During his experiment, he used two different strains of bacteria named Streptococcus pneumoniae; one was strain-III S (smooth) and second was Strain-II R (rough). III S was virulent and II R was non-virulent.
Once he applied heat killed smooth strain (III S) into mice, which was non-virulent and then he applied the mixture of rough strain (II R) and heat killed smooth strain, which was virulent.
This showed the transformation of type II-R into type III-S, which enabled the R-strain to synthesize a smooth polysaccharide coat and become virulent.
