January 2011
Volume 52, Issue 1
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Immunology and Microbiology  |   January 2011
Biocidal Efficacy of Silver-Impregnated Contact Lens Storage Cases In Vitro
Author Affiliations & Notes
  • Jaya Dantam
    From the Brien Holden Vision Institute, Sydney, Australia; and
    School of Optometry and Vision Science, University of New South Wales, Sydney, Australia.
  • Hua Zhu
    From the Brien Holden Vision Institute, Sydney, Australia; and
    School of Optometry and Vision Science, University of New South Wales, Sydney, Australia.
  • Fiona Stapleton
    From the Brien Holden Vision Institute, Sydney, Australia; and
    School of Optometry and Vision Science, University of New South Wales, Sydney, Australia.
  • Corresponding author: Jaya Dantam, Brien Holden Vision Institute, Level 5, North Wing, RMB, Gate 14, Barker Street, University of New South Wales, Sydney 2033, NSW, Australia; j.dantam@brienholdenvision.org
Investigative Ophthalmology & Visual Science January 2011, Vol.52, 51-57. doi:10.1167/iovs.09-4809
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      Jaya Dantam, Hua Zhu, Fiona Stapleton; Biocidal Efficacy of Silver-Impregnated Contact Lens Storage Cases In Vitro. Invest. Ophthalmol. Vis. Sci. 2011;52(1):51-57. doi: 10.1167/iovs.09-4809.

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Abstract

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 103, 104, 105, and 106 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 106), Serratia marcescens (3.3 ± 0.9 log reduction at 106), D. acidovorans (2.8 ± 0.1 log reduction at 103), and Fusarium solani (0.5 ± 0.2 at 103). 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 103). 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.

Microbial contamination of contact lens (CL) storage cases is common in asymptomatic wearers 1 7 and in CL wearers with sterile corneal infiltrates 8 and corneal infections. 9 12 In microbial keratitis, the causative organism may be traced to the storage case. 11 Storage case contamination occurs irrespective of disinfection system, 2 and up to 80% of microorganisms found in contaminated cases are potential ocular pathogens. 7 Hence, limiting storage case contamination is likely to enhance safe CL wear. 13  
Factors that may contribute to compromised microbial efficacy of the disinfecting solutions, and consequently storage case contamination, include improper cleaning of CLs and cases, topping up rather than replacing the solution daily in cases, and variable storage times between disinfection and insertion of the CLs. 3,14 Inadequate case cleaning 15 is the most common observation, occurring in 72% of CL wearers. Opportunistic contaminants are derived from the environment, CLs, ocular surface, hands, body, and contaminated disinfecting solutions. Interestingly, patient's own fingers are frequently cited as a source of contamination. 16  
The need for effective means of reducing storage case contamination has been identified in several studies. 5,15 Diverse approaches have been suggested, including development of easily cleanable case designs to avoid biofilm formation, 5,17 application of materials that discourage the adherence of bacteria, 14 disinfectant-enhancing agents such as sodium salicylate 18 that penetrate and neutralize the biofilm, and use of minimal concentrations of macrolide antibiotics to inhibit the production of glycocalyx by sessile organisms. 19 These proposed modes primarily address contamination at the surface level. Another approach may be to enhance the activity of care products, including the formulation of disinfecting solutions and the application of antimicrobial agents such as silver, polyquats, polymeric pyridinium compounds, selenium, nitric oxide, furanones, defensins, and lactoferrin. 20  
Silver compounds have wide-ranging applications because of their activity against a broad spectrum of bacteria and low toxicity to mammalian cells. 21 In the medical field, silver is widely used as a biocide in dental work, catheters, and wound burns. 22,23 There are several mechanisms through which silver slows adherence and colonization by microorganisms. These include inhibition of DNA and RNA replication, disruption of the cell membrane, interference with cell respiration, and alteration of enzyme conformation. 20,22 24  
Silver impregnation has recently been introduced as a strategy to reduce microbial contamination of lens storage cases. However, the in vitro performance of silver lens cases has undergone limited evaluation. Thus, in the present study, we aimed to evaluate the antimicrobial activities of silver lens cases in vitro under defined and repeatable experimental conditions. Additionally, to explore the interaction between silver-impregnated cases and different microorganisms and to understand the mechanisms of action, we monitored the amount of silver released during 10-hour cycling over 28 cycles. 
Materials and Methods
CL Storage Cases
Three commercially available silver-impregnated cases—MicroBlock (CIBA Vision, Atlanta, GA), i-clean (Sauflon Pharmaceuticals Ltd., London, UK), and Nano-case (Marietta Vision, Marietta, GA)—were evaluated for their antimicrobial efficacy in comparison with regular lens cases without any silver incorporation (CIBA Vision). 
Challenge Organisms
The group of organisms tested consisted of the panel of five organisms recommended in ISO14729 for testing of CL care products and two clinical isolates recovered from lens storage cases (Table 1). 
Table 1.
 
