Importance of Escherichia coli Strains Producing Verotoxins

Abstract

The production of a heat-labile protein unrelated to cholera toxin was described in the 1960s in studies of Escherichia coli isolated from animals (1,2). However, it was not until 1982 when Riley et al. reported an outbreak of haemorrhagic colitis associated with an E. coli that produced the same heat-labile toxin, and that these strains were considered hazardous to humans (3). A second interesting outcome of the study by Riley et al. was that E. coli strains isolated during this outbreak belonged to a serotype that had not been previously associated with human disease. Over the subsequent years, this 'new' serotype, O157:H7, became the predominant causal pathogen worldwide for both haemorrhagic colitis and haemolytic-uraemic syndrome. Outbreaks of haemorrhagic colitis and haemolytic-uraemic syndrome associated with O157:H7 E. coli strains have been reported in Canada, the United States, Europe, and Japan (4). The production of heat-labile cytotoxin is not restricted to E. coli O157:H7, with studies showing that other E. coli strains have this character (5), and the common trait among these bacteria is the ability to produce heat-labile cytotoxin. Such strains have been grouped according to specific characteristics: Verocytotoxin-producing E. coli, which relates to the capacity to destroy monolayers of vero cells in culture; enterohaemorrhagic E. coli for their capacity to cause haemorrhagic colitis and haemolytic-uraemic syndrome; and more recently, Shiga-toxin (Stx)-producing E. coli (STEC) for the similarity of the toxin produced by these E. coli strains with that produced by Shigella dysenteriae type 1 (4). Given the severity of the diseases associated with infection by these strains and their importance as the leading cause of acute renal failure in children aged less than five years in developed countries, an international network of interested laboratories has been established to conduct surveillance and determine the burden of disease associated with these infections. Concerned laboratories in less-developed areas of the world have also joined this international endeavour to determine the importance of such strains as part of the spectrum of diarrhoeal diseases in their own environments. Studies conducted in less-developed areas of the world have already shown little participation of STEC strains as causal agents of diarrhoeal disease in these countries (4). What is interesting is that the conditions for transmission of these strains from animals to humans exist, as do the inherent problems relating to basic sanitation and safe drinking-water. Therefore, the question that remains to be answered is why there has been such a lack of isolation of STEC in these areas. The reason is not due to a lack of technical capacity in participating laboratories or to inadequate surveillance systems in these areas. Active surveillance at the community level has been conducted in several longitudinal surveys of diarrhoeal disease in less-developed countries in which other types of pathogenic strains of E. coli have been properly identified and characterized by state-of-the-art methods (4). Therefore, there has to be another explanation for the lack of participation of STEC as a cause of major outbreaks of disease in these areas of the world. In this issue of JHPN, Voravuthikunchai et al. report the results of a study conducted in Thailand to determine whether the presence of a humoral immune response to O157 lipopolysaccharide (LPS) could be a reason for the lack of disease associated with infection by these strains in people with diarrhoea in that country (6). An IgM response to the O157 LPS was found by these authors in 12% of 332 serum samples obtained from healthy blood donors and patients with diseases unrelated to diarrhoea. In the case of an IgG response, 23% of these same samples showed a positive response to O157 LPS. The authors conclude that possible exposure to cross-reacting antigens in these subjects could be the reason for finding this response in their sera, and a possible protection against colonization and disease associated with O157:H7 strains. …