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Difficulty: Very hardInfectious Disease Mechanisms

An infection control team is investigating a cluster of severe pseudomembranous colitis cases at a long-term care facility. Several residents developed profound, foul-smelling diarrhea, fever, and leukocytosis (with white blood cell counts exceeding 25,000/mm325,000/\text{mm}^3) after receiving broad-spectrum antibiotics for various indications. Stool assays confirm the presence of a spore-forming, anaerobic, Gram-positive bacillus. The pathogen's major cytotoxin (Toxin B) causes massive disruption of the intestinal epithelial cytoskeleton through a highly specific intracellular cascade. Following the initial binding of this cytotoxin to host cell surface receptors (such as Frizzled-1 or CSPG4), arrange the subsequent pathophysiological events in the correct chronological sequence leading to cytoskeletal collapse.

  1. 1Internalization of the toxin-receptor complex into an endocytic vesicle
  2. 2Acidification-dependent conformational shift and insertion of the translocation domain
  3. 3Activation of the autoproteolytic cysteine protease domain by host cytosolic inositol hexakisphosphate
  4. 4Cleavage and release of the active catalytic glucosyltransferase domain into the cytoplasm
  5. 5Covalent modification and inactivation of host Rho, Rac, and Cdc42 GTPases

Answer

The correct sequence of events is: endocytosis of the complex, acidification and insertion of the translocation domain, InsP6-mediated autoproteolysis, release of the catalytic domain, and finally glucosylation of host GTPases.
The pathogenesis of Clostridioides difficile Toxin B (TcdB) involves a highly coordinated multi-step cascade. It begins with receptor binding, followed by receptor-mediated endocytosis. Once inside the endosome, the acidic pH induces a conformational change allowing the delivery (translocation) domain to insert into the endosomal membrane. The cysteine protease domain (CPD) and glucosyltransferase domain (GTD) are then translocated across the membrane. Binding of host cytosolic inositol hexakisphosphate (InsP6) to the CPD triggers autoproteolytic cleavage. This critical step releases the GTD into the cytosol, where it covalently mono-glucosylates Rho, Rac, and Cdc42 GTPases. This inactivation disrupts the actin cytoskeleton, leading to cell rounding, apoptosis, and the profound epithelial damage characteristic of pseudomembranous colitis.

Step-by-Step Solution

1
Identify the pathogen and specific virulence factor from the clinical presentation.
The clinical scenario points to Clostridioides difficile, and the primary cytotoxin disrupting the cytoskeleton is Toxin B (TcdB).
Recognizing the specific toxin is essential to determining its molecular mechanism of action.
2
Determine the method of cellular entry.
Receptor binding is followed by endocytosis into an intracellular vesicle.
Most large multidomain exotoxins must be internalized into endosomes before their catalytic domains can reach the host cytosol.
3
Analyze the mechanism of endosomal escape and activation.
Endosomal acidification causes the translocation domain to insert into the membrane, followed by InsP6 binding which triggers autoproteolysis and release of the active domain.
The low pH of the endosome is the universal trigger for membrane insertion, while cytosolic InsP6 provides a host-specific activation signal.
4
Identify the final enzymatic target and action.
The freed glucosyltransferase domain glucosylates Rho family GTPases.
This terminal enzymatic action inactivates the proteins regulating the actin cytoskeleton, leading to the cytopathic effects seen in the disease.

Key Concept

Molecular pathogenesis and intracellular processing of Clostridioides difficile Toxin B (TcdB)
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Topics

Infectious Disease Mechanisms

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