## **Core Concept**
The concept of "gene transformation" refers to the process by which genetic material is transferred from one bacterium to another through the uptake of free DNA molecules from the environment. This phenomenon was first discovered in the 1920s and has since been extensively studied.

## **Why the Correct Answer is Right**
The correct answer, *Streptococcus pneumoniae*, is historically significant because Frederick Griffith's experiments in 1928 demonstrated the transformation of non-virulent *S. pneumoniae* into a virulent form through exposure to heat-killed virulent bacteria. This experiment laid the foundation for the understanding of bacterial transformation and the role of DNA in genetic inheritance. The Avery-MacLeod-McCarty experiment in 1944 later identified DNA as the molecule responsible for this transformation.

## **Why Each Wrong Option is Incorrect**
* **Option A:** *Escherichia coli* is a commonly used model organism in molecular biology, but it was not the bacterium used in the initial discovery of gene transformation.
* **Option B:** *Bacillus subtilis* is known for its ability to undergo transformation, but it was not the organism used by Griffith in his seminal experiments.
* **Option D:** *Haemophilus influenzae* also exhibits transformation, but like *E. coli* and *B. subtilis*, it was not the bacterium in which gene transformation was first discovered.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the discovery of bacterial transformation was crucial for establishing DNA as the genetic material. This concept is foundational to molecular biology and genetics, and understanding its history and implications is essential for postgraduate medical students.

## **Correct Answer:** B. Streptococcus pneumoniae.