What ingredients in macconkey agar supplies carbon?

Peptones and beef extract

MacConkey agar is a selective and differential culture medium for bacteria designed for isolation of Gram negative and usually enteric  bacilli, based on their ability to ferment lactose. It contains bile salts, crystal violet, neutral red dye. There are many variations of MacConkey agar depending on the need. This medium was developed by Alfred Theodore MacConkey. This agar can distinguish gram negative bacterias that can ferment. This is known as the indicator medium.