SHIGA TOXIN MODE OF ACTION IN E. COLI O157:H7 DISEASE

Tom G. Obrig

Department of Microbiology and Immunology, University of Rochester, Box 672, 601 Elmwood Avenue, Rochester, NY 14642

Received 12/2/97 Accepted 12/8/97

2. THE SHIGA TOXINS (Stx)-OVERVIEW

2.1 Production by gram-negative pathogenic bacteria

Discovery of the Shiga toxins was a result of a need to know more about how gram negative bacterial pathogens cause disease (1,2). This class of toxins has been proven to be fundamental to the pathogenicity of both Shigella dysenteriae type 1 and to the more well-known E. coli O157:H7. Because the E. coli -derived toxins were cytotoxic to Vero cells, the term "verotoxin" was suggested. Most recently, there has been an attempt to unify the terminology of the Shiga toxins such that: Stx is from Shigella dysenteriae type 1, and Stx1, Stx2, etc. from E. coli. The E. coli toxins were previously called Shiga-like toxins, type 1 and 2. As discussed below, the Shiga toxins are responsible for the systemic complications of infections and are not required for the initial colonization of the gut by these bacteria. Shigella dysenteriae type 1 is a tropical pathogen endemic in some areas of Indo-Asia, such as Bangladesh. In contrast, the enterohemorrhagic E. coli (EHEC), of which E. coli O157:H7 is the prototype, are common to more temperate areas of the world. Recent outbreaks of EHEC have occurred, for example, in the U.S., Canada, Germany, England, Switzerland, France, and Japan. Although Shigella and Escherichia represent different genera of bacteria, they are considered to be very close, both genetically and biochemically. The isoforms of Shiga toxins produced by these different pathogens are also very closely related in structure and function. However, some of the differences among members of the Shiga toxin group may be related to specific clinical developments in EHEC-related disease. Overall, the emphasis here is that these unique toxins are required for the vascular complications which occur following infection of humans with these bacterial pathogens.

2.2 Evolution of Stxs and the "emerging pathogen", E. coli O157:H7

Although the exact origin of O157:H7 is not known, a thorough comparison of the enzyme-encoding genes common to virtually all E. coli revealed the closest relative to be another pathogen, E. coli O55:H7, that is capable of colonizing the gut, but which causes only a mild diarrhea in infants (3). It is important to note that in addition to a difference in serotypes (a property of surface lipopolysaccharides), the O157:H7 bacteria now harbor the genes for Shiga toxins. The Shiga toxin genes are encoded by bacteriophage in EHEC, but reside on the chromosome in Shigella dysenteriae type 1. The evolutionary origin of Shiga toxin genes remains somewhat of a mystery. However, as mentioned below a portion of Shiga toxin protein structure is closely related to other proteins found in higher plants, all of which inactivate eukaryotic ribosomes enzymatically (4). It appears that the fate of these protein toxins in animals and humans is determined by the additional associated protein subunits which have an affinity for specific carbohydrate molecules expressed on the surface of various eukaryotic cells.

2.3 Stx is a vascular acting toxin

A comprehensive review of both clinical and basic science aspects of this topic has appeared recently (5). Although the virulence of E. coli O157:H7 without its toxin genes, i.e. an isogenic tox-minus mutant, has yet to be characterized in animals, it is likely that this pathogen would more resemble the virulence ability of E. coli O55:H7. A study of this type was conducted with Shigella dysenteriae 1, the results clearly showing that an isogenic tox-minus mutant could colonize lower primates, but had lost the ability to cause systemic vascular damage (6). Direct evidence of Shiga toxin action at the endothelial level was provided by a study in which purified Stx was administered to rabbits (i.v.) with ensuing vascular damage similar to that observed in humans, although this was restricted to the colon and brain in rabbits (7).