Contact Lens Care & Compliance
Lens Bacterial Biofilm Formation
BY MICHAEL A. WARD, MMSC, FAAO
Biofilm generally refers to an aggregate of microbial cells in a polymeric matrix of DNA, proteins, and polysaccharides. Biofilms form when bacteria adhere to surfaces in aqueous environments and begin to excrete a slimy, glue-like substance that can anchor them to a surface material. A biofilm is a microbial community; inhabitants of a community survive better by living in close proximity, communicating, and collectively creating forms of shelter and defenses to protect themselves from external threats.
Biofilms can be both beneficial and detrimental to the host. Those of normal bacterial floral organisms help to protect us against foreign pathogenic bacterial invaders. For example, the presence of Staphylococcus epidermidis on a soft contact lens surface significantly reduces the adhesion of Pseudomonas aeruginosa. On the other hand, microbial biofilms can harbor pathogens in a lens storage case and aid in their persistent survival.
How Biofilms Form
Biofilm formation begins when planktonic (free-floating) bacteria contact a surface (e.g. contact lens, storage case, ocular tissue) and are held in place by weak, reversible bonds called van der Waals forces. Initial adhesion between bacteria and non-living surfaces is usually mediated by non-specific (e.g. hydrophobic) interactions, whereas adhesion to living surfaces is usually accomplished through specific molecular binding mechanisms.
At this initial stage they may be easily washed off. The initial colonists may adhere to the surface by forming more complex, irreversible bonds that will act as anchors in creating a scaffold for further bacterial attachments and micro colony formation. These additional irreversible bonds are created and held together by a matrix of excreted polymeric substances. This scaffold creates more diverse attachment sites, allowing additional bacteria to eventually form a matrix that will hold the biofilm together.
Quorum sensing (QS)—chemical message regulation of gene expression in response to fluctuations in cell-population density— allows the initial organisms to communicate with pathogens of other bacterial genera and species. Additional pathogens are invited to join the community through QS communication.
As the biofilm community matures, it grows through a combination of cell division and microbial recruitment. Developed biofilm communities provide enhanced communication among their inhabitants and enhanced protections against external threats such as antimicrobials. Developed biofilms can simultaneously contain multiple types of microbes including bacteria, fungi, and amoebae. Dispersion is the necessary final stage of the microbial biofilm life cycle. Dispersal allows the microbes to separate, spread, and produce new colonies.
Bacteria on Contact Lenses
A literature review by Dutta et al (2012) highlighted some interesting factors associated with bacterial attachments to contact lenses. Types of bacteria as well as both physical and chemical lens material properties influence rates and types of bacterial adhesions. The leading adhesive nature of Pseudomonas is its surface hydropho-bicity, allowing rapid adhesion to contact lenses in as early as one hour and biofilm formation within 24 hours. Studies have concluded that P. aeruginosa has greater adhesion than other tested bacteria do to soft lens materials (Dutta et al, 2012; Giraldez et al, 2010). Bacterial adhesion increases inversely to water content. However, lens polymer type and surface hydrophobicity can trump the effects of water content. Hydrophobic lens surfaces attract greater numbers of bacteria compared to hydrophilic lens surfaces (Santos et al, 2007). CLS
For references, please visit www.clspectrum.com/references.asp and click on document #205.
Mr. Ward is an instructor in ophthalmology at Emory University School of Medicine and Director, Emory Contact Lens Service. He is also an advisor to B+L and Alcon as well as a member of the GPLI Advisory Panel. You can reach him at email@example.com.