What would the predicted result be if Griffith injected mice

What would the predicted result be if Griffith injected mice with a mixture of heat-killed R bacteria and living S bacteria?

a. All mice would live, but only if DNAse was added to the mixture prior to injection

b. All mice would die.

c. All mice would live.

d. All mice would die, but only if DNAse was added to the mixture prior to injection.

Solution

Griffith’s Experiment- A Quick Review: Fredrick Griffith, an English physician, first reported transformation of one strain of Streptococcus pneumoniae into other strain in 1928. The bacteria cause pneumonia. A strain of this bacterium produces capsule and gives a smooth, shiny colony on culture media, thus called S-strain (smooth strain). The S-strain exhibits pathogenicity in laboratory mice. Another strain without capsule produces rough and coarse colony on culture medium, thus called R-strain (rough strain). The R-strain is avirulent (non-pathogenic).

Mice injected with

Outcome

1

Live S-strain (virulent)

Pneumonia mice dead

2

Live R-strain (avirulent)

No pneumonia mice alive

3

Heat killed S-strain + live R-strain

Pneumonia mice dead

When injected with live S-strain bacteria, mice developed pneumonia and died. When injected with live R-strain bacteria, mice did not develop pneumonia and survived. When heat killed S-strain injected to mice, they remain unaffected. However, when heat killed S-strain bacteria mixed with live R-strain injected to mice, they developed pneumonia and died. He was also able to isolate and culture S-strain bacteria from affected mice. Since heat killed bacteria could have no way been revived, he concluded that some molecules from heat killed S-strain had transformed the R-strain into S-train.    

Now,

The Current Perspective: Presence of living S-strain causes pneumonia development in mice killing them at last. It is noteworthy that transformation of R-strain into S-strain occurs only when there is alive R-strain and fragments of DNA from heat-killed S-strain.

Option A: Incorrect. DNAse is a large protein incapable of entering into bacterial cytoplasm through bacterial membrane of living S-strain. So, this treatment further cleaves DNA fragments of heat killed R-strain BUT leaves the S-strain unaffected. So, injecting the mixture to mice kills them as live S-strain causes pneumonia.   

Option B: Correct. Presence of living S-strain causes pneumonia development in mice killing them at last.

Option C: Incorrect. All mice die because S-strain cause pneumonia to them.

Option D: Incorrect. All mice would die, BUT NOT because of addition of DNAse to the mixture. As mentioned in option A explanation, DNAse does not penetrate into live S-strain, thus leave them unaffected. Therefore, mice die irrespective of DNAse treatment.