Question

In the 1928 Griffith experiment he was able to show that there is a transforming principle. Describe the 4 scenarios of these experiments (A-D) and describe what happened to the mice after infecting them with different strains. Provide explanation for each experiment.

Answer 1

Answer:

In 1928, the English bacteriologist Frederick Griffith, performed laboratory experiments with two strains of Streptococcus pneumoniae, agent and known to cause pneumonia, severe bacteraemia and meningitis.

Explanation:

One of the strains, when growing in a solid medium enriched with blood-gel, produced small rough colonies, when examined through the reflected light. This serotype-II strain was called R. The other serotype-III strain formed bright and smooth colonies L. Having compared the microscopic morphology of the two strains, certainly a difference: the L-strain was surrounded by a thick and viscous capsule, while the R-strain lacked it. Separately, the R and L strains were suspended inside tubes with liquid medium, which were then used to be inoculated subcutaneously, in laboratory mice.

a) Experiment 1: Mouse inoculated with strain L: This strain, virulent, bacteraemia, pneumonia and death, means that the capsule conferred protection against phagocytosis and the bacteria multiplied in the blood and lungs of the mouse.

b) Experiment 2: Mouse inoculated with strain R: This strain, without capsule, was rapidly phagocytized preventing its multiplication; as a consequence, the disease was avoided and the mouse survived

c) Experiment 3: Mouse inoculated with a small amount of culture R (serotype II), together with a large inoculum of strain L (serotype III), previously inactivated by heat: Mice frequently died from infection. However, only type L pneumococcus in pure culture was recovered in the blood obtained from the dying animals. This means that dead bacteria L, have retained a certain capacity to transmit their ability to produce viscous capsules, and that capacity is transmitted to the descendants of strain R.

d) Experiment 4: Mouse inoculated with strain L previously inactivated by heat: The mice survived meaning that the dead virulent strains were phagocytosed and as a consequence the disease was avoided.

Answer 2

Answer:

A complete description of Griffith´s experiment is provided below.

Explanation:

Griffith was a british scientist who tried to find a vaccine for pneumonia, working with two different strains of Streptococcus pneumoniae (R and S, respectively). He performed four differents assays injecting the strains to mice and observing the results:

- Assay 1: he injected strain R in mice and saw that mice did not develop any symptom of pneumonia.

- Assay 2: he injected strain S and saw observed that mice developed pnemonia and died before 1 or 2 days.

- Assay 3: he injected a dead strain S. In this case, mice remain healthy.

-Assay 4: he mixed dead strain S with strain R (alive) and injected it to mice, expecting that mice keep healthy (because strain R did not cause pneuminia and strain S was dead). He saw the same results of Assay 2 (illness and dead). When Griffith made the autopsy of mice he found living strain S. He thought that something contained in the dead strain S transformed strain R into strain S, that way causing pneumonia and death. He called it the transforming principle.