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Key Words cell-cell signaling, biofilm phenotype, prokaryotic development, microbial communities, integrated communities, hydrodynamics
* Abstract Prokaryotic biofilms that predominate in a diverse range of ecosystems are often composed of highly structured multispecies communities. Within these communities metabolic activities are integrated, and developmental sequences, not unlike those of multicellular organisms, can be detected. These structural adaptations and interrelationships are made possible by the expression of sets of genes that result in phenotypes that differ profoundly from those of planktonically grown cells of the same species. Molecular and microscopic evidence suggest the existence of a succession of de facto biofilm phenotypes. We submit that complex cell-cell interactions within prokaryotic communities are an ancient characteristic, the development of which was facilitated by the localization of cells at surfaces. In addition to spatial localization, surfaces may have provided the protective niche in which attached cells could create a localized homeostatic environment. In a holistic sense both biofilm and planktonic phenotypes may be viewed as integrated components of prokaryote life.
INTRODUCTION
In just two decades we have learned that biofilms comprise highly structured matrix-enclosed communities (10) whose cells express genes in a pattern (51) that differs profoundly from that of their planktonic counterparts. Because direct observations show that biofilms constitute the majority of bacteria in most natural (8) and pathogenic (11) ecosystems, it seems unwise to continue to extrapolate from planktonic cultures in studies of these systems. A new mindset is clearly required because direct observations of structural complexity and unequivocal demonstrations of one or more distinct biofilm phenotypes presage a new concept in which biofilms are seen as complex differentiated communities. Observations of biofilms formed in pure cultures of the gamma proteobacteria group of bacteria and of mixed species biofilms in natural ecosystems show a basic organization in which cells grow in matrix-enclosed microcolonies separated by a network of open water channels. The importance of these complex structures, which are seen in direct observations of living biofilms by scanning confocal laser microscopy (SCLM), is that they demonstrate a level of differentiation that requires a sophisticated system of cell-cell signals and a degree of cellular specialization. The simple maintenance of open water channels in multispecies biofilms requires interspecies signaling to direct growth and exo-polysaccharide production away from the...