In a more indirect way, the study of the multilayered

protective mechanisms also seems to lead to alterations in genetic expression. The earliest protective mechanisms that were studied included physical protection (typically by diffusion limitation/reduction) and physiological protection (through heterogeneous PF-02341066 price growth rates and nutrient concentrations within the biofilm). These mechanisms offer only transient protection (Cogan et al., 2005). Therefore, other mechanisms likely play a role. (3) What is the basis for biofilm persistence? Bacterial populations produce ‘persister’ cells that neither grow nor die in the presence of antibiotics. This phenomenon can lead to failure of antibiotic treatment in clinical situations. Persisters are different than drug-resistant mutants because their antibiotic tolerance is nonheritable and reversible (Lewis, 2007). These specialized cells, which are extremely tolerant to antibiotic application, can arise from a variety of

processes including gene expression, senescence, or niche expansion. Recent evidence indicates that this subpopulation may actively repress the expression of targets that are normally inhibited by antibiotics. This pathway is triggered in part by the SOS response and appears to involve toxin–antitoxin systems (Dorr et al., 2010; Kim & Wood, 2010). The process of persister cell formation has been incorporated into several mathematical models, sometimes indicating the predicted spatial location (Roberts & Stewart, 2005; Cogan, 2010), temporal find more stability (De Leenheer & Cogan, Progesterone 2009) or dynamic response to disinfection (Cogan, 2006). This is an area where the direct comparison of mathematical models and experimental studies has been explored helping to validate experimental hypotheses and suggest potential biological mechanisms (Balaban et al., 2004; Rotem et al., 2010). (4) How does the biofilm community contribute to ecological processes? The final question that

we will address is that of the developing ecology of the biofilm and its community. Understanding the phenotypic mosaic of developing biofilms is of importance in a variety of situations. For example, bioremediation often requires some control on the formation and elimination of an engineered biofilm. Also, biomineralization and other studies require detailed knowledge of the distribution of various species/phenotypes within the biofilm as well as their interactions. In general, ecological studies require the models to incorporate direct or indirect interaction between the biofilm components. In this way, the newest generation of models typically includes multiple species/phenotypes and often multiple substrates. It should be noted that the earliest models addressed some of these factors (Wanner & Gujer, 1986); however, based on the intermediate models it is clear that transport processes, mechanical structure, chemistry, and physics may be much more important than was initially assumed.