RESUMEN
Enterohaemorrhagic Escherichia coli (EHEC) can cause a variety of human illnesses ranging from uncomplicated diarrhoea to haemorrhagic colitis and haemolytic uremic syndrome. The serotype O157:H7 has been associated with numerous outbreaks worldwide, but in Brazil the infection is rare. Brazilian adults present antibodies reactive with the principal virulence factors of enteropathogenic E. coli (EPEC) that have many genetic and antigenic similarities with EHEC. Lipopolysaccharides (LPS) are components of outer membranes and important virulence factors of Gram-negative bacteria. LPS O111 is present in EPEC and EHEC strains. LPS O157 is found only in EHEC strains, but it has some structural similarities with LPS O55 present in EPEC strains. This study investigates the levels of IgG and IgM seric antibodies reactive with EHEC O157:H7, EHEC O111:H-, EPEC O111:H- and the levels of anti-LPS O111, LPS O157 and LPS O55 antibodies in healthy adults living in São Paulo, Brazil. The antibody levels were determined by an enzyme-linked immunosorbent assay for 100 individual serum samples, and the presence of anti-bacterial and anti-LPS seric antibodies was confirmed. Positive correlations were found among the three kinds of antibodies. The concentrations of IgM anti-LPS were significantly higher than those of IgG, and surprisingly, the concentrations of anti-LPS O157 were high in view of the infrequent isolation of O157 bacteria in Brazil. Our results suggest that there is a cross-reacting immunity to EHEC in the Brazilian population, which may be a result of the immunity to EPEC antigens. Alternatively, Brazilians may be exposed to EHEC more frequently than has previously been thought.