In his work with pneumonia-causing bacteria and mice, Griffith found that:

• A. heat-killed virulent bacteria alone could cause disease in mice
• B. only live virulent bacteria could cause transformation in harmless bacteria
• C. harmless bacteria could be transformed into disease-causing bacteria by exposure to heat-killed virulent bacteria
• D. mice injected with harmless bacteria always survived, regardless of any treatment

Answer: (C)

Verified step by step guidance

1. Understand the context of Griffith's experiment: Griffith was studying pneumonia-causing bacteria (Streptococcus pneumoniae) and their ability to cause disease in mice. He worked with two strains: a virulent strain (S strain) with a smooth capsule that caused disease, and a harmless strain (R strain) without a capsule.
2. Review the key observation: Griffith found that when he injected mice with heat-killed virulent bacteria (S strain), the mice survived. However, when he mixed heat-killed virulent bacteria with live harmless bacteria (R strain) and injected the mixture into mice, the mice developed pneumonia and died.
3. Analyze the transformation process: Griffith concluded that some 'transforming principle' from the heat-killed virulent bacteria was taken up by the harmless bacteria, enabling them to produce a capsule and become virulent. This transformation allowed the harmless bacteria to cause disease.
4. Clarify the significance of the experiment: Griffith's work demonstrated that genetic material could be transferred between bacteria, laying the foundation for understanding DNA as the molecule responsible for heredity.
5. Relate the correct answer to the experiment: The correct answer is that harmless bacteria could be transformed into disease-causing bacteria by exposure to heat-killed virulent bacteria. This aligns with Griffith's findings and highlights the concept of bacterial transformation.