Microorganisms Used in the Study
Table 1.
 
Microorganisms Used in the Study
Organism Source
Pseudomonas aeruginosa ATCC 9027 Otic infection
Serratia marcescens ATCC 13880 Pond water
Staphylococcus aureus ATCC 6538 Human isolate
Candida albicans ATCC 10231 Bronchomycosis
Fusarium solani ATCC 36031 Corneal ulcer
Delftia acidovorans 001 Lens case from an asymptomatic CL wearer
Stenotrophomonas maltophilia 002 Lens case from an asymptomatic CL wearer
Preparation of Inoculum
Each challenge bacterial strain stored at −80°C was inoculated onto tryptic soy agar (TSA) plate and incubated at 37°C for 18 to 24 hours. The harvested bacterial cells were suspended in 10 mL sterile phosphate buffered saline (PBS) in a 50-mL polypropylene test tube and centrifuged for 10 minutes at 3000 rpm to collect bacterial cells after the supernatant was discarded. The suspensions of organisms were prepared in PBS at an optical density of 0.1 at 660 nm (108 CFU/mL), then serially diluted (10-fold) in PBS to the designated inoculum concentrations. 
Each challenge fungal strain, stored at −80°C, was inoculated onto potato dextrose agar (PDA) plates and incubated at 25°C for 7 to 10 days for Fusarium solani and 24 hours at 37°C for Candida albicans. F. solani was harvested and suspended into sterile PBS. The suspension was filtered through a 70-μm strainer to remove the hyphal fragment and then resuspended to an optical density of 2.6 at 660 nm (108 CFU/mL). For C. albicans, one to two colonies from pure culture were suspended in PBS and filtered through a 40-μm strainer. The optical density of C. albicans was then adjusted to 1.45 to 1.50 (108 CFU/mL). The suspension was serially diluted in PBS to the designated inoculum concentrations. All the experiments were repeated three times. 
An inoculum size of 106 CFU/mL is recommended in ISO14729 for testing of antimicrobial efficacy of CL care products. To determine the minimum inoculum concentration at which the lens cases display bactericidal activity, four different inocula (106 CFU/mL, 105 CFU/mL, 104 CFU/mL, and 103 CFU/mL) were used for the three ISO panel bacterial strains (Pseudomonas aeruginosa ATCC 9027, Staphylococcus aureus ATCC 6538, and Serratia marcescens ATCC 13880). For fungal strains and clinical isolates of bacteria, the challenge concentration was 103 CFU/mL. 
Exposure of Challenge Organisms into CL Storage Case
Each well of the lens cases was filled with 2 mL inoculum suspension. At timed storage intervals (i.e., 6, 10, and 24 hours), an aliquot of 50 μL bacteria or 100 μL fungi was removed and serially diluted (1:10) in neutralizing broth. Dilutions were plated onto the TSA with Tween 80 and lecithin for recovery of bacteria and PDA for recovery of fungi after incubation at appropriate temperatures and durations. CFUs were then enumerated, and differences in log CFU were calculated for each storage time compared with time zero recoveries. 
Quantification of Silver Release from the Silver-Impregnated CL Storage Cases
The MicroBlock, i-clean, and Nano-case were tested for silver release. Regular lens cases were included as control. Each well of the lens case was filled with 3 mL Dulbecco's PBS (DPBS; without calcium and magnesium; catalog no. 141900; Invitrogen, Carlsbad, CA) and then incubated at 25°C. DPBS was collected after 6, 10, and 24 hours' incubation. This experiment was repeated three times. Another set of lens cases was then cycled daily with 3 mL fresh DPBS for 10 hours over 28 days to approximate routine use of the CL storage case. Silver analysis was performed on the duplicate samples collected on days 7, 14, 21, and 28 at the analytical center at the University of New South Wales using inductively coupled plasma mass spectrometry. 
Statistical Analysis
Data analysis was conducted (SPSS, version 16; SPSS, Chicago, IL), and statistical significance was set at 5%. Log differences were summarized as mean ± SD and were compared among the four different cases using a univariate ANOVA for each test organism at each incubation time; Bonferroni correction was used for post hoc multiple comparisons. 
Results
In Figures 1 to 4, the bar indicates the mean of the log difference, and the error bars indicate SD. A negative value on the y-axis indicates log reduction, a positive value on the y-axis indicates growth, and the x-axis represents the three (6, 10, and 24 hours) different time points. 
Figure 1.
 
