One very important group of bacteria is the Gram-negative, facultatively anaerobic rods. Many of them cause diseases of the intestinal tract, as well as other organs. One important family of this group is called Enterobacteriaceae or enterics, as they are commonly called, includes bacteria that inhabit the gastrointestinal tracts of humans and other animals. Enterobacteriaceae are divided into two groups: a group of bacteria which ferments lactose with gas formation (coliforms) and a group of bacteria that does not ferment lactose with gas formation. Differentiation of potential indicator organisms of the Enterobacteriaceae is based on specified biochemical tests. The traditional indol, methyl red, Voges-Proskauer, and citrate utilization (IMViC) test is useful for differentiation, but does not provide complete identification, and additional biochemical tests are necessary. The oxidative-fermentative group of Gram-negative rods resembles the Enterobacteriaceae in its colonial and microscopic morphology. However, it differs from them biochemically in that carbohydrates are utilized principally by oxidation and, only occasionally, by fermentation.