Bacterial Interactions with Polyaryletheretherketone - PEEK Biomaterials - page 93

In addition to broad variation between bacterial
species, each individual species contains many strains
that differ from each other in a small but definable
way. For example, S. aureus infections may be present
in two patients; however, one of these infections could
be susceptible to commonly used

8.2.2.1 Nonspecific Adhesion
Nonspecific adhesion describes the physical
attraction of a bacterium toward a surface driven by
three factors: electrostatic, Lifshitz e van der Waals,
and Lewis acid e base forces [34 e 36] .
Electrostatic forces arise due to the relative
electrical charges of the bacterium and the biomaterial
surface. In nature, most surfaces and bacteria have
a negative charge and therefore the electrostatic force is
normally repulsive [37] . In vivo electrostatic forces are
dampened by the presence of aqueous ions commonly
present in physiological media such as Na þ or Cl
[35,38 e 41] . A charged surface, for example a bioma-
terial or bacterium, is quickly surrounded by oppositely
charged ions that minimize the range of the electro-
static forces and reduce the relative importance of
electrostatic charge in vivo ( Fig. 8.3 ). The second
factor, the Lifshitz e van der Waals forces, are found
acting between two molecules containing dipoles [42] ;
these forces are relatively long range and are believed
to initiate bacterial adhesion. Finally, Lewis acid e base
interactions describe the forces generated between two
highly polar molecules containing hydrogen. The
Lewis acid e base force between a hydroxyl group and
another polar molecule is also known as hydrogen
bonding, one of themost important bond types found in
nature, contributing to 90% of noncovalent bonding.

-lactam antibi-
otics, whilst the other is resistant, for example meth-
icillin-resistant S. aureus (MRSA). Other intraspecific
variations can include the proteins used for adhesion,
the ability to produce a certain organic product, or the
propensity of the organism to cause disease or
resist treatment. This therefore makes it difficult to
characterize bacterial e biomaterial interactions
without studying an array of clinically relevant
species and strains, and furthermore, drawing general
universal

b

conclusions on these interactions is

challenging.

8.2.2 Bacterial Adhesion
Mechanisms
Before discussing the details of bacterial adhesion
to biomaterials, including PEEK, it is important to
understand the progression of events leading up to
bacterial colonization of the surface. Upon implan-
tation, a biomaterial is immediately confronted with
an aqueous physiological milieu consisting of ions,
organic molecules including proteins, and cells.
Within this milieu, the first molecule to interact with
the biomaterial surface is water [30,31] . Following
water, dissolved chemicals come into contact with
the surface; these include ions, proteins, and other
organic molecules. The dissolved molecules may
then interact with and adsorb to the surface in
a manner that is dependent upon the chemistry and
topography of the implanted biomaterial. Therefore,
and very importantly, in vivo, bacteria do not interact
with biomaterial surfaces directly, but instead with
a complex layer of proteins, molecules, and ions
associated with the biomaterial surface. For
more details about biomaterial e biological molecule
interactions, one should refer to the literature [32,33] .
For a biomaterial-associated infection to occur,
a contaminating organism must first adhere to the
foreign implanted material. The adhesion process can
be broken down into two mechanisms: nonspecific
and specific adhesion. Both these mechanisms are
influenced by the biomaterial, the phenotype of the
adhering bacteria and the surrounding physiological
media.

Figure 8.3 Diagram illustrating the effect of immersing
a negatively charged surface in an electrolyte-containing
medium. Positively charged ions become directly associ-
ated with the surface, additional positive ions are
attracted toward the surface in a density that decreases
with distance. This can, to a large extent and dependent
on ionic concentration, neutralize the electrostatic force
over longer ranges.