Biomedical Engineering Reference
In-Depth Information
Cell surface properties such as the presence of cell surface molecules (e.g.,
proteins or carbohydrates), pili, flagella, as well as cell surface hydrophobicity
and charge have been shown to play a role in microbial attachment. Motil-
ity, chemotaxis, cell buoyant density, size, and shape are cell properties that
can influence the transition of cells from the planktonic to the attached state
(Bouweretal . 2000). Many of these parameters can change with the physio-
logical state of the microbes, and recent studies have demonstrated that bacte-
rial starvation can enhance bacterial transport in porous media (Cunningham
et al . 2007).
Solute characteristics such as solution ionic strength, pH, temperature,
and the presence of organic compounds, such as nutrients or surfactants, can
influence the tendency of microbes to remain in suspension or to attach to a
surface in porous media (Bouwer et al . 2000). In natural systems, the ability
to control these parameters is somewhat limited because of the large amount
of (ground) water that would have to be manipulated.
The same is true for porous media characteristics. Pore size and pore
size distribution; grain and grain size distribution; mineralogy; roughness; the
presence of sorbed, dissolved, or suspended organic matter; and porous media
hydrophobicity have been shown to influence microbial attachment and trans-
port in porous media, but these parameters might be dicult to control or
manipulate in natural systems (Bouwer et al . 2000).
Many of the models describing microbial transport in porous media are
based on the advection dispersion equation and attempt to correlate one or
multiple of the parameters listed above to the collision eciency factor to
create a link between the fairly well-understood hydrodynamics and the less
well-understood microbe-surface interactions. However, apparent scale depen-
dencies of these parameters, possibly due to the heterogeneity of porous media
and microbial populations, have made it dicult to reliably couple our knowl-
edge of the micro- and nano-scale interactions between microbial cells and the
porous media with models describing the advective transport of microorgan-
isms (see discussions and references in Ginn et al . [2002] and Thullner and
Baveye [2008] for more information).
5.2.2 Biofilm Growth
Once attached to a surface, the development of a biofilm structure will depend
on a number of parameters, including availability of growth-limiting nutrients,
presence of inhibitors, as well as the prevailing hydrodynamics.
As will be discussed later in detail, direct observations of biofilm formation
and growth in porous media is a formidable challenge due to the opaque nature
of most porous media. Hence, most of the existing knowledge is based on
very simple flow cell experiments with plane (nonporous) surfaces to facilitate
microscopic investigations.
The type and extent of biofilm formation depends on the ability of the
attached microorganisms to grow and reproduce. Major factors contributing
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