When Griffith injected mice with living IIR (not disease-causing) and heat killed IIIS (disease-causing) bacteria, he found that the mice died of pneumonia. What kind of bacteria did he then isolate from the dead mice that suggested transformation had occurred?

The correct answer is: Living IIIS (disease causing) bacteria.

Explanation:

According to the question, Griffith injected two strains of bacteria in the mice, which are:Living IIR strain which are incapable of causing any disease.Heat killed IIIS strain that can cause a disease.Inside the injected mice,the heat killed IIIS (disease-causing) strain of the bacteria releases a factor that was not affected by heat.This factor was taken up by the living IIR (not disease causing) bacteria. This caused it to get transformed from the non-disease causing IIR strain to the disease causing IIIS strain.After the process of transformation, the disease causing IIIS strain causes pneumonia in the mice and hence kills it.This factor is now known to be the DNA or Deoxyribonucleic Acid.