262 BACTERIOLOGICAL EXAMINATIONS 



ing bouillon with i percent of glucose. Into a series of these tubes, 

 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 bacillus. 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 quantities 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 loB. 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 streptococci of the 

 fecal type as pollution indicators. This is not absolutely 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 solution 

 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 media of Drigalski and Conradi. This media is made of 3 



