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.
