Transformation of bacterial genome

Historically, the discovery of transformation in bacteria preceded the other two modes of gene transfer. The experiments conducted by Frederick Griffith in 1928 indicated for the first time that a gene-controlled character, viz. formation of capsule in pneumococci, could be transferred to a non­-capsulated variety of these bacteria. The transformation experiments with pneumococci eventually led to an equally significant discovery that genes are made of DNA.

In these experiments, Griffith used two strains of pneumococci (Streptococcus pneumoniae): one with a polysaccharide capsule producing ‘smooth’ colonies (S-type) on agar plates which was pathogenic. The other strain was without capsule producing ‘rough’ colonies (R-type) and was non-pathogenic.

When the capsulated living bacteria (S-bacteria) were injected into experimental animals, like laboratory mice, a significant proportion of the mice died of pneumonia and live S-bacteria could be isolated from the autopsied animals.

When the non-capsulated living pneumococci (R-bacteria) were similarly injected into mice, they remained unaffected and healthy. Also, when S-pneumococci or R-pneumococci were killed by heat and injected separately into experimental mice, the animals did not show any disease symptom and remained healthy. But an unexpected result was encountered when a mixture of living R-pneumococci and heat-killed S-pneumococci was injected.

A significant number of injected animals died, and, surprisingly, living capsulated S-pneumococci could be isolated from the dead mice. The experiment produced strong evidence in favour of the conclusion that some substance came out from the heat-killed S-bacteria in the environment and was taken up by some of the living R-bacteria converting them to the S-form. The phenomenon was designated as transformation and the substance whose nature was unknown at that time was called the transforming principle.

With further refinement of transformation experiments carried out subsequently, it was observed that transformation of R-form to S-form in pneumococci could be conducted more directly without involving laboratory animals.

An outline of these experiments is schematically drawn in Fig. 9.96:

At the time when Griffith and others made the transformation experiments, the chemical nature of the transforming principle was unknown. Avery, Mac Leod and McCarty took up this task by stepwise elimination of different components of the cell-free extract of capsulated pneumococci to find out component that possessed the property of transformation.

After several years of painstaking research they found that a highly purified sample of the cell-extract containing not less than 99.9% DNA of S-pneumococci could transform on the average one bacterium of R-form per 10,000 to an S-form. Furthermore, the transforming ability of the purified sample was destroyed by DNase. These findings made in 1944 provided the first conclusive evidence to prove that the genetic material is DNA.

It was shown that a genetic character, like the capacity to synthesise a polysaccharide capsule in pneumococci, could be transmitted to bacteria lacking this property through transfer of DNA. In other words, the gene controlling this ability to synthesise capsular polysaccharide was present in the DNA of the S-pneumococci.

Thus, transformation can be defined as a means of horizontal gene transfer mediated by uptake of free DNA by other bacteria, either spontaneously from the environment or by forced uptake under laboratory conditions.

Accordingly, transformation in bacteria is called:

a. Natural and

b. Artificial.

It may be pointed out to avoid misunderstanding that the term ‘transformation’ carries a different meaning when used in connection with eukaryotic organisms. In eukaryotic cell-biology, this term is used to indicate the ability of a normal differentiated cell to regain the capacity to divide actively and indefinitely. This happens when a normal body cell is transformed into a cancer cell. Such transformation in an animal cell can be due to a mutation, or through uptake of foreign DNA.