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Test your knowledge
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Section 5 |
Structure changes geometry of the
diffusion problem
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Much has been made over the last decade of the heterogeneous
architecture of some biofilms (3, 36). These biofilms are
described as clusters of microbial cells that are interspersed
with water channels through which liquid flows. It is natural
to wonder whether such conduits might ameliorate or even
eliminate limitation of diffusive solute transport.
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Figure 4-8.
Flow through channels in a microbial biofilm. This movie
follows the movement of fluorescently-tagged latex beads
(small, bright spots) through channels and around cell
clusters. Image credit: P. Stoodley (36).
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Flow through water channels can improve
solute transport in the immediate lining of the channel, but
it does not assure access to the interior of cell clusters.
Perhaps the best demonstration of this fact is the direct
experimental measurement of oxygen penetration to the base of
a biofilm in a void area but failure of the oxygen to
penetrate in an adjacent cell cluster (8). Water may course
through channels, but it does not percolate the cell clusters
themselves (see
example in previous section). The dense aggregation of bacterial cells and their extracellular polymers within cell clusters precludes fluid
flow. This means that water channels can expose the surfaces
of clusters or channels but they do not allow free access of
solutes to the interior of cell clusters.
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Structural heterogeneity in a biofilm changes the geometry of
the diffusion problem, but it does not alter the fundamental
phenomena. The illustrative calculations presented in this
article have all been framed in terms of simple shapes, but
the ability to calculate diffusion phenomena is not limited by
biofilm geometry in this era of fast computers. Several
mathematical models have been described that calculate
reaction-diffusion interactions in heterogeneous biofilm
structures in two dimensions (12,
15,
23).
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Test your knowledge
| Proceed to
Section 5 |
