Biocidal efficacy of silver-impregnated contact lens storage cases in vitro.

Purpose: Silver-impregnated contact lens (CL) storage cases are designed to reduce microbial contamination during use, but there are limited data on their effectiveness. This study evaluated early antimicrobial activity of silver-impregnated CL cases and silver-release characteristics in vitro.

Methods: Three silver-impregnated CL storage cases-MicroBlock (CIBA Vision, Atlanta, GA), i-clean (Sauflon Pharmaceuticals Ltd., London, UK), and Nano-case (Marietta Vision, Marietta, GA)-were evaluated. Test organisms included the ISO14729 panel and two clinical isolates, Delftia acidovorans and Stenotrophomonas maltophilia. Each well of the case was challenged with 2 mL of the organism in phosphate-buffered saline at 10(3), 10(4), 10(5), and 10(6) CFU/mL. Survivors were recovered after 6, 10, and 24 hours' incubation at 25°C. Inductively coupled plasma mass spectrometry was used to quantify the release of silver from the cases for similar incubation conditions and for time points up to 28 days.

Results: Significant differences in antimicrobial activity were observed between cases (P ≤ 0.001). Activity was apparent only after 24 hours. MicroBlock showed the highest activity against Pseudomonas aeruginosa (2.4 ± 0.5 log reduction at 10(6)), Serratia marcescens (3.3 ± 0.9 log reduction at 10(6)), D. acidovorans (2.8 ± 0.1 log reduction at 10(3)), and Fusarium solani (0.5 ± 0.2 at 10(3)). The i-clean case was most effective against Staphylococcus aureus (5.4 ± 1.1 log reduction), whereas Nano-case showed the greatest activity against S. maltophilia (0.2 ± 0.3 log reduction at 10(3)). MicroBlock was the only case to demonstrate silver release over 28 days.

Conclusions: Current silver-impregnated CL storage cases show variation in their in vitro antimicrobial activity. Broadly, the MicroBlock case demonstrated robust activity against most Gram-negative bacteria, whereas the i-clean case was more effective against S. aureus. Silver-release data suggest different modes of action for different cases.