Studies on Escherichia coli serotype O157:H7 strains containing a 60-MDa plasmid and on 60-MDa plasmid-cured derivatives

Seventeen strains of Escherichia coli serotype O157:H7 producing Shiga-like toxin were examined for the presence of plasmids and for the ability to adhere to HEp-2 and Intestine 407 cells. All of the strains possessed a common 60-MDa plasmid. To determine the role of the 60-MDa plasmid, plasmid-cured strains were compared with the parent strains for their ability to produce pili and to adhere to epithelial cells in culture. The total cell lysate protein and outer-membrane protein (OMP) profiles were also compared. Both the parent strains and their plasmid-cured derivatives produced pili. Immunofluorescence assay results indicated that the plasmid-cured and parent strains adhered equally well to HEp-2 and Intestine 407 cells; overall adherence was greater with intestinal cells than HEp-2 cells. SDS-PAGE of polypeptides synthesised in an E. coli system in vitro showed that plasmid DNA encodes c. 35 proteins. SDS-PAGE of OMP preparations demonstrated that the 60-MDa plasmid appears to be involved in the synthesis of a 33-kDa OMP. Two strains cured of the 60-MDa plasmid, one that possessed no plasmids and one that still contained a 2.2-MDa plasmid, produced exopolysaccharide (EPS) when cultured on solid medium at 25 degrees C. Two other strains, which were cured of the 60-MDa plasmid but contained a 4.5-MDa plasmid, did not produce visible amounts of EPS. Gas chromatography analysis showed that the EPS consisted of fucose, glucose and galactose in an approximate molar ratio of 2.0:0.9:1.1 and also had 7% of a uronic acid sugar as part of its structure.