Lectin-binding epitopes at the surface of Escherichia coli K-12: examination by electron microscopy, with special reference to the presence of a colanic acid-like polymer

The presence and distribution of lectin-binding epitopes at the surface of Escherichia coli K-12, strain W1655, was studied by electron microscopy after lectin-gold labeling and negative staining. A comparison was made between the lectin-binding capacity of cells cultivated at 20 degrees C and 37 degrees C (in broth or on agar). A variety of pre-treatment protocols were applied prior to labeling. The gold-conjugated lectins used were wheat germ agglutinin (WGA), soybean agglutinin (SBA) and Ulex europaeus lectin (UEA-I). For all culture conditions, the bacteria had moderate exposure of WGA-binding sites, and this was not changed after pre-treatment. Cells cultivated at 37 degrees C had exposed SBA- and UEA-I-binding epitopes apparently associated with the cell surface. These significantly increased in number after boiling the cells for 10 min. With bacteria cultivated at 20 degrees C these two lectins recognized sites situated on exopolysaccharide filaments. Affino dot-blot experiments with isolated polysaccharides of the strain identified the K-12 lipooligosaccharide as the source of WGA-binding epitopes, and the exopolysaccharide, colanic acid (CA) as the source of SBA- and UEA-I-binding sites. The interaction with these two lectins of bacteria cultivated at 37 degrees C could be due to altered translocation of CA from the cytoplasm to the environment. This suggestion was supported by the demonstration by electron microscopy of SBA and UEA-I binding at the surface of hot phenol-water extracted cell walls.