Question

A culture plate of Lactobacillus shows blue-coloured colonies and colourless colonies. Explain the principle involved in the formation of such variance in the colour of colonies.

Answer 1

1. The variation in colour of colonies is due to the principle of insertional inactivation.
2. In this, a recombinant DNA is inserted within the coding sequence of an enzyme, β-galactosidase. This results in the inactivation of the enzyme, which is referred to as insertional inactivation.
3. The presence of a chromogenic substrate gives blue-coloured colonies if the plasmid in the bacteria does not have an insert.
4. Presence of insert results into insertional inactivation of the β - galactosidase and the colonies do not produce any colour, these are identified as recombinant colonies.