter circle on the rotating plate of a standard microwave oven (2,450MHz/650W).

The cases were exposed to high-intensity microwave irradiation for zero to 15 minutes. None of the six challenge microorganisms evaluated survived two minutes or longer of microwave exposure. Thus, microwave disinfection of up to 40 soft contact lenses at one time has been reported to be a convenient, rapid, and effective method of disinfection. It is also adaptable as an in-office soft contact lens disinfection procedure.

Contact lens cases that have been contaminated are a predisposing risk factor for Acanthamoeba keratitis. A study evaluated what effect microwave irradiation has on Acanthamoeba trophozoites and cysts (Hiti et al, 2001). Various types of contact lens cases were contaminated with trophozoites and cysts of three different Acanthamoeba species (i.e., A. comandoni, A. castellanii, and A. hatchetti) and were irradiated in a microwave for three, five, and eight minutes, respectively.

All cysts of the different Acanthamoeba strains and trophozoites were effectively killed, even after only three minutes of microwave irradiation. There were not any negative effects of irradiation on the contact lens cases themselves. Hence, the authors concluded that microwave treatment is an effective, simple, and inexpensive method to keep contact lens cases free of Acanthamoeba, to substantially reduce the risk of Acanthamoeba keratitis.

Biofilm formation with infrequent contact lens case replacement and inadequate disinfection with multipurpose solutions on biofilm removal is associated with high rates of bacterial keratitis infections. A study evaluated the use of microwave irradiation to eliminate biofilm in contact lens cases (Goble et al, 2022). Staphylococcus aureus biofilms were cultured in contact lens cases, visualized with crystal violet staining, and then exposed to four treatment regimens (no treatment, 45 seconds of microwave irradiation, tap water, and multipurpose solution). CFU assay using streak dilutions was utilized to assess bacterial survival.

Crystal violet staining demonstrated that biofilms coalesce between the ribs of the contact lens case. After treatment with microwave irradiation, in 5 out of 8 cases, no CFUs were cultivated. Microwave irradiation demonstrated the most consistent and highest rate of bacterial eradication of the four treatment regimens.

Further investigation is required to determine whether microwave irradiation is a viable at-home and in-office disinfecting option. CLS

REFERENCES

1. Centers for Disease Control and Prevention. Infection Control: Other Sterilization Methods. 2016 Sept 18. Available at cdc.gov/infectioncontrol/guidelines/disinfection/sterilization/other-methods.html#:~:text=Microwaves%20are%20used%20in%20medicine,%2C%20do%20not%20melt)%20931 . Accessed Aug. 27, 2022.
2. Harris MG, Rechberger J, Grant T, Holden BA. In-office microwave disinfection of soft contact lenses. Optom Vis Sci. 1990 Feb;67:129-132.
3. Hiti K, Walochnik J, Faschinger C, Haller-Schober EM, Aspöck H. Microwave treatment of contact lens cases contaminated with acanthamoeba. Cornea. 2001 Jul;20:467-70.
4. Goble BJ, Boyd JD, Grady ME. Exploring microwave irradiation as a method to disinfect contact lens cases. Cont Lens Anterior Eye. 2022 Aug;45:101522.