Enterobacteriaceae (enterics) are facultative Gram-negative rod-shaped bacteria that inhabit the intestinal tracts of humans and many animals.  Enterics that can ferment lactose (lactose positive) are called coliforms and usually are considered to nonnpathogenic in the intestinal tract (e.g. Escherichia coli, Enterobacter aerogenes).  (Note that in recent years certain strains of E. coli such as O157:H7 have been implicated in causing bloody diarrhea.) The enteric pathogens Salmonella and Shigella are unable to ferment lactose (lactose negative). Two types of selective and differential media are commonly employed for isolating and identifying Enterobacteriaceae: eosin methylene blue (EMB) agar and MacConkey agar. Lactose positive colonies are visibly different from lactose negative colonies on both types of media.

EMB agar

Eosin methylene blue agar, as the name suggests, contains the dyes eosin and methylene blue.  EMB agar is selective because the aniline dyes in this purple media inhibit growth of Gram-positive organisms.  Lactose fermenters metabolize the lactose in the media and produce acid byproducts, causing a color change in the colony.  Thus, EMB is also a differential medium. Strong acid production by organisms such as E. coli results in a metallic green sheen. Weaker fermentation of lactose results in colonies with a pinkish-purple color.  Colonies of nonlactose fermenters remain colorless, or at least are no darker than the color of the media.

MacConkey Agar 

MacConkey agar contains lactose, bile salts, neutral red and crystal violet.  It is a selective media because Gram-positive organisms are inhibited by the bile salts and the crystal violet.  When bacteria ferment lactose and produce enough acid products to reduce the pH below 6.8, the neutral red turns from colorless to red.  Thus, MacConkey is a differential media on which lactose fermenting colonies appear red (or pink).  Nonlactose fermenters are colorless. 

1. EMB agar	2. MacConkey agar