The virulence of Bacillus anthracis is mediated by two key plasmid-encoded factors. Which of the following is the correct biochemical composition?

1. A

   A. pXO1 poly-D-glutamate capsule

2. B

   B. pXO1 polysaccharide capsule

3. C

   C. pXO2 polysaccharide capsule

4. D

   D. pXO2 poly-D-glutamate capsule

Appeared in: INI-CET EXAM -2026

Explanation

• The virulence of Bacillus anthracis is determined by two plasmids: pXO1 and pXO2.
• The pXO2 plasmid carries the genes responsible for synthesizing the antiphagocytic capsule.
• This capsule is unique because it is composed of poly-D-glutamic acid, a polypeptide, which protects the bacterium from being engulfed by immune cells.
• Therefore, the correct association is the pXO2 plasmid with the poly-D-glutamate capsule.

Why Other Options Were Wrong

• Option A: This is incorrect because the pXO1 plasmid is responsible for encoding the three components of the anthrax toxin (Protective Antigen, Edema Factor, and Lethal Factor), not the capsule.
• Option B: This option is incorrect for two reasons: the pXO1 plasmid encodes the toxin, not the capsule, and the capsule's composition is a polypeptide (poly-D-glutamate), not a polysaccharide.
• Option C: This is incorrect because although the pXO2 plasmid does encode the capsule, the capsule's biochemical composition is poly-D-glutamate, not polysaccharide. Most other bacterial capsules are polysaccharide-based, making this a key distinguishing feature of B. anthracis.

Related Visual

Clinical Relevance

• Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Virulence factors of Bacillus anthracis as background academic context rather than a clinical decision trigger.
• Understanding that B. anthracis virulence depends on both the pXO1-encoded toxin and the pXO2-encoded capsule is fundamental to microbiology and infectious disease management.
• The unique poly-D-glutamate nature of the capsule makes it a poor immunogen but a critical factor in evading the host's immune response, influencing treatment and vaccine strategies.
• The live attenuated Sterne vaccine strain, widely used for livestock, is a variant that lacks the pXO2 plasmid. It can produce toxin but is avirulent because it cannot form a protective capsule and is easily cleared by the immune system.

How to Approach the Question

• First, identify the key terms in the question: Bacillus anthracis, virulence, plasmids (pXO1, pXO2), and biochemical composition.
• This is a factual recall question. Access your knowledge about the specific roles of the two major plasmids in B. anthracis.
• Recall that pXO1 is responsible for the three-part anthrax toxin.
• Recall that pXO2 is responsible for the antiphagocytic capsule.
• Recall the unique composition of the anthrax capsule: it's a polypeptide (poly-D-glutamate), not a polysaccharide like most other bacterial capsules.
• Evaluate each option based on these facts: Options A and B incorrectly link pXO1 to the capsule. Option C correctly links pXO2 to the capsule but states the wrong composition (polysaccharide). Option D correctly links the pXO2 plasmid to the poly-D-glutamate capsule.

Concept Tested & Keywords

• Concept Tested: Virulence factors of Bacillus anthracis
• Stem keywords: Bacillus anthracis, virulence, plasmid-encoded factors, biochemical composition
• Lead-in keywords: correct

Question ID

QTaPJ3cmLpsrevFaDABxVa

Copy ID

Reference Book

E6 Pathology- ROBBINS & COTRAN PATHOLOGIC BASIS OF DISEASE 10TH Ed p. 370-372