242 Bacteriology of Water 



plate. His method is to add to each of several fermentation-tubes 

 containing i per cent, dextrose-bouillon a certain quantity of water. 

 The evolution of 50-60 per cent, of gas by the third day is a strong 

 indication that the colon bacillus is present. The presence of gas 

 in a fermentation tube constitutes the "presumptive test" for the 

 colon bacillus. It is not an infaUible indication of their presence. 

 A careful study of its usefulness has been made by Ruediger and 

 Slyfield* who found that in making quantitative determination of 

 B. coli in polluted waters by means of the fermentation tubes, the 

 most accurate results were obtained by the use of neutral-red lactose 

 bouillon. Gas appeared earlier in the neutral-red lactose tubes 

 than in lactose bile broth tubes, and B. coli were more easily isolated, 

 by plating, from the former than from the latter. The finding of 

 B. coli in the fermentation tubes is greatly facilitated by making 

 plates soon after the appearance of the gas. When the fermenta- 

 tion of the sugar and the appearance of gas in the tube occurs, some 

 bacteriologists are satisfied that B. coli are present, and go no fur- 

 ther, but a careful workman will always take pains to confirm the 

 indications of their presence by plating and isolating the bacillus in 

 pure culture. 



It was at one time thought that the occurrence of the colon bacOlus 

 in water was sufficient to condemn its potability, but the evidence 

 accumulated in recent years, showing that this organism may reach 

 streams from manured soil, may enter it with the dejecta of domestic 

 animals, wild animals, birds, and perhaps even of fishes, makes it 

 doubtful whether anything but an exceptionally large number of the 

 organisms should be looked upon as indicative of sewage pollution 

 and proof that the water is not potable. 



In determining the species of bacteria found in the water reference 

 must be made to the numerous monographs upon the subject and to 

 special tables. An excellent table of this kind, arranged by Fuller,t 

 is given on pages 240 and 241. 



Filtration with sand, etc., diminishes the number of bacteria for a 

 time, but, as the organisms multiply in the filter, the benefit is not 

 permanent and the filters must frequently be subjected to bacterio- 

 logic tests and the sand washed, spread out to dry and the filters 

 renewed. Porcelain filters seem to be the only positive safeguard, 

 and even these, the best of which seems to be the Pasteur-Chamber- 

 land, allow the bacteria to pass through if used too long without proper 

 attention. 



For those whose special line of work is the bacteriology of water, 

 the report of the Committee on Standard Methods of Water Analysis 

 to the Laboratory Section of the American Public Health Association, 

 published in Supplement No. i of the "Journal of Infectious Dis- 

 eases," May, 1905, will prove indispensable. 



*Jour. of the American Public Health Association, 1911, i, No. 11, p. 828. 

 t "Public Health and Journal of Experimental Medicine." 



