Rapid microbial quantification of disinfected contact lens surfaces

The efficacy of disinfection of contact lenses is difficult to determine. We have monitored microbial contamination on surfaces and in fluids by intrinsic fluorescence of microbes and distinguished their metabolic states (viable cells, nonviable cells, and endospores). This approach is sensitive (10 cells), requires no added reagents or sample contact, and measurements can be made in near real time. The disinfection performance of AMO Easy Rub™ and Alcon No Rub™ contact lens solutions was compared with CVS-brand saline for contact lenses on contact lenses contaminated with Pseudomonas aeruginosa (ATCC 10145). Our aim is to show that intrinsic fluorescence measurements can yield real-time, critical information about the efficacy of contact lens decontamination products and protocols.

Intrinsic fluorescence of Acuvue™ contact lenses (samples and controls) was measured before and after incubation with P. aeruginosa. Inoculated lenses were then cleaned and disinfected, according to directions for AMO Easy Rub™ and Alcon No Rub™, rubbed and rinsed with saline for contact lenses (the brand made a difference in the experiment), followed by measurement of the effectiveness of the disinfection procedure with the intrinsic fluorescence instrument. Both the sample lenses after disinfection and the control lenses were immediately placed in LB broth for outgrowth and measured by standard cell-counting methods. The experiments were performed in triplicate.

The concentration of P. aeruginosa formed on the lenses by the above methods varied from 10(6)-10(13) cells/mL. All lenses that were cultured following cleaning yielded no colonies. However, 10(5)-10(9) cells/mL of the bacteria on the treated lenses remained viable when analyzed by intrinsic fluorescence methodology.

These results indicate that a medically significant amount of bacteria remained on the contact lenses after disinfection and/or cleaning, which were viable but nonculturable and are likely to go undetected when using standard culture methods.