Question

i undertake an experiment for the isolation of lac mutants with MacConkey agar/lactose.

In the wild type, it appears with red colour as the lac operon functions properly.

while, mutants display yellow colour. Can you explain why it shows yellow?

I know that the basic principle of MacC. where the production of acid product from utilization of lactose turn netural red to red.

Answer 1

pH indicator present in the MacConkey agar is neutral red. This indicator gives a red or pink color under acidic conditions and yellow color under alkaline conditions. As you have also pointed out correctly that fermentation of lactose causes acidic production and turns McConkey to red; being a mutant, bacteria may not ferment lactose which might be due to the absence of a functional copy of either beta-galactosidase or permease. Both these enzymes required for the break down of lactose. In the absence of lactose degradation enzyme; mutant bacteria will replenish its energy demand through the break down of peptides and amino acids. The degradation of amino acids and peptide leads to alkaline pH and turns neutral red to yellow color.