Examples of non-lactose fermenting bacteria include salmonella, shigella, proteus and pseudomonas aeruginosa. These microorganisms can be differentiated from lactose-fermenting bacteria by using differential bacteriological media, such as MacConkey agar and eosin-methylene blue agar, or EMB.
Lactose is a type of sugar present in carbohydrates. Some bacteria are unable to process lactose and instead ferment peptone. In the laboratory, these organisms are identified using a culture medium, where growth of bacteria is either supported or inhibited. Using a MacConkey agar, the presence of nonlactose fermenting bacteria is indicated by the development of colorless or white colonies. The colonies may also appear gold or brownish in color with darkened centers. When an EMB agar is used, nonfermenting bacteria form light-purple or colorless colonies.