Introduction

Worldwide, millions of people use contact lenses as an alternative to spectacles. It has been shown contact lenses wear, especially extended wear, is a major risk factor for microbial keratitis and corneal ulcers.1, 2, 3, 4, 5 Although the incidence rates of contact lens-related microbial keratitis is very low, this complication is an important health concern because a very large population is at risk and because of the potential for poor visual outcome and blindness.6, 7

Investigations have documented that contact lens-related microbial keratitis is most commonly caused by bacteria, such as Pseudomonas aeruginosa and Staphylococcus aureus.8, 9, 10, 11, 12, 13, 14, 15

P. aeruginosa is a Gram-negative bacterium that is commonly found in many environments, including water. It is an opportunistic pathogen and innately resistant to dilute solutions of disinfectants. Also P. aeruginosa keratitis associated with contact lens wear is difficult to treat because P. aeruginosa can display multiple resistance to antibiotics.16, 17, 18 Studies showed that extended wear of soft contact lenses increases the adherence of P. aeruginosa to the epithelial cells of its wearers.19, 20

S. aureus is an aerobic Gram-positive bacterium carried by 50–60% of normal population on the hands, face, nose, and skin as a commensal bacterium, and can readily find access to the eye. Generally, Staphylococcal ocular infection is most likely due to hand-to-eye transfer. One study showed that S. aureus is the most common bacterial cause of contact-lens-induced peripheral ulceration.13

The goal of this laboratory-based study is to evaluate and compare the antibacterial activity of the three multipurpose contact lens disinfecting solutions available in Iran when inoculated with the clinical isolates and standard ATCC strains of P. aeruginosa and S. aureus, based on the ISO 14729 Stand-alone procedure for disinfecting products.

The ISO 14729 guidelines are the used standard in industry to demonstrate the activities of contact lens disinfecting solutions against microorganisms. According to this guidelines for Stand-alone primary criteria, an active contact lens disinfecting solution must be able to reduce the viability of starting concentration of bacterial species (S. aureus, Serratia marcescens, P. aeruginosa) by 3 log (99.9%) and fungal species (Fusarium solani and Candida albicans) by 1 log (90%) at minimum disinfecting time as specified according to the manufacturer's label.21, 22

Materials and methods

Test solutions

Three multipurpose lens care solutions commercially available in the Iranian market, namely ReNu Multiplus (Bausch & Lomb, Rochester, NY, USA), Solo Care Aqua (CIBA Vision, Duluth, GA, USA), and All-Clean Soft (Avizor, Spain) were evaluated. 0.9% normal saline was used as control solution. The formulation and recommended disinfection times for test solutions are shown in Table 1. Three lots of each disinfectant and triplicate samples from each lot were tested. Products were within expiration dating and were tested according to manufacturer's labeled instructions for minimum disinfection time.

Table 1 The formulation and recommended disinfection times for test solutions
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Test microorganisms

The test microorganisms included the ISO standards and clinical isolation of P. aeruginosa and S. aureus. The standard strains were obtained from Iranian Research Organization for Science & Technology provided from the American Type Culture Collection (ATCC) and included P. aeruginosa (ATCC 9027) and S. aureus (ATCC 6538). Clinical isolates of these organisms were obtained from asymptomatic contact lens wearers after handling. All of the bacterial strains were grown for 18–24 h on tryptone soya agar (TSA) at 30–35 °C and then collected, using a procedure based on the ISO 14729 standards. The inocula were centrifuged and suspended in normal saline and adjusted to have final concentration of 1.0 × 107–1.0 × 108 colony-forming units per milliliter (CFU/ml)

Test procedure

Test methods were based on the procedures described in ISO 14729 Stand-alone acceptance primary criteria. Growth conditions were as described previously. According to this test method, 0.1 ml of 1.0 × 107 CFU/ml test organisms suspension was added to 10 ml of each test solution. Then, the test solutions were mixed to disperse evenly the test organisms. Inoculated test solution was stored at room temperature and sampled for viable microorganisms at the labeled minimum recommended disinfection time. Then 0.1 ml aliquot was taken from each test tube, diluted with Dey–Engley neutralizing broth and permitted to stand at ambient temperature for at least 10–15 min to neutralize the preservative and then plated in TSA and incubated at 30–35 °C for 2 to 4 days for recovering the bacteria. After incubation, the number of surviving test microorganisms was determined and then the mean logarithmic reduction was calculated.21, 22

Results

The mean log reduction at the manufacturer's minimum recommended disinfection time for each of the multipurpose solution against all test organisms is reported in Table 2.

Table 2 Mean log reduction after minimum recommended disinfection time (4 h)
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Based on the data in Table 1, all test solutions exceeded the required 3.0 log reduction for the recommended standard ATCC strain of P. aeruginosa and S. aureus and, therefore, all solutions met the current ISO Stand-alone primary acceptance criteria for standard type of these organisms.

Similarly, all these solutions achieved the required 3.0 log reduction in bacterial count, when they were tested against the clinical isolate of S. aureus. But, Solo Care and All-Clean had maximum efficacy.

Although there were differences in their efficacy, Solo Care and All-Clean met and exceeded the required 3.0 log reduction for clinical isolate of P. aeruginosa, but ReNu Multi-Plus failed to meet the ISO Stand-alone primary acceptance criteria for this strain (Figure 1).

Figure 1
figure 1

Antibacterial efficacy: comparison between clinical and standard strains. Horizontal line at the 3 log mark to indicate that this is the level that a solution must attain to meet the ISO criteria for contact lens care products.

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Discussion and conclusions

In the present study, the ISO Stand-alone test showed differences in susceptibility to three contact lens disinfectants among clinical isolates and laboratory strains of P. aeruginosa and S. aureus.

According to previous studies, one of the test hypotheses was that the clinically isolated would be more resistant than laboratory strains.23, 24, 25, 26, 27, 28 This hypothesis is confirmed by our study.

Therefore, it is clear that the ISO Stand-alone procedure does not ensure disinfectant efficacy during normal use, as demonstrated by the many reports of microbial contamination.29, 30, 31

Although it has been shown that the cleaning and rinsing steps of contact lens care regimen can remove more than 90% of the microbial contaminations from the lenses before the disinfection step,32 non-compliance is very high among contact lens wearers. According to the report of Turner et al,33 30% of contact lens users do not always clean their lenses before disinfection and 44% of them do not wash their hands before handling lenses; this indicates the importance of a good disinfection.

Similar to previous studies,23, 24, 25, 26, 27, 28 the present study showed that the efficacy of a contact lens disinfecting solution is dependent on the type of strains that are used as the challenge organism. Currently clinical strains of microorganisms do not get evaluated by standard test methods. There is a need for developing a new standard protocol for testing contact lens disinfecting solutions. Additionally, despite the use of contact lens disinfecting solution, it is still essential that contact lens wearers are recommended to comply with the manufacturer's labeled instructions for care regimen.