Log difference of P. aeruginosa at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case.
Figure 1.
 
Log difference of P. aeruginosa at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case.
Figure 2.
 
Log difference of S. marcescens at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case.
Figure 2.
 
Log difference of S. marcescens at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case.
Figure 3.
 
Log difference of S. aureus at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. †P ≤ 0.007, ‡P ≤ 0.02 for i-clean against CIBA control, MicroBlock, and Nano-case.
Figure 3.
 
Log difference of S. aureus at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. †P ≤ 0.007, ‡P ≤ 0.02 for i-clean against CIBA control, MicroBlock, and Nano-case.
Figure 4.
 
Log difference of C. albicans (A), F. solani (B), D. acidovorans (C), and S. maltophilia (D) at challenge inoculum of 103 CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case. §P < 0.02 for Nano-case against the other cases.
Figure 4.
 
Log difference of C. albicans (A), F. solani (B), D. acidovorans (C), and S. maltophilia (D) at challenge inoculum of 103 CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case. §P < 0.02 for Nano-case against the other cases.
A significant (P ≤ 0.001) log reduction for the MicroBlock compared with the other lens cases was clearly revealed in Figures 1, 2, and 4 against the test strains P. aeruginosa ATCC 9027, S. marcescens ATCC 13880, Delftia acidovorans 001, and F. solani ATCC 36031 after 24 hours of storage at all the challenged inocula. As can be seen in Figures 1 and 2, MicroBlock exhibited complete kill effect at lower inocula (Figs. 1A, 1B) after 24 hours of storage against P. aeruginosa ATCC 9027 at 103 and 104 CFU/mL and S. marcescens ATCC 13880 at 103 CFU/mL (Fig. 2A). Additionally, at higher inocula (Figs. 1C, 1D, 2C, 2D), these cases showed a log reduction of 3.2 ± 1.4 (mean ± SD) and 2.4 ± 0.5 against P. aeruginosa ATCC 9027 at 105 and 106 CFU/mL and 3.8 ± 0.3, 4.2 ± 1.3, 3.3 ± 0.9 against S. marcescens ATCC 13880 at 104, 105, and 106 CFU/mL, respectively, after 24 hours of storage. Noticeably, after 6 and 10 hours of storage, MicroBlock demonstrated significantly (P < 0.001; Fig. 4) higher activity than the other storage cases against D. acidovorans 001 and F. solani ATCC 36031, and log reductions of 2.8 ± 0.1 and 0.5 ± 0.2 were observed, respectively, after 24 hours of storage. In addition, these cases revealed significant (P < 0.02; Fig. 3) activity against S. aureus ATCC 6538 compared with CIBA control by showing log reductions of 1.3 ± 0.9 at 103 and 2.4 ± 1.1 at 106 CFU/mL after 24 hours' storage. MicroBlock showed no effect on S. maltophilia 002 (Fig. 4). 
Interestingly, i-clean was more effective against S. aureus ATCC 6538 (P < 0.007) than other cases at all the inocula after 24 hours (Fig. 3). In addition, log reductions of 2.8 ± 1.5, 3.7 ± 1.9, and 5.4 ± 1.1 were observed at 104 CFU/mL, 105 CFU/mL, and 106 CFU/mL, respectively, after 24 hours. Against this test strain, at the lowest challenge inocula, i-clean cases demonstrated significantly greater (P ≤ 0.02) activity when compared with the other cases at 6 hours and 10 hours, with further complete bactericidal activity after 24 hours. There are no noteworthy observations with i-clean against P. aeruginosa ATCC 9027, S. marcescens ATCC 13880, C. albicans ATCC 36031, F. solani ATCC 36031, D. acidovorans 001, and S. maltophilia 002
The Nano-case showed significant (P < 0.02) activity against S. maltophilia 002 compared with other cases at 24 hours, with a small log reduction of 0.2 ± 0.3. Nano-case showed no appreciable activity against all other strains. 
Figure 5a shows the silver release data for 3 cases tested at 6, 10, and 24 hours, and Figure 5b shows the silver release over a month at 7, 14, 21, and 28 days. MicroBlock, with a range of 16–45 μg/L over 28 days, was the only storage case to demonstrate measurable release of silver into the DPBS. 
Figure 5.
 
Quantification of silver release (μg/L) in three commercially available silver-impregnated contact lens storage cases compared with control case with no silver components. (A) Over 1 day. (B) Over 1 month.
Figure 5.
 
