Bacilli Calmette Guerin contains the avirulent strains of
**Core Concept**
The Bacillus Calmette-Guérin (BCG) vaccine is a live, attenuated vaccine that contains an avirulent strain of Mycobacterium bovis, a close relative of Mycobacterium tuberculosis. The BCG vaccine is used primarily to protect against tuberculosis (TB) and has been shown to be effective in preventing severe forms of TB, such as TB meningitis.
**Why the Correct Answer is Right**
The BCG vaccine contains an avirulent strain of Mycobacterium bovis that has been attenuated through a process of serial passage in culture. This attenuation reduces the virulence of the bacteria, making it safe for use in humans. The vaccine is prepared by culturing the attenuated bacteria in a nutrient-rich medium, such as glycerol, and then lyophilizing (freeze-drying) the culture to produce a stable vaccine. When administered, the BCG vaccine stimulates an immune response that provides protection against TB.
**Why Each Wrong Option is Incorrect**
* **Option A:** Mycobacterium leprae - This bacterium is responsible for leprosy, a different infectious disease that affects the skin, nerves, and mucous membranes.
* **Option B:** Mycobacterium avium - This bacterium is a type of nontuberculous mycobacterium (NTM) that can cause lung disease in people with weakened immune systems, but it is not the strain used in the BCG vaccine.
* **Option C:** Mycobacterium tuberculosis - While the BCG vaccine is used to protect against TB, it contains an avirulent strain of Mycobacterium bovis, not Mycobacterium tuberculosis.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the BCG vaccine is not a guarantee against TB infection, but it can significantly reduce the risk of severe forms of the disease, such as TB meningitis. The vaccine's effectiveness varies depending on the population and geographic region, and it's often used in areas where TB is common.
**Correct Answer:** D. Mycobacterium bovis