Microbial Biofilms
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CHAPTER 3.10 l a i bor c M i
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Microbial Biofilms DIRK DE BEER AND PAUL STOODLEY
Introduction Biofilms are usually thought of as the slimy layer of microorganisms that covers solid surfaces. However, there are a number of features that distinguish biofilm populations from their planktonic (suspended or free floating) counterparts, namely: the association with a surface, high population densities (on the order of 1010 cells per ml of hydrated biofilm), an extracellular polymer (EPS) slime matrix, and a wide range of physical, metabolic and chemical heterogeneities. However, some biofilms may not have all features. Indeed a concise universal definition of biofilms has yet to emerge; in part this is because of the wide diversity of biofilm populations. Although much of contemporary microbiology is based on the study of planktonic “cells,” it is now thought that biofilms are the primary habitat for many microorganisms. Microbial mats associated with sediment and suspended microbial flocs or aggregates, although different in appearance from conventional biofilms, have many important features in common and thus are included in the definition of “biofilm.” Often biofilm cells are embedded within a highly hydrated EPS matrix, and in the absence of corrosion products or scale, biofilms are estimated to be primarily water. The physical properties of the biofilm are largely determined by the EPS, while the physiological properties are determined by the bacterial cells. Characklis (1990a) identified up to eight processes in the development of biofilms. These can be condensed to three main processes: the attachment of cells to a surface (colonization), growth of the attached cells into a mature biofilm, and the detachment of single cells (erosion) or large pieces (sloughing). Since free convection is hindered within biofilms, the chemical environment to which the cells are exposed differs from the surrounding water phase. Also, mass transfer to the cells often limits conversion rates. All natural biofilms, mats, aggregates, and flocs can consist of complex microbial communities, and their function is characterized by interactions between different
populations within these communities. Many experimental methods to study aggregated biomass are similar, i.e., microscopic and staining techniques. We will refer therefore in this review to all types of aggregated microorganisms. Table 1 gives some examples of biofilm types. Biofilms, flocs, and microbial mats are responsible for most microbial conversions in natural environments. Natural biofilms can develop on solid surfaces under all conditions facilitating microbial growth thus biofilms are ubiquitous in nature, covering rocks and plants in seawater and freshwater, sediment grains, and sediment surfaces. Microbial mats are formed on most sediments, especially under extreme conditions (temperature, salinity) that inhibit the activity of grazers (Karsten and Kühl, 1996). Flocs are highly fragile structures suspended in fresh and seawater (called river- and marine snow) an
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