little is known about the role and numbers of bacteria in aquatic ecosystems. The coliform bacteria that are particularly well known to public health officers and sanitary engineers are the facultative, gram-negative, rod-shaped, nonspore-forming ones that can ferment lactose. They are associated with sewage since many, but not all, are derived from the digestive tract of mammals. Although there will always be colifrom bacteria present in natural waters, high levels provide evidence of water pollution from sewage.