UC Berkeley

Bacterial adhesion to soft-contact lenses is a major risk factor for microbial keratitis. Microbial keratitis, a potentially blinding condition, is the most serious complication associated with soft-contact-lens wear. Thus, curtailing adhesion of bacteria to soft contact lenses is a method of preventing microbial keratitis. One of the most implicated bacteria in soft-contact-lens related microbial keratitis is Pseudomonas aeruginosa. However, binding of bacteria to solid substrates is complex and incompletely understood. This thesis investigates and elucidates the mechanisms of P. aeruginosa binding to soft-contact lenses in order to advance antifouling science.

This study consists of three main parts. First, the bacterium's motility appendages, which have been implicated in bacterial adhesion, were studied. Changes in bacteria surface association due to modifications of the appendages were investigated. Following this, the influence of anionic membrane surfaces on bacteria association was studied. Finally, utilizing what was learned, surface modifications via surfactants were explored for antifouling efficacy.

Most current studies involving binding to contact lenses report end-point analyses with little understanding of the mechanisms of adhesion. Under phase-contrast microscopy, a parallel plate flow chamber, which provides a well-defined flow field, was utilized. P. aeruginosa binding was visualized and investigated, and its binding mechanisms were dissected. P. aeruginosa swimming motility increased attachment rates by increasing the bacterium's effective diffusion but did not change the bacterium's ability to bind. Additionally, pili are not necessary for attachment. Hyperpiliation, infact, hinders attachment. By direct observation individual bacteria, it is shown that P. aeruginosa has multiple binding sites. Removal of one binding site givs way to binding via other binding sites.

Many contact lenses are anionic. Membranes with addition of anionic poly(methacrylic) and poly(acrylic acid) were investigated. Wild-type P. aeruginosa strain PAK exhibits an accumulation burst-attachment phenomenon on anionic hydrogel membranes when the suspending medium contains divalent cations and the membrane initially does not. Early-time accumulation rates of are larger than on counterpart nonionic hydrogels. The initial high accumulation rate is followed by spontaneous bacterial detachment to a low number of strongly bound bacteria. The bursting phenomenon arises from reversible divalent-cation bridging between the anionic bacterium and anionic charged sites on the hydrogel surface. Few strongly bound PAK bacteria remain on the gel surface is most likely due to increased water wettability compared to the nonionic hydrogels.

Soft-contact-lens surface wettability can be increased by addition of surfactants. The ability of polymeric surfactants to modify contact lenses and prevent adhesion was studied. A polyethylene oxide/polypropylene oxide surfactants (Pluronic F127) and ethylene oxide/butylene oxide (EOBO) polymeric surfactants were capable of increasing the wettability of soft-contact lenses. Surfactants absorbed into the lens matrix. Over time, the surfactants leach from the lens matrix and adsorb at the external lens surface. The surfactants increase the lens wettability by decreasing the air/water surface tension while also increasing the adhesion tension of the lens surface. A highly wettable surface slightly reduces bacterial adhesion of PAK bacteria in agreement with previous studies. However, prevention of PAO1 bacteria was not possible, as surface wettability did not play a role.

This study emphasizes the delicate balance of forces at play during bacterial adhesion to substrates. In addition, the many factors such as substrate, bacterium, and adhesive factors that play a role stress the difficulty of preventing adhesion.