Biomedical Engineering Reference
In-Depth Information
FIGURE 5.2
Schematic of possible spatial organization in a natural biofilm community.
Depending on the availability of terminal electron acceptors (e.g., oxygen,
nitrate, sulfate), aerobic organisms might establish themselves in the top
layer of the biofilm, followed by nitrate-reducing organisms whose activity
will increase once oxygen availability decreases, followed by sulfate-reducing
organisms, which require a much lower redox potential (indicated by E h ). In
the absence of these electron acceptors, anaerobic hydrolysis of complex car-
bon sources, fermentation, as well as methanogenesis might occur, which can
produce sugars, organic acids, alcohols, and other small organic compounds,
which can serve as electron donors and carbon sources for the respiratory
organisms (aerobes, nitrate- and sulfate-reducing organisms) in the top layers
of the biofilm. Carbon cycling not shown in this figure to reduce complexity.
It can be imagined, for instance, that in a natural environment a number of
aerobic organisms establish themselves in the top layers of a biofilm consuming
the available oxygen and provide anaerobic conditions for nitrate-reducing,
sulfate-reducing, and fermentative organisms. The fermentative organisms
in turn might produce short-chain organic acids and alcohols, which could
become inhibitory in a solely fermentative community but are being con-
sumed by the nitrate-, sulfate-, and oxygen-reducing organisms. Experimental
evidence of such stratification has been published (Kuhl and Jorgensen 1992;
Ramsing et al . 1993; Okabe et al . 1999) and is outlined in Figure 5.2.
Even without the protection from other community members, the EPS
can present a diffusion barrier for solutes allowing organisms deeper inside
a biofilm community potentially more time for the initiation of a protective
stress response. For reactive solutes, such as oxidative antimicrobials (e.g.,
chlorine), EPS can also present a reactive barrier. Owing to the high-reaction
rate of chlorine with extracellular matrix components, its penetration can
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