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A Disc Diffusion Assay of Three Contact Lens Disinfectants
BY ERIN CLAIRE KAIRYS AND DAVID J. KAIRYS, OD,MS

There is controversy among eyecare professionals over the comparative efficacy of various contact lens disinfecting solutions. Dannely et al ("How Dangerous Is Noncompliance with Multi-purpose Solutions?" Contact Lens Spectrum, January 2000) found that Opti-free Express  killed a strain of Pseudomonas aeruginosa more effectively than did Bausch & Lomb ReNu MultiPlus. They used a log-3 (1000-fold) reduction of bacterial numbers as the successful kill criterion.

Using the same strain but starting numbers (10,000,000 per cubic ml) approved by the International Standards Organization, Miller et al ("Antimicrobial Activity of ReNu MultiPlus Against Pseudomonas," May 2000) could not replicate Dannely's results, finding both disinfectants equal in their bacteriocidal abilities.

We selected the most direct, simple and time-honored tool of microbiologists to try to resolve this issue: the disc diffusion assay.

Methods

We used a commercially available strain of Pseudomonas, growing it in standard tryptic soy broth to concentrations about 10,000,000 to 100,000,000 per cubic ml. We spread the bacteria on the surface of standard 5 percent defibrinated sheep-blood agar, then overlayed four sterile paper discs, each impregnated with one of four solutions: saline, Opti-Free Express, ReNu MultiPlus and CIBA Vision Solo-care.

Incubation for 24 hours at 36 degrees C permitted growth over the blood agar except where the antibacterial solutions diffused out of the discs; we measured efficacy by measuring the band width of non-growth around each disc.

Table 1 shows the results from 12 samples. The three disinfectants did not differ from each other in their antibacterial efficacy. ReNu MultiPlus registered slightly higher in the total band width when all 12 samples were pooled, but this was not statistically significant when the data was submitted to a statistical analysis.

All three disinfectants differed from the control (saline) in antibacterial activity (T greater than 1.78 in all three comparisons).

Discussion

It is possible that sampling error affected the data. Though a few samples showed ragged edges in the bacterial inhibition zone, we were able to fairly estimate the true band width by using a symmetrically circumferential area of the band for measurement.

It is also possible that the agar used presents an artifact not present within the medium where lens disinfection takes place. The agar may impede some disinfectants more than others as they diffuse outward from the disc. However, biofilms present on a contact lens (or within the case) may also alter one disinfectant's movement toward target bacteria. We feel this assay has merit in the pursuit of an ideal measurement of bacteriocidal activity.

Ms. Kairys is a pre-med student. Dr. Kairys practices in DuBois, PA, and has taught microbiology and biochemistry at Penn State University.