Quantification of silver release (μg/L) in three commercially available silver-impregnated contact lens storage cases compared with control case with no silver components. (A) Over 1 day. (B) Over 1 month.
Discussion
Contamination of CL storage case remains common in the community and is a risk factor for microbially driven adverse events in CL wear. Additionally, good compliance with hygiene recommendations does not guarantee contamination-free CL storage cases. 12 This poses a challenging situation for CL practitioners and wearers. Silver-impregnated cases have been introduced recently as a possible solution to persistent microbial contamination. Although we anticipate that the antimicrobial storage cases should demonstrate significant antimicrobial activity in vitro, there is no available standard for testing the efficacy of these products. We have elected to use a modified version of the stand-alone test procedure recommended by the International Organization for Standardization (ISO) for testing of CL care products (ISO 14729) to determine the antimicrobial efficacy of the lens cases in the present study. The sampling time points selected represent recommended disinfection time with multipurpose solution and the routine period for storage of CLs. 
Using three currently available commercial silver products, we have demonstrated significant antibacterial activity in vitro after 24 hours' exposure. MicroBlock cases were effective against P. aeruginosa ATCC 9027 and S. marcescens ATCC 13880. Indeed, for the lower inocula, complete microbial killing was achieved. This is relevant given that these two organisms persist as the main Gram-negative bacteria associated with CL-associated microbial keratitis. 25 31 The MicroBlock plastic storage case contains ionic silver in a glass powder additive, distributed through the bulk polymer. 32 Silver ions are released from the surface in the presence of moisture. Previous in vitro studies with this product have indicated complete kill of isolates of P. aeruginosa and Citrobacter amalonaticus at 103 CFU/mL in PBS after 24 hours. 32 Compared with non-silver–impregnated storage cases, the MicroBlock case showed a significant reduction in recoverable organisms after 24 hours' incubation for a range of Gram-negative bacteria. 32 The present study showed limited activity for the other antimicrobial storage cases against P. aeruginosa and S. marcescens, which is consistent with findings of a previous report. 33  
Bacterial biofilm formation in CL storage cases has been implicated in persistent microbial contamination in CL storage cases, particularly when there has been good compliance with lens hygiene. 12,17,19,34 Although not specifically tested in this study, it would be anticipated that the major effect of a surface antimicrobial such as silver would be to limit microbial adhesion and biofilm formation. In a study of the impact of silver storage cases and multipurpose solution on biofilm formation, fewer viable cells were recovered from a P. aeruginosa biofilm formed on MicroBlock cases than on non-silver impregnated cases, though no significant effect was demonstrated for S. aureus biofilms. 35  
In the present study, i-clean cases demonstrated significant activity against S. aureus ATCC 6538 compared with the other test cases. Although Gram-positive bacteria are less commonly implicated in CL-related corneal infections, 31,36,37 they are a common cause of CL storage case contamination. 2,6,38,39 The lack of planktonic S. aureus activity for the MicroBlock case is consistent with the findings of a study on activity against bacterial biofilm. 35 A clinical study, however, has demonstrated some reduction in contamination by coagulase-negative staphylococci after 1 month of use of silver-impregnated (MicroBlock) and non-silver–impregnated storage cases in a contralateral study design. 32  
Activity was noted for the MicroBlock storage case against the clinical isolate D. acidovorans 001 and limited activity (0.5 log unit reduction) for the ISO strain of F. solani. The recent outbreaks of Fusarium keratitis associated with daily CL use and the presence of Fusarium contamination of CL storage cases 40 42 illustrates the importance of ongoing antimicrobial activity against such environmental contaminants. 
S. maltophilia showed low susceptibility to most silver cases, and Nano-case showed the greatest activity of <0.5 log unit reduction. No cases showed activity against the yeast C. albicans. Given that the three cases appear to use broadly similar silver impregnation technology, the wide differences in spectrum and degree of antimicrobial activity are intriguing, notwithstanding that the susceptibility of microorganisms to silver may vary with structural characteristics of the species. 43  
