Biofilms and Contact Lenses
BY LORETTA B. SZCZOTKA-FLYNN, OD, MS, FAAO
Ongoing research into the causes of microbial keratitis continues to inspire new ideas. The concept of contact lenses acting as vectors for pathogens to gain access to the cornea has been explored since the soft lens became available. However, recent studies have been assessing firmly attached bacterial and fungal biofilms, rather than loosely adherent microbial cells, on lenses.
What is a Biofilm?
Biofilms form when microbes adhere to surfaces in an aqueous environment. The microbes secrete substances and upregulate genes that cause a shift in their phenotype. In essence, they create on a surface their own community in which they communicate, seek nutrients, and excrete substances to shield and protect themselves from the environment. The organisms, particularly bacteria and fungi in lens-related biofilms, begin to excrete a polymeric substance that can anchor them to some living or inert surfaces.
Biofilms are present in many different industrial and biomedical settings. For example, the plaque that forms on teeth is a type of bacterial biofilm, and the slimy coating on rocks in rivers or that which spoils pipes and tanks can be composed of a mixture of fungal, protozoal, and bacterial biofilms.
Biofilms have been known to form on contact lenses during lens wear. In the late 1990s, two studies showed dense biofilms formed on lenses that were obtained from patients who had microbial keratitis. The process of biofilm formation on lenses or cases (or any biomedical surface) begins when free-floating (planktonic, Figure 1) bacteria or fungi first settle on the surface, where the attachment is initially weak. However, if the microbes are not almost immediately removed, some strains will attach themselves with cell adhesion molecules and they will start to secrete their matrix. In fact, Stapleton (1993) has shown that glycocalyx formation can occur within 30 minutes of cell surface attachment. The biofilms then flourish, divide and disperse to re-colonize elsewhere (Figure 2).
Figure 1. Planktonic bacteria.
Figure 2. Biofilms.
Formed biofilms are much more resistant to antibiotics, biocides, disinfectants, and the host immune response. Therefore, there is interest in the ability of various lens surfaces to attach biofilm, the disinfecting properties of contact lens care solutions against biofilms, and the potential of antimicrobial lens and case surface treatments to diminish or prevent formation.
Recent Research on Contact Lens Biofilms
Selan et al published a study in the January 2009 American Journal of Ophthalmology in which they explored the impact of phosphorylcholine on the ability of biofilm to form on poly (2-hydroxyethyl methacrylate, pHEMA) lenses. They also explored the differences between silicone hydrogel and phosphorylcholine-based and traditional hydrogels.
Their hypothesis was that the phosphorylcholine molecule, which is naturally found in human cell membranes, enhances the biocompatibility of the lens surface by binding water tightly and resisting deposition of lipids, proteins, and cells.
They grew biofilms of Pseudomonas aeruginosa and Staphylococcus epidermidis on omafilcon A (CooperVision), etafilcon A (Vistakon) and balafilcon A (Bausch & Lomb) to represent a phosphorylcholine-based lens, a traditional pHEMA lens, and a silicone hydrogel lens, respectively. They also grew planktonic cultures. They used antibiotic susceptibility assays to determine the biofilm state of the bacteria by assuming that increased antibiotic susceptibility was associated with diminished bacterial adhesion (and biofilm formation) (Table 1).
In summary, the phosphorylcholine-based lens was more resistant to bacterial adhesion and colonization compared to pHEMA and silicone hydrogel lenses because the biofilm formed on the phosphorylcholine-based lens behaved similarly to the planktonic cells during antibiotic susceptibility testing. These findings were also confirmed by confocal microscopy.
In 2008, two groups published on the effect of multipurpose solutions against biofilms formed on contact lenses or lens cases: Imamura et al in Antimicrobial Agents and Chemotherapy and Vermeltfoort et al in the Journal of Biomedical Materials Research Part B: Applied Biomaterials.
Vermeltfoort et al published on a model to determine efficacy of multipurpose solutions (MPSs) against biofilms grown in lens cases. They also used the model to determine the efficacy of solutions on bacterial transmission from the biofilm-laden case to silicone hydrogel lenses stored in the case. They found that a biguanide-preserved care system was the most effective MPS tested overall for both the biofilm and planktonic bacteria, and silver impregnation of lens cases combined with their prescribed solution increased the killing efficacy for P. aeruginosa biofilms.
They found that Opti-Free Express (Alcon) was the most effective in reducing transfer of bacterial cells from a biofilmladen case to a silicone hydrogel lens soaked for eight hours within the case.
Imamura showed that although common MPSs were able eradicate Fusarium solani planktonic cells, they were significantly less effective against F. solani biofilms grown on contact lenses, with only about a 50-percent reduction of biofilm activity at a four-hour recommended soak time. This is typical of what occurs with most biofilms; that is, biofilms are more resistant to biocides and disinfectants compared to free-floating cells.
Summing it All Up
Together, these studies establish models of lens and case-associated bacterial and fungal biofilms. Biofilms can form differentially on materials and are more resistant to antibiotics, biocides, and common lens care products. They have the propensity to induce more corneal disease compared to free-floating microbial cells. Preventing such biofilms is difficult, but hopefully will be targeted in the future with better products and care practices. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #162.
Dr. Szczotka-Flynn is an associate professor at the Case Western Reserve University Dept. of Ophthalmology & Visual Sciences and is director of the Contact Lens Service at University Hospitals Case Medical Center. She has received research funding from CIBA, Vistakon, Alcon, and CooperVision.