Shiga toxins: a review of structure, mechanism, and detection

Authors: Silva, Christopher - Chris; Brandon, David; Skinner, Craig; He, Xiaohua

Submitted to: Complete Book
Publication Type: Book / Chapter
Publication Acceptance Date: 9/15/2016
Publication Date: 3/3/2017
Citation: Silva, C.J., Brandon, D.L., Skinner, C.B., He, X. 2017. Shiga Toxins: a Review of Structure, Mechanism, and Detection. Cham, Switzerland: Springer International Publishing. 118 p.

Interpretive Summary: The E. coli that cause food poisoning are called STEC because they are Shiga toxin producing E. coli. Shiga toxins are large proteins made up of five smaller proteins (identical B subunits) and one larger protein (A subunit) that is responsible for the toxic effects of the toxins. The STEC bacteria often infect people when they eat food that is contaminated with those E. coli. Once inside a person, the STEC can replicate and produce Shiga toxin, which sickens the person. The Shiga toxins are responsible for an undue amount of human suffering. Each year approximately 175,000 Americans are sickened by STEC. The costs related to taking care of the people suffering from Shiga toxin-related disease is estimated to be 1 billion US dollars. Scientists have determined how Shiga toxins move from the STEC bacteria into the human patient. They have determined what Shiga toxin looks like. Scientists have made antibodies to the toxins and used sophisticated instruments, such as mass spectrometers, to detect Shiga toxins. These tests are used by regulators to detect Shiga toxins and STEC, so that contaminated food can be identified before people can eat it. Unfortunately there is no treatment for Shiga toxin poisoning. They are using this information to develop molecules that may lead to the treatment of the disease in the future.

Technical Abstract: Although Shiga toxin (Stx)-producing Escherichia coli (STEC) are responsible for a minority of foodborne disease outbreaks in the United States, they account for a disproportionately large number of hospitalizations, serious sequelae, and deaths associated with foodborne illness. Diseases caused by STEC sicken approximately 175,000 Americans and cost as much as 1 billion USD a year in direct costs, in addition to substantial indirect costs. The primary virulence factor of STEC is the Shiga toxin or toxins they produce. These toxins belong to AB5 group of toxins that contain 5 identical B subunits non-covalently bound to a single A subunit. The toxicity of Stx resides in the A subunit, which possesses N-glycosidase activity. The B subunits bind to specific gangliosides on the surface of a target cell and facilitate toxin uptake. Shiga toxins enter target cells by endocytosis and are then retrogradely transported from the cell membrane to the cytoplasm. During this process a characteristic motif of the A subunit is proteolyzed by furin (a target cell enzyme) and a disulfide bond is cleaved in the endoplasmic reticulum (ER) to release the enzymatically-active A1 domain from the holotoxin. The A1 fragment is translocated from the ER to the cytoplasm where it cleaves the N-glycosidic bond of adenosine 4324 of the 28S ribosomal RNA. This results in the inactivation of the 60S subunit of the ribosome and complete inhibition of protein synthesis. A single active A1 fragment is sufficient to kill a cell. Although outbreaks are associated with STEC, the production of Shiga toxins is controlled by temperate lambdoid phages that infect those strains. Since these phages are stable in the environment and the Shiga toxins they encode do not affect domestic livestock such as sheep and cattle, they can readily pass into new E. coli or even other bacteria. It is therefore not surprising that at least 400 strains of STEC have been isolated from human patients. With ongoing evolution of STEC as well as changes in dietary and food production practices, we can expect that future outbreaks will differ in their sources and impact from those we now experience. Basic and applied research is advancing our understanding of these toxins and has led to the development of a number of novel diagnostic tests. Recently developed antibodies have enabled more specific and rapid tests for Stx in a variety of matrices. More recently, mass spectrometry has been used to detect and distinguish among the Shiga toxins at extremely low levels (in the attomole, 10-18 mol range or about 8 µL of 1 femtomole/mL). The development of new toxin-binding molecules, including small molecules, antibodies, and nanobodies, has provided strategies to prevent the binding of toxin to target cells. Small molecules that interfere with the catalytic activity of the A1 fragment have also been developed. Antibiotics undergo systematic evaluation to identify those that will kill STEC without inducing the production of Stx carried by STEC-borne phages. Thus, many new tools for early detection of Stx and prevention of Stx-associated diseases are on the horizon.