In an attempt to understand the differences in activity between the test CL storage cases, silver ion release was quantified for each experimental condition. MicroBlock was the only storage case to demonstrate measurable release of silver into the DPBS over 28 days. Interestingly, the pattern of release for 24 hours was consistent with the antimicrobial activity against the Gram-negative species reported here. The activity of the other two cases, specifically of the i-clean storage case against S. aureus ATCC 6538, does not appear to be related to the degree of silver release into the solution. One possible approach may be to investigate the surface activity and biofilm formation for each type of storage case or to precondition cases before exposure. However, we would speculate that the manufacturing method and the form of silver in the lens cases are the determinants of the mechanism of action of these lens cases. This study endeavored to establish the antimicrobial effect of the storage case only. In clinical use, the storage cases are used with the respective multipurpose solution, presumably to enhance the activity of the overall system. The effect of the lens solution combination requires further investigation. 
Based on these results, it is apparent that most activity of the silver lens cases against the test strains was demonstrated at 24 hours, which might benefit occasional CL wearers. Nevertheless, in a practical situation, wearers tend to store the lenses between 8 and 12 hours. All cases appear to show limited activity in vitro at the 6-and 10-hour time points representing the regular storage time. Conceivably, with preconditioning, cases might exhibit efficacy in the earlier time points, which may be closer to the real-time storage of the CL wearers. There is discrepancy regarding the replacement of CL storage cases. Usually, manufacturers recommend replacing the CL storage case every month or using a new pack of multipurpose disinfecting solution; the FDA suggests replacing the storage case every 3 to 6 months, and eye care professionals suggest replacing the case every 1 to 6 months. Future studies are needed in this direction to establish optimal instructions. 44  
This study determined activity against planktonic organisms, and silver-impregnated cases that release silver ions into a fluid-filled storage case would be likely to perform better than those with predominantly surface activity. Limited information is available to describe the mode of action of two of the cases evaluated in this study. Evaluation of microbial adhesion, biofilm formation, surface characteristics, impact of care solution, and performance in vivo will ultimately provide a more comprehensive evaluation of the mode of action and clinical benefit of this technology. 
In conclusion, this study has developed a reliable test method for comparing the efficacy of antimicrobial CL storage cases, based on a modified version of the stand-alone test procedure recommended by ISO for the testing of CL care products (ISO 14729). MicroBlock cases were effective against most of the Gram-negative bacteria except S. maltophilia in vitro, whereas i-clean cases may be more active against Gram-positive bacteria. These experiments were conducted in PBS to elicit the efficacy of the silver lens storage cases alone. Further studies are in progress with silver-impregnated cases in conjunction with multipurpose solutions and CLs to better model the in vivo situation. Clinical studies comparing the different technologies will assist in determining their efficacy in use and any impact on microbially driven adverse events. Thorough understanding of the benefits and optimal use of these products will enable practitioners to make appropriate recommendations to CL wearers based on the best evidence. 
Footnotes
 Supported by a tuition fee remission scholarship from University of New South Wales through School of Optometry and Vision Science and faculty research stipend from the Brien Holden Vision Institute, Sydney, Australia.
Footnotes
 Disclosure: J. Dantam, None; H. Zhu, None; F. Stapleton, None
The authors thank Thomas John for valuable advice on statistical analysis, Rabeya Akter for help in processing the samples using ICP-MS, and research associate Rani Bandara for microbiology assistance. 
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Figure 1.
 
