BACTERIOLOGY OF WATER 279 



varying amounts of water are run by means of a sterile pipette, 

 2 c.c., i c.c., .5 c.c., .1 c.c., .01 c.c., of water being used. After a 

 stay of twenty-four hours in the incubator, if gas appears, the 

 bouillon should be examined by plate cultures for the colon bac- 

 illus. Lactose litmus agar is used, and where colonies appear that 

 redden the litmus and resemble the colon colonies in appearance, 

 they are planted in milk, fermentation tubes, peptone solution, 

 neutral red agar, nitrate solution, and gelatine, and the various 

 reactions in the various media noted. Some idea of the numerical 

 presence of colon bacilli can also be obtained. Definite quanti- 

 ties of the raw water, similar to those used in the fermentation 

 tubes, may be plated directly without previous incubation. A 

 deeply tinted litmus lactose agar is used and upon this medium 

 colon bacillus colonies appear, small, pink, round or whetstone- 

 shaped surrounded by a pink zone or halo. Such pink colonies 

 are fished out into the different media as above. If there were 

 twenty pink colonies of the colon type upon a plate of litmus 

 lactose agar that had been seeded with i c.c. of water and of 

 these eight were fished and determined, with the discovery that 

 four only were true B. coli, we would assume that in i c.c. of 

 raw water half the pink growing colonies were those of B. coli 

 and that the water contained ten B. coli per cubic centimeter. 



The significance of the colon bacilli is often overestimated. 

 They are found in all rivers, and often reach streams, wells, and 

 even springs by contamination from the barnyard, or manured 

 fields. Attempts to separate colon bacilli from human and animal 

 sources have been unsuccessful. Some authorities use strepto- 

 cocci of the faecal type as pollution indictors. This is not abso- 

 lutely reliable. 



Typhoid bacilli have been found in water. One way that is 

 sometimes successful is to take 25 c.c. of a 4 percent peptone solu- 

 tion and add this to a litre of the water to be examined; from this, 

 after twenty-four hours in an incubator, plates may be prepared 

 with the agar medium of Endo as already given on page 120. 



