Shigella organisms may be very difficult to distinguish biochemically from Escherichia coli. Brenner (1) considers Shigella organisms and E. coli to be a single species, based on DNA homology. Nonetheless, Shigella species are Gram-negative, facultatively anaerobic, nonsporulating, nonmotile rods in the family Enterobacteriaceae. They do not decarboxylate lysine or ferment lactose within 2 days. They utilize glucose and other carbohydrates, producing acid but not gas. However, because of their affinity to E. coli, frequent exceptions may be encountered, e.g., some biotypes produce gas from glucose and mannitol. Neither citrate nor malonate is used as the sole carbon source for growth, and the organisms are inhibited by potassium cyanide.