This image illustrates the growth of gram-negative bacteria that cannot ferment lactose on eosin methylene blue (EMB) agar. EMB agar contains bile salts and dyes which inhibit growth of gram-positive bacteria. Growth on EMB agar is a useful diagnostic tool to distinguish between lactose fermenters and nonfermenters which will appear colorless. This image can be used to describe the use of metabolism in identification of bacteria. Salmonella and Shigella, both nonlactose fermenting pathogens, can be distinguished from the more common intestinal flora which ferment lactose.
 

　　lnfemb-an.jpg Lactose Nonfermenter on EMB
　　Pat Johnson, Palm Beach Community College, Lake Worth, Fla., USA