Log difference of P. aeruginosa at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case.
Figure 1.
 
Log difference of P. aeruginosa at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case.
Figure 2.
 
Log difference of S. marcescens at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case.
Figure 2.
 
Log difference of S. marcescens at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case.
Figure 3.
 
Log difference of S. aureus at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. †P ≤ 0.007, ‡P ≤ 0.02 for i-clean against CIBA control, MicroBlock, and Nano-case.
Figure 3.
 
Log difference of S. aureus at challenge inocula of 103 (A), 104 (B), 105 (C), and 106 (D) CFU/mL for the test time points. †P ≤ 0.007, ‡P ≤ 0.02 for i-clean against CIBA control, MicroBlock, and Nano-case.
Figure 4.
 
Log difference of C. albicans (A), F. solani (B), D. acidovorans (C), and S. maltophilia (D) at challenge inoculum of 103 CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case. §P < 0.02 for Nano-case against the other cases.
Figure 4.
 
Log difference of C. albicans (A), F. solani (B), D. acidovorans (C), and S. maltophilia (D) at challenge inoculum of 103 CFU/mL for the test time points. *P ≤ 0.001 for MicroBlock against CIBA control, i-clean, and Nano-case. §P < 0.02 for Nano-case against the other cases.
Figure 5.
 
Quantification of silver release (μg/L) in three commercially available silver-impregnated contact lens storage cases compared with control case with no silver components. (A) Over 1 day. (B) Over 1 month.
Figure 5.
 
Quantification of silver release (μg/L) in three commercially available silver-impregnated contact lens storage cases compared with control case with no silver components. (A) Over 1 day. (B) Over 1 month.
Table 1.
 
Microorganisms Used in the Study
Table 1.
 
Microorganisms Used in the Study
Organism Source
Pseudomonas aeruginosa ATCC 9027 Otic infection
Serratia marcescens ATCC 13880 Pond water
Staphylococcus aureus ATCC 6538 Human isolate
Candida albicans ATCC 10231 Bronchomycosis
Fusarium solani ATCC 36031 Corneal ulcer
Delftia acidovorans 001 Lens case from an asymptomatic CL wearer
Stenotrophomonas maltophilia 002 Lens case from an asymptomatic CL wearer
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