Question

In Bacillus anthracis, the poly-D-glutamic acid (poly-\(\gamma\)-D-glutamate) capsule is encoded by which plasmid?

• A. pXO1 (toxin plasmid)
• B. pXO2 (capsule plasmid)
• C. Chromosomal genes only
• D. pXO1 and pXO2 jointly

Hint:

Two plasmids: one for toxin, one for the protein capsule - capsule is on pXO2.

Answer 1

The Correct Option is B

Step 1: Identify the virulence factors of Bacillus anthracis.
Full virulence of B. anthracis requires two plasmids: pXO1 and pXO2. Loss of either renders the organism attenuated.

Step 2: Map each plasmid to its product.
• pXO1 (~182 kb) carries the genes for the tripartite anthrax toxin - protective antigen (PA, pagA), lethal factor (LF, lef) and edema factor (EF, cya).
• pXO2 (~95 kb) carries the capBCADE operon that synthesises the antiphagocytic poly-D-glutamic acid capsule.

Step 3: Apply to the question.
The capsule is therefore encoded by pXO2. The poly-glutamate capsule is unusual because most bacterial capsules are polysaccharide; this protein/peptide capsule resists phagocytosis and is poorly immunogenic, which is why the live attenuated Sterne vaccine strain (pXO1\(^+\), pXO2\(^-\)) is non-capsulated.

Step 4: Why the other options are wrong.
pXO1 codes for toxin, not capsule. The capsule is plasmid-borne, not chromosomal. Both plasmids are needed for full virulence, but the capsule specifically maps to pXO2 alone.

Key fact: pXO1 = toxin, pXO2 = capsule (poly-D-glutamate).