Question

The following results were obtained after cultivating four different microbes on MacConkey medium: gram-positive, lactose positive = no growth gram-negative, lactose positive = dark colonies with pink halos gram-positive, lactose negative = no growth gram-negative, lactose negative = pale colonies with no halos Based upon these results, what to what category of media does MacConkey belong?

Answer 1

Mc Conkey agar is a selective media and also an indicator media . It differentiates non lactose fermenters from lactose fermenters and gram negative bacteria from gram positive bacteria by inhibiting the growth of gram positive bacteria.

From the above scenario it is clear that gram negative bacteria which are lactose fermenters grow on the media while gram positive and non lactose fermenters are inhibited.

Selective media because it contains substances which inhibits all but a few types of bacteria and helps in isolation of a particular species.

It is also an indicator media because due to fermentation of lactose there is acidic pH which forms the pink colonies because of presence of neutral red indicator.