The concept that nucleic acids are the genetic material is based on the discovery of transformation by Fredrick Griffith in 1927, medi-cal officer in the British Ministry of health, was studying Pneumococcal infection in mice. Different strains of the BacteriumDiplococcuspneumoniae, now named as Streptococcus pneumoniae were usedfor his experiments. There are two types of bacterial strains, one viru-lent and other avirulent. The virulent strains cause pneumoniae in hu-mans and mice, however, avirulent strains do not cause pneumoniae. The virulence is related to the presence of a polysaccharide capsule of the bacterium. The capsulated virulent cells cannot be phagocytosed by the circulatory system, where as non-encapsulated avirulent strains are destroyed by the phagocytic cells.
The capsulated bacteria form smooth shiny surface colony (S) when grown on agar culture plate, whereas non encapsulated strains produce rough colonies (R). Each bacterial strain may be differentiated based on serotype. The specificity of the serotype is due to the chemi-cal structure of the polysaccharide is identified by immunological tech-niques. They are usually designated by Roman numerals. There are different types such as I, II and III of Pneumococci that have different capsular polysaccharide.
In 1920â€™s, in United States, type I and type II were common in causing pneumoniae. Griffith book type II and type III for his experi-ments that led him to the concept about genetic material.
The characteristics of bacteria used by Griffith for his original transformation experiment is given below.
Some type of interaction was there between living II R and heat killed III S cells. It was concluded by Griffith that the heat killed III S bacteria somehow was responsible for converting live avirulent II R cells and into virulent IIIS cells.
Griffith suggested that some transforming principle from the dead III S cells was responsible for the conversion of II R cells into III S cells. This phenomenon is called transformation. This is called as Griffith effect or more popularly called as Bacterial transformation.