Question

·       In one experiment Lederberg used Gal⁺ and Gal^– strains of E. coli. One half of a plate that contained medium with galactose as the only carbon source was inoculated with phages that came from lysed cells of the Gal⁺ strain. The other half of the plate was left uninoculated. A Gal^- strain was then placed on the entire plate. Numerous colonies appeared on the half of the plate inoculated with the phage, whereas only a few appeared on the half that was not inoculated with phage. Explain what happened using appropriate terminology.

Answer 1

This experiment demonstrates the presence or absence of both prototrophs and auxotrophs.

If a culture of E. coli cells are plated on a petriplate having the complete media, then both auxotrophs and prototrophs would grow.

Any strain of bacteria that can utilize the sugar galactose as an energy source is designated as gal+ and the one that cannot would be called gal-. THe gal- strain would not grow on minimal media if galactose is the sole carbon source, but will grow if a sugar other than galactose is present.

In this experiment, they have used a minimal media which has only galactose as the only catbon source. One half was inoculated with phages that came from lysed cells of Gal+ strain and the other half was left uninoculated. There was growth on both the sides, but the quantity varied.

The first side had many colonies grown, which means that the cells of Gal strains were successfully transduced by the phages into Gal+ strains. But in the second half, which was left uninoculated had fewer number of colonies which means that Gal strain from the E. coli had few auxotrophic strains which would not grow in the given galactose minimal media and only the Gal+ starin cells could grow.

Further investigation of finding out which was the auxotrophic strain, a replica plating can be done and the prototrophs and auxotrophs can be differentiated.