In an experiment done by Griffith, it was found that when virulent capsulated smooth 'S' bacteria were injected into healthy mice, the mice dies of pneumonia. When mice were injected with non virulent 'R' forms of same bacteria, they survived and did not die to pneumonia. In another case when S-forms were killed by heating at above `60` and injected, rats did not get pneumonia, but when killed, s-forms and R-forms of bacteria were mixed and injected, the rats suffered from pneumonia and died. which one of the following conclusions can be drawn from the above experiment?

Options

A. Heat killed virulent and heat killed a virulent at times can cause disease
B. virulent bacteria alone can cause disease
C. Whole body of virulent bacteria is nearest to cause disease
D. Heat resistant component of virulent bacterial cells in combination with avirulent strain can produce the disease

Answer

**Step-by-Step Solution:**

1. **Understanding the Experiment**: In Griffith's experiment, two strains of *Streptococcus pneumoniae* were used: the virulent smooth strain (S strain) and the non-virulent rough strain (R strain). The S strain has a polysaccharide capsule that makes it virulent, while the R strain lacks this capsule and is non-virulent.

2. **Injection of S Strain**: When mice were injected with the virulent S strain, they developed pneumonia and died. This shows that the S strain is capable of causing disease.

3. **Injection of R Strain**: When mice were injected with the non-virulent R strain, they survived. This indicates that the R strain does not cause disease on its own.

4. **Heat-Killed S Strain**: When the S strain was killed by heating (at temperatures above 60°C) and then injected into the mice, the mice survived. This suggests that the heat killed the bacteria and rendered them non-virulent.

5. **Mixing R Strain with Heat-Killed S Strain**: When the heat-killed S strain was mixed with the live R strain and injected into the mice, the mice died. This was unexpected since both components individually did not cause disease.

6. **Conclusion of Transformation**: The key finding was that the DNA from the heat-killed S strain was taken up by the live R strain bacteria. This process is known as transformation, where the R strain bacteria acquired the virulent characteristics from the S strain's DNA, leading to the mice developing pneumonia and dying.

7. **Identifying Correct Conclusions**: From the experiment, we can draw several conclusions:
- Virulent bacteria (S strain) can cause disease.
- The heat-resistant component (DNA) from the virulent strain can transform non-virulent bacteria (R strain) into virulent forms.
- The whole body of the virulent bacteria is not necessary to cause disease; the DNA and the polysaccharide coat are sufficient.

**Final Conclusion**: The correct conclusions drawn from Griffith's experiment are:
- Virulent bacteria alone can cause disease.
- The heat-resistant component of the virulent bacteria (DNA) in combination with the avirulent strain can produce disease.

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