Lactose is a MacConkey ingredients. The purpose of lactose in this medium is

To act as a fermentable source of carbohydrate.

Explanation:

In microbiology, certain media are used to culture specific micro-organisms. One of those media is Macconkey medium agar, which was prepared and named after a man called Alfred Theodore MacConkey. Macconkey medium is used for the selective and differential growth of gram negative bacteria.

The medium consists of ingredients such as peptone, lactose, bile salt, water, agar, etc. The lactose component of the agar is used as the source of fermentable carbohydrate and also serves to differentiate between lactose fermenting bacteria and lactose non-fermenting bacteria, which is one of the uses of the agar medium.