Biomedical Engineering Reference
In-Depth Information
nonflagellar appendages contribute to cell-surface hydrophobicity and play
a role in cell attachment by overcoming the initial electrostatic repulsion
barrier, which exists between segments of cell surface and the substratum
(Rosenberg and Kjelleberg 1986). Other cell-surface components may also
facilitate attachment: Several studies provided evidence for the impact of pro-
teins on attachment (Bashan and Levanony 1988). The O antigen component
of lipopolysaccharide (LPS) was also shown to confer hydrophilic properties
in gram-negative bacteria (Williams and Fletcher 1996). Electrostatic force
was also shown to participate in microbial attachment. Cations were shown to
cross-link with the anionic groups of polysaccharides, resulting in contraction
(Fletcher et al. 1991). Beech and Gaylarde (1989) found that lectins inhib-
ited but did not prevent attachment. Binding of lectins with the cells would
minimize the attachment sites and interfere with cell attachment mediated by
polysaccharides (Zottola 1991).
In view of these findings, cell-surface components such as fimbriae, various
proteins, LPS, EPS, and flagella, all play an important role in the attachment
process. Cell-surface polymers with nonpolar sites such as fimbriae, proteins,
and components of certain gram-positive bacteria (mycolic acids) appear to
dominate attachment to hydrophobic substrata, while EPS and LPSs are more
important in attachment to hydrophilic materials. Flagella are also important
in attachment, though their role may be to overcome repulsive forces rather
than to act as adhesive (Korber et al. 1989).
4.2.2 Biofilm Growth
Cells irreversibly attached to the supporting surface will undergo cell divi-
sion, form microcolonies, and produce the extracellular polymers (EPS) act-
ing as the intercellular adhesive matrix of the biofilm. These polysaccharides
are highly hydrated (
>
95% water) and tenaciously bound to the underlying
surface.
The structure of the biofilm is not merely a homogeneous slimy mono-
layer but a heterogeneous one, changing in space and over time. Biofilms
have a composite structure comprising “water channels” that allow diffusion-
controlled transport of essential nutrients and oxygen to the cells embedded
within (Figure 4.3). Biofilms may also act as filters to entrap suspended par-
ticles of various kinds, including minerals and host components such as fibrin,
red blood cells (RBCs), and platelets.
4.2.2.1
Quorum Sensing
Quorum sensing is a term used to describe intercellular signaling in bacteria.
Although several quorum-sensing systems are known, the most thoroughly
investigated systems are the acyl-homoserine lactone (acyl-HSL) system of
many Gram-negative species and the peptide-based signaling system of many
Gram-positive species (Sturme et al. 2002; Parsek and Greenberg 2005). For
Search WWH ::
Custom Search