From the bacteriological point of view, the count of the number of colonies produced in the usual method of investigation is an index of the number of bacteria which may be found. The number, however, is not any more important than the kinds, and if the bacterial examination reveals the presence of a number of species allied with those of the pathogenic character, the water would be pronounced bad, even if no germs of any given specific disease were discovered. If the investigations of the analysis show the presence of the colon bacillus or colon group of organisms, it is considered as satisfactory evidence of sewage contamination. It is possible that there may be instances where the colon bacillus exists which has not been due directly to a growth from the excreta of an animal, but even if these bacteria be transmitted through several stages of growth outside of the animal and afterward enter the water, it is evident that the contamination is one of degree only, and is to be avoided.

1 First Report of the Lake Michigan Water Commission, 1909, page 68.