The Biology of the Escherichia coli Extracellular Matrix.

Biofilm formation can increase bacterial fitness in both host and non-host environments (37, 38). In this review we will use the general definition of a biofilm as a group of surface-associated bacteria enveloped in a self-produced extracellular matrix (39). The E. coli extracellular matrix contains a major protein polymer called curli and the carbohydrate polymer, cellulose (40–42). Although curli and cellulose are typically the most abundant biofilm constituents, the extracellular matrix of E. coli can also include type 1 pili, flagella, antigen 43, DNA, β-1,6-N-acetylglucosamine (β-1,6-GlcNAc), capsule sugars, and colonic acid (43). Most pathogenic strains of E. coli form robust biofilms, however, some laboratory strains of E. coli are attenuated in their ability to produce biofilms. The K12 strain of E. coli was first isolated from Stanford in 1922, and was subsequently passaged for more than 50 years (44). This passaging led to evolutionary adaptation to the laboratory growth conditions, and to the loss of certain traits that influence biofilms (45). K12 E. coli therefore requires extended periods of time to adhere to surfaces and form biofilms (40). On the other hand, a host of pathogenic, environmental, and commensal E. coli isolates readily form biofilms in the laboratory and therefore make great model organisms for biofilm formation studies (46–50). Biofilm formation correlates with resistance to a variety of environmental stresses, including antibiotics, the immune system, and predation (38). Resistance is conferred through at least two distinct mechanisms. First, the extracellular matrix forms a physical barrier that can resist shear stress and recognition and phagocytosis by immune cells (38). Secondly, bacteria within biofilms often assemble into subpopulations that have distinct physiological characteristics (46, 51, 52). Subpopulation development can be triggered by mutations, stochastic gene expression, or chemical gradients that develop during biofilm formation (37, 52–54). For instance, bacteria at the biofilm surface are exposed to more oxygen, stimulating a higher rate of aerobic respiration (37, 55, 56). Metabolic changes often coincide with resistance to different stresses (54, 57). A biofilm community with multiple subpopulations, each resistant to different stresses, therefore demonstrates resistance to a broader range of environmental pressures to the biofilm community as a whole (37, 54, 57).