Abstract
Purpose.:
To evaluate microbial contamination in silver-impregnated contact lens (CL) storage cases while establishing the effect of “wet ” and “dry ” case maintenance and to determine its association with clinical signs, symptoms, and compliance.
Methods.:
Two noncontemporaneous prospective studies were conducted. Regular storage cases in study 1 (n = 40) and silver-impregnated cases in study 2 (n = 41) were used in conjunction with CL solution and CLs (balafilcon A). Cases were replaced monthly and collected at 1, 3, and 4 (for silver-impregnated cases only) months. Regular cases and the fourth-month silver-impregnated cases were maintained dry, and the other cases were maintained wet between uses. At collection, storage cases were sampled and cultured for microbial identification and enumeration. Ocular clinical findings, subjective responses to CL wear, and compliance were recorded at each visit.
Results.:
The percentages of microbial contamination for silver-impregnated and regular cases were 71% and 82% respectively. There were significantly (P < 0.005) fewer organisms in silver-impregnated cases (1.7 log CFU per well) than in regular cases (4.1 log CFU per well). In particular, silver-impregnated cases showed lower levels of Gram-negative bacteria (P = 0.04), Gram-positive bacilli (P = 0.03), and fungi (P = 0.006). Maintaining the silver-impregnated cases wet resulted in a lower percentage of contamination (71%; P < 0.01) than maintaining them dry (94%). There was no association between any clinical signs, symptoms, or compliance and microbial contamination of storage cases.
Conclusions.:
More than 70% of the storage cases used in daily wear CL care for a month was contaminated irrespective of the types of cases. However, silver-impregnated cases were colonized by reduced levels of Gram-negative bacteria. (www.anzctr.org.au number, ACTRN12609000165280.)
Microbial keratitis is a rare, devastating ocular complication associated with contact lens (CL) wear.
1 –6 There are approximately 125 million CL wearers in the world,
7 and approximately 5 per 10,000 wearers per year develop microbial keratitis.
1,3,8 –10 Microbial contamination of the CL care system
11 –15 has been identified as a risk factor for CL-associated microbial keratitis. Other established factors include soft CL
4 and an extended-wear schedule.
4,8,10,16 –18 Among contact lens care products, contamination occurs most frequently
19 and is highest in CL storage cases.
20 Recommendations to wearers for lens and case hygiene vary among practitioners, the US Food and Drug Administration, and the industry,
21 and inadequate case cleaning has been observed in 61%
22 to 72%
23 of CL wearers. The frequency of microbial contamination of storage cases can range from 24% to 81% of wearers.
14,24 –33 Unlike CL contamination, which is usually bacterial in nature, storage case contamination is often polymicrobial.
25,27 Microbial isolates recovered from the contaminated storage cases are often identical with those isolates recovered from the cornea in microbial keratitis.
15,34
Storage cases may be reservoirs for pathogenic microorganisms, including
Pseudomonas aeruginosa,
2,3,8,35,36 Serratia marcescens,
3,12 fungi,
14,26,27 and protozoa.
25 –28 Contaminants derived from the environment, multipurpose solutions, and the hands/fingers of the CL wearers might be introduced into the CL storage cases and eventually to the CLs. This would facilitate access of microorganisms to the ocular surface.
37 The transmission of
Staphylococcus aureus from the CL storage cases to porcine corneas, mediated by CLs,
38 has been reported. The transmission of microbes from the storage case to CLs was not significantly different for the type of bacteria (Gram-negative and Gram-positive)
39 ; however, the surface properties of CL (hydrophilic or hydrophobic) and the type of bacteria can influence microbial transmission from CL to cornea.
37 In such circumstances, a pre-existing insult or compromise of the ocular surface might allow opportunistic pathogens to initiate infection. Limiting storage case contamination would seem to be an overlooked but important part of safe CL wear.
23
The suggested methods of reducing case contamination include the development of antibacterial cases, appropriate case hygiene, and frequent replacement of cases.
21 The latter two are dependent on patient compliance with instructions. Even then, contamination of storage cases occurs.
40 Information on the performance of antibacterial storage cases is sparse. In vitro studies evaluating antimicrobial efficacy of silver-impregnated storage cases demonstrated their robust activity against Gram-negative bacteria
41 and increased killing efficacy against
P. aeruginosa in biofilm when used in combination with the lens care solution.
39 However, there are limited available data on the clinical performance of silver-impregnated storage cases. The objectives of this study were to compare the microbial contamination percentages and levels between silver-impregnated and regular cases in clinical studies and to establish appropriate maintenance conditions for the silver-impregnated cases. This study also evaluated the association between microbial contamination of cases with ocular clinical signs, symptoms of wearers during lens wear, and compliance of wearers with hygiene instructions.
