**Core Concept**
Johne's bacillus is a specific type of bacteria associated with Johne's disease, a chronic gastrointestinal infection primarily affecting ruminant animals. Johne's disease is characterized by malabsorption, diarrhea, and weight loss in infected animals.

**Why the Correct Answer is Right**
Mycobacterium paratuberculosis (M. paratuberculosis) is the causative agent of Johne's disease. This bacterium belongs to the Mycobacterium genus, which also includes M. tuberculosis, the agent of human tuberculosis. M. paratuberculosis is a slow-growing, acid-fast bacterium that primarily infects the intestinal epithelium of ruminant animals, leading to chronic inflammation and tissue damage.

**Why Each Wrong Option is Incorrect**
**Option A:** Corynebacterium pseudotuberculosis is the causative agent of caseous lymphadenitis, a different type of bacterial infection affecting ruminant animals. It is not associated with Johne's disease.
**Option C:** Haemophilus agrippinum (H. agrippinum) is a type of Haemophilus bacterium, which is not typically associated with Johne's disease or gastrointestinal infections in ruminant animals.
**Option D:** Micrococcus is a genus of Gram-positive bacteria that are commonly found on the skin and in the environment, but they are not typically associated with Johne's disease.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that Johne's disease is a zoonotic infection, meaning it can potentially be transmitted from animals to humans. While the risk of transmission is low, it's crucial to practice proper hygiene and handling procedures when working with animals suspected of having Johne's disease.

**✓ Correct Answer: B. M. Paratuberculosis**