Which one of the following microorganisms uses antigenic variation as a major means of evading host defences –
**Core Concept**
Antigenic variation is a mechanism employed by certain microorganisms to evade the host's immune system by changing the surface antigens, making it difficult for the immune system to recognize and target them. This concept is crucial in understanding how some pathogens survive and persist within a host.
**Why the Correct Answer is Right**
The bacterium *Borrelia* (such as *Borrelia burgdorferi*, the agent causing Lyme disease) utilizes antigenic variation as a primary strategy to evade the host's immune defenses. This is achieved through a process called gene conversion, where the bacterium switches between different antigenic variants of its surface proteins (e.g., VlsE) to maintain a "moving target" for the immune system. This mechanism allows *Borrelia* to repeatedly infect cells and evade clearance by the host's immune response.
**Why Each Wrong Option is Incorrect**
* **Option A:** *Trypanosoma brucei*, the causative agent of African trypanosomiasis, employs antigenic variation as a major means of evading the host's immune defenses. However, this is not the correct answer as it is not one of the options provided.
* **Option B:** *Mycoplasma pneumoniae* is a bacterium that can cause respiratory infections, but it does not primarily use antigenic variation to evade the host's immune system. Instead, it has a very small genome and relies on its ability to adhere to and invade host cells.
* **Option C:** *Streptococcus pneumoniae* is a bacterium that can cause a range of infections, including pneumonia. While it does have mechanisms to evade the host's immune system, antigenic variation is not its primary means of doing so.
**Clinical Pearl / High-Yield Fact**
Antigenic variation is a key concept in understanding the pathogenesis of certain bacterial infections, particularly those caused by *Borrelia* species. It highlights the importance of developing vaccines that can target conserved regions of the pathogen's surface proteins.
**Correct Answer:** C.