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Bacteria: S. typhi and typhoid toxin


Typhoid fever is caused by the ingestion of food and water contaminated with the gram-negative bacterium Salmonella enterica serovar Typhi. Typhi bacteria are deadly because they produce a unique type of AB toxin. Typically AB toxins consist of two subunits: The “A” subunit is the active toxin and the “B” subunit binds to cell surface receptors, triggering endocytosis. Typhoid toxin is known as A2B5 because there are two different A units, CdtB and PltA, and a pentameric B subunit comprising 5 PltB peptides. In epithelial cells, typhoid toxin binds to podocalyxin-like protein 1 (PODXL) receptors.
Researchers performed the following experiments to investigate S. typhi binding and toxicity.
Experiment 1
Researchers introduced shRNA (short hairpin RNA) and found that it decreased the toxin-binding ability of epithelial cells. In addition, they found that typhoid toxin binds to other cell types with the aid of carbohydrate moieties on the surface of glycoproteins. After removing the carbohydrates from the surfaces of the proteins, the binding ability of typhoid toxin was reduced.
Experiment 2
Researchers attempted to determine which subunit or subunits cause typhoid toxicity. For this investigation, they used mice, who do not have the receptors needed to initiate endogenous production of typhoid toxin but nevertheless respond to the toxin itself. Researchers injected experimental groups with high levels of normal typhoid toxin, CdtB only, PltA only, or PltB only. Researchers used weight loss—a symptom of typhoid fever—as a gauge of toxicity levels. Results are shown in Figure 1.
Figure 1 Results of exposure to high levels of typhoid toxin, various typhoid toxin subunit proteins and to no toxin
Experiment 3
Researchers investigated the cell cycle status of cells treated with wild type S. Typhi or another variant of S. Typhi in which a disulfide bond in the junction between PltA and CdtB is broken. Cells in each stage of the cell cycle were quantified; results are shown in Figure 2.
Figure 2 Cell cycle profiles; peaks correspond to specified transitionary points
Sources: All figures information was adapted from J. Song, X. Gao, J.E. Galán Structure and function of the Salmonella Typhi chimaeric A2B5 typhoid toxin Nature, 499 (2013), pp. 350–354.
Which subunit or subunits of the typhoid toxin is most likely the actual cause of typhoid fever?
Choose 1 answer: