Typhoid toxin provides a window into typhoid fever and the biology of Salmonella Typhi

Abstract

Salmonella Typhi is the cause of typhoid fever, a disease that has challenged humans throughout history and continues to be a major public health concern. Unlike infections with most other Salmonellae, which result in self-limiting gastroenteritis, typhoid fever is a life-threatening systemic disease. Furthermore, in contrast to most Salmonellae, which can infect a broad range of hosts, S. Typhi is a strict human pathogen. The unique features of S. Typhi pathogenesis and its stringent host specificity have been a long-standing puzzle. The discovery of typhoid toxin not only has provided major insight into these questions but also has offered unique opportunities to develop novel therapeutic and prevention strategies to combat typhoid fever.

Keywords: Salmonella Typhi; bacterial pathogenesis; bacterial toxins; cell autonomous immunity; typhoid fever.

Fig. 1.

Typhoid toxin induces cell cycle arrest in intoxicated cells. A diagram of the genetic locus that encodes the typhoid toxin genes is depicted (Top). Culture epithelial cells were infected with WT S. Typhi or a typhoid toxin-deficient mutant derivative (∆cdtB), and, 72 h after infection, the infected cells were examined by phase contrast microscopy (Middle) or processed to measure DNA content by flow cytometry (Bottom). The peaks corresponding to cells in G0-G1, S, or G2 are indicated.

Fig. 2.

Atomic structure of typhoid toxin. (A) Two views of the overall structure of the typhoid holotoxin complex shown as a ribbon diagram and related by 90° rotation about a vertical axis. CdtB, PltA, and PltB are shown in blue, red, and green, respectively. (B) Bottom view of the channel formed by the PltB pentamer (in green), depicting the PltA C-terminal α-helix (in red) within it. (C) Atomic interface between CdtB and PltA. (Inset) A detailed view of a critical disulphide bond between PltA^Cys 214 and CdtB^Cys 269 (adapted from ref. 24).

Fig. 3.

Model for typhoid toxin secretion and export. Typhoid toxin expression is stimulated after S. Typhi enters into host cells, where it receives the specific cues to initiate its synthesis. The toxin subunits are secreted into the bacteria periplasm (Inset) via the sec machinery, which recognizes canonical secretion signals (secSP) in each one of the polypeptides. After assembly within the bacterial periplasm, the typhoid holotoxin is translocated through the peptidoglycan layer with the help of TtsA, an N-acetyl-β-d-muramidase that shares amino acid sequence similarity to a novel class of bacteriophage endolysins. Like its homologs TtsA lacks a typical secretion signal therefore it is expected that it reaches the periplasm aided by an as yet unidentified holin, a family of small membrane proteins that share the property of forming a protein channel through which endolysins reach the periplasmic space. After secretion from the bacterial cell into the Salmonella-containing vacuole, typhoid toxin is packaged into vesicle carrier intermediates, which transport the toxin to the plasma membrane for its delivery to the extracellular space. The toxin can then reach its cellular targets via paracrine or autocrine pathways.

Fig. 4.

Atomic structure of typhoid toxin B subunit bound to its glycan receptor. (A) The atomic structure of the PltB pentamer in complex with the GalNAcβ1-4(Neu5Acα2-8Neu5Acα2-3)Galβ1-4Glc oligosaccharide is shown as a ribbon diagram with each protomer depicted in a different color. Cyan, blue, and red sticks represent the sugar carbon, nitrogen, and oxygen atoms, respectively. (B) Surface charge distribution of the PltB pentamer structure and sugar-binding pockets. (C) Comparison of the sugar-binding sites of PltB and SubB bound to Neu5Ac and Neu5Gc, respectively. Critical residues that differ between SubB (Tyr78) and PltB (Val103) are highlighted as sticks. Other interacting amino acids and sugars are shown in lines. PltB is shown in yellow, Neu5Ac in Cyan, SubB in Green, and Neu5Gc in light purple (adapted from ref. with permission from Elsevier; www.sciencedirect.com/science/journal/00928674).

Fig. 5.

Model for the Rab32/BLOC-3–dependent cell-autonomous defense pathway that restricts the growth of S. Typhi in nonhuman hosts and the mechanisms evolved by the broad-host S. Typhimurium to counter it. In nonpermissive hosts, Rab32 (and its exchange factor BLOC3) coordinate the delivery of an antimicrobial factor to the Salmonella Typhi-containing vacuole. The broad host S. Typhimurium counters this host-defense pathway by delivering two type III secretion effector proteins, GtgE and SopD2, which exert their function as specific protease and GAP for Rab32, respectively (adapted from ref. with permission from Elsevier; www.sciencedirect.com/science/journal/19313128).