102 BACTERIOLOGICAL EXAMINATION OF WATER. 



chemical composition, and B. coli forms very characteristic 

 colonies in it. 



The methods above mentioned will be found discussed in 

 greater detail in the section devoted to the B. typhosus. My 

 own experience has shown that if a pure water supply, i.e., one 

 containing few water-bacteria, has been recently polluted with 

 fresh sewage, it is comparatively easy to isolate the B. coli by 

 any of the methods described. If, however, the water contains 

 a large number of water-bacteria and the pollution is not recent 

 or has been caused by old sewage, it is sometimes by no means 

 easy to isolate the B. coli from the supply. The B. coli 

 does not live indefinitely in water ; the numbers, as a rule, di- 

 minish rapidly unless considerable quantities of nitrates are 

 present. Consequently, if some time has elapsed since the 

 pollution, it may be necessary to examine considerable quan- 

 tities of the water in order to isolate the bacillus ; in other 

 words, the water has to be concentrated by pumping one or two 

 litres through a Berkefeld bougie, and the deposit on the candle, 

 after diffusion in 10 c.c. of sterile water, must then be examined. 

 But under these conditions the water-bacteria are present in 

 enormous numbers, and I have found that carbolised media are 

 quite unable to sufficiently restrain the growth of these organ- 

 isms unless about 0.15 to 0.2 per cent, of carbolic acid is used : 

 but if this amount is employed there is always a chance that 

 the B. coli, enfeebled by existence in sewage, may not be able 

 to develop. Under these conditions, I have obtained the best 

 results with Fakes 1 method of cultivation, which combines 

 anaerobic conditions with incubation at 42 C. But even with 

 this procedure many organisms may be found besides B. coli, so 

 that it is necessary to make several dilutions, i.e., at least three 

 or four gelatine plates, from the glucose-formate broth tube 

 after twenty-four hours incubation at 42 C. It is also very 

 useful to plate the broth in litmus-lactose-agar and then sub- 

 culture the colonies which turn the blue litmus red. 



In order to deal with larger quantities of water, Abba sug- 

 gested the following solution : 



Lactose . ... . 200 grammes. 



Peptone. . . . . 100 



Sodium chloride ... 50 



Water , 1.000 c.c. 



