This method was first discovered by Griffttth in1928 in Streptococcus pneumonia which is responsible for causing Pneumonia in children. The pneumonia bacteria are of different types . They are named as type I,II,III,IV and so on . These pneumococcus are of two types, one with capsule and another without capsule.The one with capsule is called Smooth type or S-type and the other without capsule is called Rough type or R-type . The smooth type are more infectious or virulent then the R-type .This R-type is converted to S- type when they get suitable condition during transformation.
The pneumonia bacteria was examined with rats. Three groups of rats were taken and they were injected by different samples of bacteria. The first group was injected by non encapsulated R-type bacteria. The second group of rat was injected
by heat killed encapsulated bacteria or R type .The third group was injected by the mixture of living R-type bacteria and heat killed non -encapsulated bacteria (S-type). The first group and second group of rats were not infected and continue to survive. The third group survive for some period but finally died and when their body was examined S- type encapsulated bacteria was isolated. This was explained by saying that heat killed encapsulated bacteria produced some substances which procided a medium for R-type non-encapsulated bacteria to produce capsulated bacteria called S-type.
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