TheErwinia chrysanthemiType III Secretion System Is Required for Multicellular Behavior

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Abstract
Enterobacterial animal pathogens exhibit aggregative multicellular behavior, which is manifested as pellicles on the culture surface and biofilms at the surface-liquid-air interface. Pellicle formation behavior requires production of extracellular polysaccharide, cellulose, and protein filaments, known as curli. Protein filaments analogous to curli are formed by many protein secretion systems, including the type III secretion system (TTSS). Here, we demonstrate thatErwinia chrysanthemi, which does not carry curli genes, requires the TTSS for pellicle formation. These data support a model where cellulose and generic protein filaments, which consist of either curli or TTSS-secreted proteins, are required for enterobacterial aggregative multicellular behavior. Using this assay, we found thathrpY, which encodes a two-component system response regulator homolog, is required for activity ofhrpS, which encodes a σ54-dependent enhancer-binding protein homolog. In turn,hrpSis required for activity of the sigma factor homologhrpL, which activates genes encoding TTSS structural and secreted proteins. Pellicle formation was temperature dependent and pellicles did not form at 36°C, even though TTSS genes were expressed at this temperature. We found that cellulose is a component of theE. chrysanthemipellicle but that pellicle formation still occurs in a strain with an insertion in a cellulose synthase subunit homolog. Since the TTSS, but not the cellulose synthase subunit, is required forE. chrysanthemipellicle formation, this inexpensive assay can be used as a high throughput screen for TTSS mutants or inhibitors.