Antimicrobial Interfaces - Biointerfaces: Where Material Meets Biology - page 406

support the progress of survival mechanism such as expression of particular
eux pumps and up-regulation of proteins (such as porin proteins) that
could quickly disseminate right through the biofilm population. 75
Biofilm associated implant infections show a high rate of recurrence that
leads to further aggressive diseases. The return of such infection is now
attributed to the presence of a protected sub-population of persister cells. 76
Persister cells might have inactivated their automatic cell death mechanisms
which help them to display resistance to the actions of antibiotics. More
importantly, after earlier—seemingly successful—therapies, multi-drug tol-
erant progeny originating from persister cells could repopulate the com-
munity and be responsible for persistent infections. The mechanism of
multi-drug tolerance among biofilm bacteria and the nature of persister cells
are still under intense research. Multi-drug tolerance occurs due to elevated
expression of toxin-antitoxin modules and related genes. This may hinder
vital cell functions that include translation, a major target for certain anti-
biotics. 76 Due to such variation and yet to be elucidated resistant mech-
anisms possessed by biofilm, a single substance or strategy to alleviate the
issues linked to implant infections appears to be a remote possibility.

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14.5 Challenges of Antimicrobial Material Design
There is an underlying need to improve the properties of existing materials
due to the incidence of healthcare associated infections, which often lead to
revision surgery. Currently, prophylaxis in the form of systemically ad-
ministered antibiotics serves as the main weapon against infection following
implant surgery. 77 The first step in bacterial colonisation that could lead to a
mature bacterial biofilm is the attachment of an individual bacterium to
material surfaces and interfaces. The key steps in bacterial adhesion to
surfaces can normally be summed up in a two-stage model. That includes an
early reversible interaction between the material surface and the bacterial
cell surface followed by a second stage which comprise of specific and
nonspecific interactions between binding molecules on the material
surface and adhesion proteins expressed on bacterial surface structures. 11
This latter stage is gradually reversible but

.

frequently deemed to be

irreversible. 78

14.5.1 Anti-adhesive surfaces
Adhesion of bacteria to materials is a multifaceted interaction that involves
varying means including inherent motility of bacteria, Brownian motion,
diffusion, convection and gravity associated movement. When a material is
implanted in the human body a thin film, termed a conditioning film, which
comes from organic matter (mainly adsorbed proteins and blood platelets)
present in the surrounding fluid, is deposited on the material surface. The
composition of this organic matter affects the physicochemical and mor-
phological properties of

the surface. The conditioning film forms an