Molecular and cellular insight into Escherichia coli SslE and its role during biofilm maturation

Abstract

Escherichia coli is a Gram-negative bacterium that colonizes the human intestine and virulent strains can cause severe diarrhoeal and extraintestinal diseases. The protein SslE is secreted by a range of pathogenic and some commensal E. colistrains. It can degrade mucins in the intestine, promotes biofilm maturation and in virulent strains, it is a major determinant of infection, although how it carries out these functions is not well understood. Here we examine SslE from the E. coli Waksman and H10407 strains and using electron microscopy (EM), small angle X-ray scattering (SAXS), nuclear magnetic resonance (NMR) spectroscopy and biochemical analyses we show that SslE has a highly dynamic structure in solution. We also directly observe acidification within mature biofilms, describe a mechanism where SslE forms unique functional fibres under these conditions and determine that these SslE aggregates can bind cellulose, a major exopolysaccharide of many E. coli biofilms. Our data indicates that the spatial organization of SslE polymers and local pH are critical for biofilm maturation and SslE is a key factor that drives persistence of SslE-secreting bacteria during acidic stress.