[1] Traditionally, MacConkey agar has been used to distinguish those bacteria that ferment lactose from those that do not.
This is important because gut bacteria, such as Escherichia coli, can typically ferment lactose, while important gut pathogens, such as Salmonella enterica and most shigellas are unable to ferment lactose.
For example, lactose fermenters turn a deep red when this pH indicator is used.
Although some authors refer to NLFs as being colourless, in reality they turn neutral red a buffish color.
Non-pathogenic strains of E. coli ferment sorbitol to produce acid: Pathogenic E. coli cannot ferment sorbitol, so this strain uses peptone to grow.