Antimicrobial Coatings to Prevent Biofilm Formation on Medical Devices - Antibiofilm Agents: From Diagnosis to Treatment and Prevention - page 183

4.2.1 Compounds That Are Released from Lenses
Furanones
These are quorum-sensing blockers. These compounds showed an ability to inhibit
biofilm on medical device polymers (Baveja et al. 2004 ) but yielded equivocal
results with contact lenses for their ability to block biofilm formation (George
et al. 2005 ). However, a more recent paper (Kuehl et al. 2009 ) showed that these
compounds (1) will inhibit biofilm formation; (2) need to be free to inhibit biofilms;
however, (3) were toxic for murine fibroblasts; and (4) at sub-inhibitory concen-
trations, showed an enhancement of S. aureus biofilm formation (Table 1 ). This
would appear to make them poor candidates for addition to contact lenses.
Polyquaternium Compounds (Polyquats)
These compounds have only been used in contact lens case solutions (Alcon, Inc.).
They are thought to kill by chelation of bacterial components (Weisbarth
et al. 2007 ). If these compounds do kill by chelation then they would have a limited
range of activity. Also, they have been shown to increase cytoxicity and inflam-
mation in human corneal epithelial cells (Paimela et al. 2012 ) (Table 1 ). They may
also kill by cell wall rupture (see below under “Covalent attachment to the Lens”).
They generally consist of long-chain molecules with cationic ends and can be
attached to surfaces. It is felt that the cationic end pierces components of the
bacterial cell wall causing it to rupture and die. This is thought to be an advantage
since it would not lead to the development of resistance in microorganisms.
However, since these long-chain compounds need to have direct contact with the
microorganisms in order to penetrate their cell walls, this would restrict the amount
of killing that could take place and the surface could become overwhelmed by
bacteria with time.
Silver
Silver is widely used since it has antimicrobial activity against a broad spectrum of
bacteria (Yin et al. 1999 ) and fungi (Wright et al. 1999 ). It is thought that silver
atoms bind to thiol groups in essential enzymes and subsequently cause their
deactivation. Silver also forms stable sulfur-silver bonds with proteins in the cell
membrane that are involved in ion transport (Klueh et al. 2000 ). It was also
proposed that Ag + enters the cell and intercalates between the purine and pyrimi-
dine base pairs disrupting the hydrogen bonding between the two antiparallel
strands and denaturing DNA molecules (Klueh et al. 2000 ). Many antimicrobial
medical devices use silver as their active agent. However, silver has been shown to
be cytotoxic to fibroblasts and keratinocytes (Atiyeh et al. 2007 ; Trop et al. 2006 ;