SUMMARYAs part of the bacteriological quality control programme of the Public Health Laboratory Service, we were asked to investigate the possibility of providing simulated water samples for distribution to the laboratories. For this purpose it was necessary to find some means whereby suspensions of coliform organisms and Escherichia coli could be kept relatively stable in number at room temperature for a period of 7-10 days. This, it was finally found, was best achieved by adding selected strains of the organisms to improved formate lactose glutamate medium (Gray, 1964) without the lactose but with added boric acid to a final concentration of 1.8 %.The procedures adopted in the successful quality control programme are described.
INTROD'UCTIONIn the preparation of simulated samples of polluted water for the purpose of the quality control programme, the main problem was to find some means of maintaining the stability of suspensions of coliform organisms and Escherichia coli at ambient temperature for periods up to 10 days.It is well known that coliform organisms have a limited survival time in water. Variations occur even after 6 hr. storage, with significant decreases in numbers occurring more frequently than increases (Reports, 1952(Reports, , 1953 Fisher (personal communication) found that some cultures of E8ch. coli after freeze-drying had lost their ability to ferment lactose at 440 C. when reconstituted in the manner described.Two methods employing chemical agents for the preservation of bacteria have been described. Hoather (1952, 1957) suggested that the addition of sodium thiosulphate to water samples was effective in the short term in stabilizing the 4 HYG 76