ABSTRACT
During the last decade it became obvious that the researchers who studied biofilms at the microscale accumulated a collection of experimental results that were impossible to interpret using the traditional conceptual model of biofilms where microorganisms are uniformly distributed in a continuous matrix of extracellular polymers. Some of these results are discussed later in this chapter. As a solution, a new conceptual model of heterogeneous biofilms has been suggested. There are several versions of that model now, all conveying the same message, viz. that biofilms consist of microcolonies separated by interstitial voids. It soon became evident that the basic declaration of the new model, that the building blocks of biofilms are microcolonies, may have implications going much further than initially expected. Reported evidences of spontaneous microbial coaggregation and cell-cell recognition (Kolenbrander and London, 1992; 1993), and cell-cell communication in biofilms (Davies et al., 1998) were quickly associated with the new biofilm model and hypotheses were suggested regarding the possible role of microcolony structure and internal cell organization in biofilm activity and survival. The journal New Scientist expanded on these hypotheses in an article published in August 1996, and emphatically compared biofilms to cities built by microorganisms. General expectation among biofilm researchers is that these elaborate microorganism-formed structures have meaning. However, notions endowing biofilms with abilities to intentionally control these structures, and their environment, should be approached with caution, as there is little experimental evidence to support them. Nevertheless, the unusual propensity of microorganisms to form complex structures on surfaces has been noted by many researchers (Keevil and Walker, 1992; Costerton et al., 1994, Massol-
Deya et al., 1994; Wolfaardt et al., 1994; Bishop and Rittmann, 1995), although the reasons for this remain unclear. In natural and engineered systems a spectrum of structurally heterogeneous biofilms is observed ranging from dense, amorphous biofilms, which are less structurally heterogeneous, to biofilms demonstrating robust, well developed structures.
