Question
All the following are false regarding C. diphtheriae toxin EXCEPT:
| A. |
It is an endotoxin |
| B. |
Toxin production depends on optimum concentration of iron |
| C. |
It acts by activating adenyl cyclase |
| D. |
Schick Test demonstrates circulating toxin |
|
Correct Answer » B Explanation |
 |
Corynebacterium diphtheriae produces a potent exotoxin responsible for the systemic manifestations of diphtheria. Understanding the toxin’s nature, regulation, and mechanism is essential for solving such MCQs.
Nature of Diphtheria Toxin
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It is a protein exotoxin, not an endotoxin.
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It is encoded by a β-bacteriophage (lysogenic conversion).
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Structurally, it is an A-B toxin:
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B (binding) subunit: attaches to host cell receptors.
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A (active) subunit: inhibits protein synthesis inside the cell.
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Mechanism of Action
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The toxin causes ADP-ribosylation of elongation factor-2 (EF-2).
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This leads to inhibition of protein synthesis, resulting in cell death.
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This mechanism is entirely different from toxins that act via adenylate cyclase (e.g., cholera toxin).
Regulation of Toxin Production (Key Concept)
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Toxin production is regulated by iron concentration via the DtxR repressor gene.
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When iron levels are high:
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DtxR is activated → toxin production is suppressed.
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When iron levels are low:
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Repression is lifted → toxin is produced.
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