The Impact Of Biofilm Formation

Biofilms are collections of microorganisms, predominantly bacteria, enmeshed within a three-dimensional gelatinous matrix of extracellular polymers secreted by the microorganisms (Fig. 1). In an aqueous environment, a support, termed a substratum, will be immediately biased by dissolved organic molecules and macromolecules that adsorb rapidly from the liquid phase. Bacterial cells present in the fluid contact the substratum by a variety of transport mechanisms. Once at the substratum, the cells can adsorb either reversibly or irreversibly. Provided the cells remain at the surface for a sufficient time, they will secrete extracellular polymers that serve to attach the cells tenaciously to the substratum. Attached cells metabolize prevailing energy and carbon sources (either dissolved within the surrounding fluid, adsorbed to the substratum surface, or existing as a constituent of the substratum itself), reduce terminal electron acceptors, grow, replicate, and produce insoluble extracellular polysaccharides, thus accumulating an initial viable biofilm community. Inert particles, bacterial cells of the same or different species, and higher life forms (e.g., algae, amoeba, protozoa) continue to be recruited from the fluid and incorporated into the biofilm community.

As the biofilm bacterial communities mature, the adherent populations will oxidize electron donors and reduce existing terminal electron acceptors in an order of decreasing redox potential. As the biofilm community becomes denser and thicker, the penetration depth of one electron acceptor will overlap that of another; thus, a succession of different microbial populations can be established. Biofilm layers near oxygen-saturated water will harbor aerobic heterotrophic or autotrophic activity, reducing the available oxygen. As the distance from the aerobic interface increases and oxygen is depleted, a sequence of terminal electron acceptors is utilized by specific microbial populations: facultative denitrifiers using nitrate and nitrite; anaerobic sulfate reducers using sulfate; fermentative microbes partially reducing organic carbon compounds; and finally, anaerobic methane producers. Depending upon the balance between mass transfer rates and microbial reaction rates, biofilms can develop stratified ecosystems, on a microscopic scale, similar to those observed in lake or marine sediments. In engineered systems, highly active aerobic activity within a biofilm can completely deplete oxygen concentrations over distances of 10-20 im, creating anaerobic layers capable of supporting obligate, anaerobic sulfate-reducing bacteria. Consequently, it should come as no surprise to discover anaerobic sulfate reduction and hydrogen sulfide corrosion of the metallic surfaces in heat exchange systems that utilize highly aerated cooling water to condense steam.

As a result of hydrodynamic forces and stresses exerted by replication, there can be a continual erosion of cells and extracellular material from the biofilm back to the bulk fluid. A more random stochastic process known as "sloughing" can also occur where either large sections or the entire biofilm becomes displaced from the substratum and enters the liquid.

Biofilms are as versatile as they are ubiquitous. Intentional use of biofilms can serve many benefits, for example, in the water and waste treatment industry, in bioremedia-tion applications, and in industrial biotechnology. Unintentionally formed biofilms can create such detriments as biofouling of heat exchange systems and marine structures; microbial-induced corrosion of metal surfaces or the deterioration of dental surfaces; contamination of household products, food preparations, and pharmaceuticals; and the infection of short- and long-term indwelling biomedical implants and devices. such detriments can range in severity from being a mere nuisance to being life threatening. This chapter will first summarize the impact of biofilms, both beneficial and detrimental, on engineered and natural processes. A detail discussion of the fundamental rate processes governing biofilm formation and persistence is then given, followed by a summary of recent advances in biofilm research.

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