The coliform group of bacteria has been the most commonly used indicator of water quality. The coliform group consists of all aerobic and facultatively anaerobic, gram-negative, oxidase-negative, non-spore forming, rod-shaped bacteria that ferment lactose in a broth medium with gas formation within 48 hours at 350C. Most coliforms also produce the enzyme $-D galactosidase which can be detected with a colour forming reagent. The group generally comprises the genera Escherichia, Klebsiella, Enterobacter and Citrobacter. The presence of these bacteria in drinking water is indicative of inadequate filtration/disinfection or in the distribution system a continuing loss of the chlorine residual.