It has been observed that during the treatment of sewage by biological filtration the numbers of coliform bacteria are considerably reduced (Allen, Tomlinson & Norton, 1944; Allen, Brooks & Williams, 1949). As it was thought that this reduction might be due to the action of bacteriophage known to be present in sewage, a short investigation to obtain evidence on this point was made at Stevenage sewage works.At this works sewage from the residential and industrial areas of the town is mixed and screened, one-third of the flow being then treated in the old section of the works and two-thirds in a modem extension. In each section the sewage is settled in continuous flow tanks, treated biologically in percolating filters, and settled again in humus tanks before it flows to a sump, common to the two sections, from which it is pumped intermittently. METHODSEnumeration and isolation of coliform organisms Counts of coliform bacteria were made from sewage taken at different stages of treatment by spreading 041 ml. of a suitable dilution in saline over the surface of Bacto eosin-methylene-blue agar and counting typical coliform colonies after incubation for 24 hr. at 370 C. This medium was almost completely specific for coliform organisms.Over a period of several weeks pure cultures of forty-nine strains of coliform organism were obtained from the influent crude sewage and a further fifty from the effluent in the final sump by plating on double strength Bacto eosin-methyleneblue agar and subculturing typical Bacterium coli colonies twice on single strength eosin-methylene-blue medium; the resulting cultures were maintained on nutrient agar. All the organisms were subsequently confirmed biochemically to be either Bact. coli, Bact. aerogenes, or intermediates.Isolation and enrichment of specific bacteriophage Ten ml. of each of the sewage samples, of which a portion had been used for inoculation on to eosin-methylene-blue agar, was immediately treated with 3 drops of toluene at 37°C. for 2 hr. This procedure killed the majority of the bacteria in the sewage without affecting the bacteriophage count. The purified bacteriophage suspensions were stored in a refrigerator until the pure culture of the coliform isolated from the same sample was available. Fifteen ml. of nutrient broth was then mixed with 5 ml. of toluene-treated suspension, drops of free toluene being
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