During fermentation of the sugar, acid is formed and the pH of the medium drops, changing the color of the pH indicator. Different formulations use different indicators: neutral red is often used. For example, lactose fermenters turn a deep red when this pH indicator is used. Those bacteria that are unable to ferment lactose, often referred to as non-lactose fermenters, or NLFs for short, use the peptone in the medium. This releases ammonia, which raises the pH of the medium. Although some authors refer to NLFs as being colourless, in reality they turn neutral red a buff-ish color.
H7 O157:H7 differs from most other strains of E. coli in being unable to ferment sorbitol. In sorbitol MacConkey agar, lactose is replaced by sorbitol. Most strains of E. coli ferment sorbitol to produce acid: E. coli O157:H7 can not ferment sorbitol, so this strain uses peptone to grow. This raises the pH of the medium allowing the O157:H7 strain to be differentiated from other E.coli strains through the action of the pH indicator in the medium.