698 BACTERIA IN AIR, SOIL, WATER, AND MILK 



human forces. The limit of safety, Rosenau states, is one colon bacillus 

 per c.c. If more are present the water should be regarded as sus- 

 picious. If more than 10 per c.c. are found the water must be re- 

 garded as dangerous and unqualifiedly condemned. 



For the purpose of isolating B. coU from water, a large number 

 of methods have been devised. In examining sewage or other pol- 

 luted waters in which the number of colon bacilli is comparatively 

 large, the direct use of lactose litmus agar plates yields excellent re- 

 sults. Varying quantities of water may be added to this mediiun 

 and the development of red colonies at incubator temperatures usually 

 indicates the presence of bacilli of this species. These colonies may be 

 fished and further identified. Copeland ^ has proposed the addition of 

 .2 per cent of phenol to this medium in order to inhibit other bacteria. 



In water less grossly polluted, some method of enrichment must be 

 employed in order to increase the number of bacteria so that they may 

 be found in plates. For this purpose glucose bouillon in fermentation 

 tubes, according to the method of Theobald Smith, may be employed.^ 

 In this medimn, at a temperature of 37.5° C, the colon bacilli grow with 

 great speed and transplants to plating media may be taken after eight 

 or more hours' incubation. 



A medium proposed by Jackson^ has been found successful. This 

 consists of undiluted ox-bile, to which has been added 1 per cent of lactose. 



For quantitative estimation of colon bacilli in water, Theobald 

 Smith * has proposed the use of dextrose bouillon in fermentation tubes, 

 to which are added varying quantities of water, ranging from 0.001 to 

 1 c.c. The appearance of gas in any tube indicates the presence of 

 B. coli, and the number can be approximately computed from the 

 smallest quantity of water by which gas formation has been produced. 



The presence of B. coU in such fermentation tubes may be deter- 

 mined without isolation and cultivation, by estimating the comparative 

 amount of CO2 in the gas.^ Whenever CO2 forms approximately 33 

 per cent of the gas present. Irons concludes that B. coli is present. 



Jackson * beUeves when lactose ox-bile is used that 25 per cent of 

 gas within 72 hours may be regarded as positive for B. coU. 



' Copeland, Jour. Boston Soc. Med. Sciences, 1901. 



^ Prescott, Science, xvi, 1902. 



3 Jackson, Jour. Inf. Dis., Suppl. 2, 1907. 



" Th. Smith, 13th Ann. Rep. N. Y. S. Board of Health. 



s Irons, Trans. Amer. Pub. Health Assn., xxvi, 1900. 



^Jackson, "Biol. Studies of Pupils of W. T. Sedgwick," Boston, 1906. 



