INTESTINAL OR COLON-TYPHOID GROUP 289 



but not in higher dilutions, it may be inferred that there are be- 

 tween 100 and 1000 B. coli present per cubic centimeter in the 

 original sample. 



A more accurate determination of the number of B. coli present 

 in a given sample may be secured by plating different dilutions in 

 agar containing 1 per cent, lactose, colored blue by litmus solution, 

 and incubating twenty-four to forty-eight hours at 37. Organ- 

 isms which can ferment lactose with acid production are surrounded 

 by a red discoloration of the litmus. Such organisms are the 

 B. coli, B. lactis acrogenes, and Streptococcus. The first two may be 

 considered together, as they come from the same source and in- 

 dicate the same facts. The colonies of these may usually be 

 readily differentiated from those of Streptococcus by their large 

 size, their shiny appearance, and the frequent gas-bubble accom- 

 panying the colony if it lies below the surface. The Streptococcus 

 colonies, on the other hand, are small, rarely larger than a pin- 

 head, and never have gas-bubbles. It is sometimes necessary 

 to make transfers from colonies and carry them through the 

 various media to complete the identification. A highly contam- 

 inated water will usually reveal the acid colonies directly upon 

 plating, but in careful work it is sometimes necessary to enrich the 

 suspension. Plates may be poured from fermentation tubes 

 that show gas-production. 



B. coli is not uncommon in nature; its constant presence in the 

 feces of most animals makes it widely distributed. It is entirely 

 probable that the presence of small numbers of B. coli in water may, 

 therefore, be without significance from the standpoint of potability. 

 It is generally regarded in America that if B. coli can be constantly 

 demonstrated in 1 c.c. samples of the water, this is an indication 

 of recent sewage contamination. When its presence may be 

 demonstrated only by the use of larger samples than 1 c.c., the 

 evidence must be regarded as inconclusive. 



Water Purification 



Self-purification of Natural Waters. Natural waters, both run- 

 ning and impounded (as in lakes and reservoirs) , gradually free them- 

 selves from organic and bacterial contamination. The rapidity 

 and efficiency of this cleansing process depend upon many factors. 



19 



