Characterization of the Structure, Regulation, and Function of CsgDmediated Escherichia coli Biofilms.

Abstract

Biofilms are communities of bacteria bound together by an extracellular matrix. Biofilm formation correlates with increased resistance to environmental stresses, both in an infection setting and in the non-host environment. Enteric bacteria such as Escherichia coli form biofilms by producing two matrix components, amyloidogenic curli fibers and the polysaccharide cellulose. I investigated the architecture, regulation, and function of rugose biofilms produced by the uropathogenic E. coli strain UTI89. Rugose biofilms are matrix-dependent, wrinkled colonies that form on agar plates. Using confocal microscopy and various molecular techniques I found that in rugose colonies, matrix production is limited to cells at the air/biofilm interface. Bacteria lining the interior of the biofilm do not produce matrix. Furthermore, the two biofilm populations can be mechanically separated by washing the biofilm in buffer. Interior cells wash easily into suspension while the matrix-encased exterior maintains its shape and stays aggregated. By investigating environmental cues that affect rugose colony development, I found that iron is a key regulator in biofilm formation. While E. coli produces matrix in both low and high iron conditions, iron induces the development of wrinkled, rugose colony biofilms. Iron-driven biofilm formation is not dependent on the presence of iron per se, but on cellular oxidation which results from iron exposure. Using low iron conditions, I screened for redox-sensitive regulators that affect rugose biofilm formation. I found that the ArcAB two-component system drives rugose development in response to redox-balance. In E. coli and many other enteric bacteria, biofilms are only produced at low temperatures (<30°C) and in low salt conditions. I hypothesized that such conditions would be prominent outside of the host, and that development of curli/cellulose-dependent biofilms could allow for environmental persistence. I therefore tested whether rugose biofilm formation could confer resistance to environmental stresses such as oxidation and predation. Indeed, rugose biofilm formation correlated with resistance to hydrogen peroxide stress and to killing by the predatory nematode Caenorhabditis elegans and the soil bacteria Myxococcus xanthus. Altogether my work outlines a model where non-host regulatory signals lead to production of a biofilm matrix that protects E. coli against environmental stresses.