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The colon group of bacteria may be characterized as nonsporing Gram-negative bacilli which ferment lactose, producing acid and gas, and which can grow aerobically. Although many bacteriologists recognize the genera Escherichia and Aerobacter, the genus Citrobacter has not been generally accepted. In 1923, Koser showed that strains of the genus Aerobacter utilized citric acid as a sole source of carbon (citrate positive), whereas those of the genus Escherichia did not (citrate negative). In 1924, Koser found that 23 of 25 coliform strains [methyl-red (+), Voges-Proskauer (-)] isolated from unpolluted soil were citrate (+). Due to their source he referred to them as "soil forms." The possible sanitary significance of this finding was immediately realized. The correlation of habitat with morphology and biochemical characteristics was, and still is, the chief hope for an improved criterion of water pollution. Bardsley (1934) summarized previous work on this point: "Most workers in tropical countries where pollution is very heavy find it practical to distinguish between the different types of coliform bacilli, and only those organisms which conform to the true B. coli group [methyl-red (+), citrate (-)], are regarded as excretal contamination .... In temperate climates the position is not so well defined." Fermentation of cellobiose and alpha-methyl-glucoside, production of trimethylene glycol from glycerol, and even Werkman and Gillen's (1932) description of the genus Citrobacter as comprising those citrate (+) coli-like strains which produce "Ace-

Nitrogen Availability as an Aid in the Differentiation of Bacteria in the Coli-Aerogenes Group