Evaluation of prevention and disruption of biofilm in contact lens cases

Purpose: The presence of biofilm in the lens case has been shown to be a risk factor for contamination of lenses and consequently microbial keratitis. This study aimed to evaluate effectiveness of solutions for rigid contact lenses in prevention and disruption of biofilm in lens cases and methods for biofilm detection. Method: This study adopted a stepwise approach to evaluate effectiveness of four rigid lens disinfecting solutions against biofilm. These included two polyhexamethylene bigiuanide (PHMB) solutions and a chlorhexidine/PHMB-based solution, as well as a novel povidone-iodine formulation. The presence of biofilm following exposure to the solutions was assessed using both crystal violet (CV) staining and MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) viability assay, taking into account the effect of lens case design. Three lens case designs, conventional flat, large bucket type, and cylindrical cases, were investigated for the ability to trap stain and allow biofilm formation. Results: Considerable differences were noted between solutions in their ability to prevent and disrupt biofilm (p < 0.001). Lens case design greatly influenced optical density (OD) measurements even in negative controls, as cylindrical cases trapped more stain, increasing OD readings. Correcting for this factor reduced variations, but could not differentiate between residues and biofilm. MTT assay revealed that both povidone-iodine and chlorhexidine-containing solutions could effectively kill > 95% of organisms, whilst PHMB-based solutions were less effective with up to 55% of staphylococci and 41% of Pseudomonas surviving at 24 h. Conclusion: Biofilm can rapidly form in lens cases and may not be killed by disinfecting solutions. Of the solutions tested, none were able to prevent biofilm formation or disrupt established biofilm, but those containing chlorhexidine or povidone iodine were able to penetrate the biofilm and kill organisms. Assessment of biofilm by CV assay may be confounded by lens case design. Whilst CV assay can demonstrate presence of biofilm, this technique should be accompanied by viability assay to determine bactericidal activity.