Two noncontemporaneous prospective, single-group, bilateral-design, open-labeled clinical studies were conducted at the Brien Holden Vision Institute under good clinical practice to evaluate the percentages and levels of microbial contamination of regular (study 1) and silver-impregnated (MicroBlock; study 2) storage cases during use. The study protocols were reviewed and approved by the Human Research Ethics Committee and were performed in accordance with the guidelines of the Declaration of Helsinki for Experimentation on Humans (1975, revised in 1983). Informed consent was given by participants before start of lens wear.
Subjects older than 18 years with normal ocular signs, no systemic or ocular contraindications to CL wear, and with or without previous lens wear experience were recruited either by email or from the database of the Brien Holden Vision Institute. Assuming the case contamination percentage of 80% with regular cases, a sample of 40 participants in each study using either storage case (regular or MicroBlock cases) with multipurpose solution (AQuify) collected at two study visits was required to demonstrate a significant reduction of 50% in case-contamination percentages (80% with regular cases vs. 40% with MicroBlock cases). The sample size for both studies was estimated at the 5% level of significance and with 90% power, assuming a 20% dropout percentage. Forty and 41 subjects were enrolled in studies 1 and 2, respectively. CLs and storage cases were replaced every month. Subjects in study 1 were instructed to maintain their regular CL storage cases “dry. ” Subjects in study 2 were instructed to maintain their silver-impregnated cases “wet ” between uses for up to 3 months and dry for the fourth month to evaluate the effect of maintenance condition on case contamination. To say cases were maintained in a dry state refers to a condition in which, after lens insertion, the old solution was discarded from the lens cases; cases were rinsed with multipurpose solution (AQuify) and then air dried. In wet case maintenance, similar steps were followed except that the storage cases were immediately capped without air drying after rinsing with multipurpose solution (AQuify) after removal of lenses for wear. All subjects were asked to return the used CL cases for microbial analysis at the end of 1-month and 3-month (for both studies 1 and 2) and 4-month (for study 2 only) follow-up visits.
Microbial contamination percentages of the storage cases were compared between two studies using logistic regression analysis with robust estimate of variance and Fishers exact test. Levels of microbial contamination (CFU) of cases were log transformed for the data analysis. A linear mixed model was used to compare the levels of contamination between different storage cases and for different storage conditions after adjusting for any intrasubject correlations.
A linear mixed model was used to compare the ocular signs of subjects from the two studies and to determine any association with the microbial contamination. χ2 tests were used to compare subjective symptoms between the studies and to verify the association of symptoms with the microbial contamination. To ascertain the association of compliance with the microbial contamination, each question was assigned a score and was categorized under lens care and case care and was compared using a linear mixed model. Statistical software (SPSS, version 16; SPSS, Chicago, IL) was used for data analysis.
Silver-impregnated CL storage cases have been introduced to limit storage case contamination. The present study compared the percentages and levels of microbial contamination between regular storage case (without silver) and silver-impregnated cases and their association with the clinical findings and subject compliance. The study showed that though there was no significant difference in microbial contamination percentages between regular and silver-impregnated cases, there was a substantial decrease in the level of microbial contamination in the silver-impregnated cases. Further, the silver-impregnated cases maintained wet between uses performed better than those maintained dry. There were no associations between symptoms, signs, and compliance scores and the level or type of microbial contamination.
In the present study, we evaluated the clinical performance of antibacterial storage cases containing silver, a well-known, broad-spectrum antimicrobial agent.
46 Silver is believed to exert its antimicrobial effects by various mechanisms, including interference with DNA and RNA replication, disruption of the cell membrane, interference with cell respiration, and inactivation and alteration of enzymes.
45,47 The manufacturer of this silver-impregnated case claims that the slow, sustained release of silver ions from these storage cases contributes to the antimicrobial activity of the cases.
45 Silver ions are incorporated into amorphous zones of polymeric material of the storage cases. The silver ions migrate to the surface of the lens case when an imbalance of vapor pressure demands equalization, providing a consistent supply of antimicrobial agent at the surface of the lens case.
48
Irrespective of the type of storage case, >70% of the storage cases were contaminated. These results are consistent with a previous study that has reported 90% contamination with the silver-impregnated CL storage cases (MicroBlock) and 100% contamination in regular cases.
49 However, another study demonstrated significant reduction in the contamination with silver-impregnated CL storage cases (MicroBlock), showing either 26% or 38% under different storage conditions.
45 These differences in the percentage of contamination could be attributed to differences in the processing time for the samples, which ranged from 4 hours
49 to a few days
45 (not specified); in the present study, the difference was 2 hours. A delay between the collection of storage cases and processing may result in prolonged contact between silver ions and microbes at the surface, leading to an overall decrease in contamination or a reduction in the survival of fastidious organisms. The location in sampling of the storage case may also influence the reported percentage of contamination.
33 In addition, the levels of case contamination vary with the use of different disinfecting solutions.
43
In the present study, a lower level of Gram-negative bacteria was observed with the silver-impregnated cases. Additionally, similar to previous reports,
45,49 there was no recovery of
P. aeruginosa and
S. marcescens, which are often associated with CL-related microbial keratitis.
2,3,8,20,35,36 CL hygiene and storage case contamination by bacteria have been associated with the occurrence of sterile or nonsterile corneal infiltrates.
