Biomedical Engineering Reference
In-Depth Information
In the presence of water, the sum of acid
e
base inter-
actions describes the wettability of a surface. When
a bacterium and a biomaterial surface are in an aquatic
environment, the tendency of water to be attracted to
the two surfaces affects how the surfaces interact with
each other. When two hydrophobic surfaces, such as
untreated PEEK and a hydrophobic bacterium, are
submerged in water they will be attracted together, as
lowering the total surface area interacting with water is
energetically favorable. Conversely, two hydrophilic
surfaces will not attract, maximizing the surface area
interacting with water. Even subtle changes in local
hydrophobicity affect protein adhesion and confor-
mation
[43
e
45]
, which in turn affects bacterial adhe-
sion, as discussed later in Section
8.3.2
.
Altogether, these three types of forces are believed
to influence nonspecific bacterial adhesion. A
summary of the strength, range, and examples of the
causative agents of these forces can be seen in
Tables
8.1 and 8.2
.
Although nonspecific adhesion is guided by phys-
ical constants brought about by the chemistries of the
bacterium, biomaterial, and the surrounding environ-
ment
[33]
, there are methods by which bacteria
manipulate these conditions to improve the chance of
adhesion. For example, many bacteria express cell
surface appendages such as flagella, which aid surface
contact
[47
e
49]
. Additionally, bacteria such as P.
aeruginosa
[50]
and E. coli use fimbriae for motility.
Motile bacteria can actively move toward biomaterial
Table 8.2
Four Common Functional Groups with
Corresponding Forces
Functional
Group
Polar/Apolar
Charge
Methyl, eCH
3
Apolar
Neutral
Hydroxyl, eOH
Polar
Neutral
Amine, eNH
2
Polar
Positive
Carboxyl,
eCOOH
Polar
Negative
Apolar functional groups can also be described as hydro-
phobic and polar groups are hydrophilic. The comparisons
made in this table are relative and dependent on the envi-
ronment the functional group is in
[33]
.
surfaces, bind to them, and even swarm across them to
find the most favorable colonization conditions
[51]
.In
addition to surface structures, many bacteria also
produce extracellular polymeric substances to coat the
bacterial cell, the biomaterial, or both
[52
e
54]
to
provide favorable adhesion conditions, or even mask
unfavorable chemistries.
8.2.2.2 Specific Adhesion
Bacterial adhesion to host tissues, cells, and
surface-bound proteins is the first step toward infec-
tion in both the presence and absence of a biomaterial.
As discussed in Section
8.2.2.1
, an implanted
biomaterial is always coated in a conditioning layer of
native proteins and it is these proteins with which
a bacterium interacts. Because of this fact, bacteria
have evolved adhesion aids, such as adhesins, which
specifically target components of typical conditioning
films
[55
e
57]
. Adhesins are molecules present on
many bacteria that form ligand
e
receptor type inter-
actions with host proteins and other molecules
(
Fig. 8.3
). Adhesins can recognize and bind to
specific carbohydrate structures, membrane proteins,
or host extracellular matrix components. Some of the
most extensively studied adhesins are those belonging
to S. aureus, and are termed microbial surface
component-recognizing adhesive matrix molecules
(MSCRAMMs)
[58]
(
Fig. 8.4
). Because each indi-
vidual adhesin is limited to targeting specific
molecules or set of functional groups, a single
bacterium will often have many different adhesin
Table 8.1
The Range and Strength of the
Intermolecular Forces Used When Modeling
Bacteria as Colloids
Range
(nm)
Strength
(kJ)
Force
Lifshitze
van der Waals
0.3e0.4
<
2
Hydrogen bond
(Lewis
acidebase)
0.2e0.3
12e16
Electrostatic
forces
Dependent on media
Lifshitzevan der Waals forces often act over the longest
range; however, polar hydrogen bonds are the strongest.
Electrostatic bonding is highly dependent on the media the
material is in
[46]
.
