Microbial biofilms

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 (Figs. 13.1 and 13.2).