50 Endotoxin released from Gram-negative bacteria is believed to be the primary cause of corneal infiltrative responses.
51,52 Most bacteria recovered from the silver-impregnated cases were Gram-positive, primarily
S. epidermidis and
Propionibacteria spp., which are part of normal ocular microbiota. The decreased efficacy of silver-impregnated cases against
S. aureus biofilm compared with
P. aeruginosa biofilm has been reported.
39 There was a lower level of recovery for Gram-positive bacilli with the silver-impregnated cases. Earlier, an association of
Bacillus spp. with keratitis, the spores of which are resistant to disinfectants, was established.
53,54 The significance of the level of microbial contamination in CL storage cases is unknown and is beyond the scope of the present study. Further studies are needed to determine whether the silver-impregnated cases offer some margin of safety against adverse events associated with these potential pathogens.
Fungal contamination decreased from 18% to 4% with the use of silver-impregnated cases under the manufacturer's recommended conditions. This is consistent with an earlier report in which the fungal contamination was 24%
27 with regular cases when used with different disinfection systems. This is the first clinical study to report reduced fungal contamination in the silver-impregnated cases. Previous work
41 has demonstrated a small but significant reduction of
Fusarium solani ATCC 36031 in the silver-impregnated cases in vitro. The toxicity of silver against
Fusarium oxysporum has been reported.
55 Another study established that silver nanoparticles were effective against
Candida spp.
56 In the present study, we did not classify the fungal isolates recovered, and the samples were not screened for
Acanthamoeba spp., which may be important to consider in future studies given the recent outbreak of atypical CL-related keratitis
57 –59 and increased environmental exposure to
Acanthamoeba.
59 However,
Acanthamoeba spp. were not recovered in a large sample of cases from wearers recruited from the same center (
n = 373).
43
Our results revealed that the silver-impregnated cases demonstrated lower contamination levels under wet storage conditions. This is in accordance with a previous study,
45 in which the activity of silver-impregnated cases was reported for different levels of moisture. Further, when the cases were air dried, there was an overall increase in contamination (94%), particularly for fungi and Gram-positive cocci. Our results confirm that silver-impregnated cases are most effective when maintained wet between use cycles.
The comparison between the storage conditions revealed weaker biofilm formation in the cases maintained in wet condition. It is possible that the silver ion release from the surfaces of the storage cases might have limited biofilm formation.
49 Earlier reports proposed that the efficacy of the silver ions would be best at the interface of storage cases and biofilm, leading to a low viability of biofilms in silver-impregnated cases when tested against the
P. aeruginosa strain under laboratory conditions.
39 A recent study of nonculturable organisms has established that the number of species recovered from the CL storage cases of wearers is associated with the severity of keratitis, which may support the hypothesis of reduced morbidity as the bacterial load of the storage case is reduced (Wiley L.
IOVS 2011;52:ARVO E-Abstract 6508).
There were no associations between symptoms/signs and the levels or types of microbial contamination of cases. A previous study has suggested that the bacterial bioburden of CL is associated with discomfort,
60 whereas the contamination of storage cases has been implicated in CL-related problems such as itching, redness, and dryness.
24 However, in the present study, there was no direct association between symptoms with microbial contamination. Despite the repeated emphasis of care instructions, variable practices (
Table 4) were observed among subjects toward CL care both after and before CL wear and in the routine care of the storage cases. This highlights the inconsistent practices common with the care regimen because of the lack of standard guidelines and even within a carefully controlled study. Eye care practitioners and the industry must be very specific with their instructions for the use of CLs and their care products. CL case contamination has been identified as a significant public health concern because it may lead to sight-threatening microbial keratitis; hence, there is need for evidence-based guidelines for the CL care regimen.
21 In the present study, the total compliance scores were not different between men and women and were not associated with the microbial contamination of the storage cases. This suggests that reduction in the microbial contamination observed with the silver case is a robust effect rather than simply a reflection of better compliance.
In conclusion, this study has demonstrated that the percentages of contamination for regular cases and silver-impregnated cases were not significantly different. However, silver-impregnated cases maintained wet showed lower numbers of microbes, particularly in Gram-negative bacteria such as P. aeruginosa and S. marcescens. For the silver cases, wet storage between use appears to reduce biofilm formation more effectively than does dry storage. Further studies are required to establish whether the use of such cases might limit the CL adverse responses associated with microbial contamination and to determine whether such benefits are maintained in community studies.
Supported by a tuition fee remission scholarship from the University of New South Wales through the School of Optometry and Vision Science and a faculty research stipend from Brien Holden Vision Institute, Sydney, Australia.
Disclosure:
J. Dantam, None;
H. Zhu, None;
M. Willcox, None;
J. Ozkan, None;
T. Naduvilath, None;
V. Thomas, None;
F. Stapleton, None
The authors thank Najat Harmis and Shamil Iskander for their kind assistance in processing the microbial samples.