Biomedical Engineering Reference
In-Depth Information
noted that the surface energy of the suspending medium may affect hydrody-
namic interactions of microbial cells with surfaces by altering the substratum
characteristics.
4.2.1.3
Hydrodynamics
Flow velocity of fluids in direct contact with substratum interface is mostly
negligible. This zone is defined as the hydrodynamic boundary layer. Its
thickness depends on the linear velocity of the outer medium; the higher the
velocity, the thinner the boundary layer. The flow in the region outside the
boundary layer is dominated by mixing or turbulence. Slow flow is charac-
terized by laminar or minimal turbulence. The hydrodynamic boundary may
enhance cell-substratum interactions. Suspended microbial cells behave as
particles in a liquid, and the rate of settling and association with a sub-
merged surface will largely depend on their carrier velocity. Under very low
linear velocities, the cells must cross the hydrodynamic boundary layer. As the
velocity increases, the thickness of the boundary layer decreases and cells will
be subjected to vigorous turbulence and mixing. Low linear velocities would
enable rapid association with the surface while high velocities will exert sub-
stantial shear forces, resulting in cell detachment (Characklis 1990b). These
effects were confirmed by studies carried out by Rijnaarts et al. (1993) and
Zheng et al. (1994). Mass transport of substrates and antimicrobial agents
into the biofilm is also affected by turbulence and boundary layer (Wanner
et al. 2006).
4.2.1.4
Characteristics of the Contacting Aqueous Medium
Other characteristics of the aqueous medium, such as pH, nutrient levels,
ionic strength, and temperature, may also play a role in the rate of micro-
bial attachment to a substratum. Several studies have shown a seasonal effect
on bacterial attachment and biofilm formation in different aqueous systems
(Donlan et al. 1994). This effect may be due to water temperature and other
seasonally affected parameters. Fletcher (1988) found that an increase in the
concentration of several cations affected the attachment of Pseudomonas fluo-
rescens to glass surfaces, presumably by reducing the repulsive forces between
the negatively charged bacterial cell's surface and the glass surfaces. Cowan
et al. (1991) showed correlation of increase in nutrient concentration with an
increase in the number of attached bacterial cells.
4.2.1.5
Cell Properties
Cell-surface hydrophobicity, presence of fimbriae and flagella, and produc-
tion of exopolysaccharide (EPS) all affect the rate and extent of micro-
bial cell attachment. The hydrophobicity of many bacterial cell surfaces
plays an important role in their adhesion along with their surface nega-
tive charge, as noted by Rosenberg and Kjelleberg (1986). Fimbriae, that is,
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