Fig. 2. A simplified schematic illustrating the concept of biofilms as diversity incubators, and the dynamic process of diversity generation in biofilms.

The biofilm mode of life and external stressors stimulate the generation of mutations in biofilm cells. The selection gradient that forms as a result of external stressors acts differentially on the various layers of cells. This allows the sequential generation of increasingly fit phenotypes under multiple rounds of mutation and increasing selection pressure. Cells with decreased fitness (as a result of initial mutations) can survive within the protective biofilm environment and accumulate mutations that can restore, or even increase their fitness. Upon dispersal from the parent biofilm, cells with decreased fitness perish without the protective shield of the biofilm, whereas cells with increased fitness have the potential to establish new biofilm communities where the further evolution of fitness can occur. Mutations leading to an increase in fitness are indicated by blue arrows followed by a ‘+’ sign, mutations leading to decreased fitness are indicated by red arrows and a ‘−’ sign. A limited number of variations is presented.