708 A MANUAL OF BACTERIOLOGY 



uncountable, ipso facto this would be sufficient to condemn 

 without an actual count. Dilution is necessary when dealing 

 with river or other water known to be polluted, and of which an 

 estimate of the number of organisms present is desired. In order 

 to count the colonies if very numerous, ink lines may be. drawn 

 across the bottom of the Petri dishes so as to divide them into 

 sectors. Ruled paper discs (Pakes's discs) upon which the dishes 

 are placed can also be obtained. The colonies in the -sectors 

 are then much more easily counted : or if the colonies be very 

 numerous and evenly distributed, the number in two or three of 

 the sectors may be counted, and the total number on the plate 

 estimated by calculation. 



SEARCH FOR BACILLUS COLI, ETC. Various media are 

 employed for the detection of the presence of B. coli, 

 and by taking varying amounts of the water a quantita- 

 tive estimate can be made at the same time. The media 

 generally used are glucose or lactose bile-salt peptone- 

 water and neutral-red broth. 



As a routine for a drinking water, a quantity of 100 c.c. 

 in all should be examined for the presence of B. coli, 

 quantities from a minimum of 0-1 c.c. to a maximum 

 of 50 or 100 c.c. being added to the tubes of culture media. 



The water should be added directly to the tubes of 

 culture medium, even with the larger amounts, and 

 should not be concentrated. The culture media may 

 be diluted with at least an equal volume of the water 

 without interfering with their cultural properties, and 

 boiling tubes or small flasks are used for the larger 

 amounts. 



In the case of glucose or lactose bile-salt peptone-water, 

 the medium may for the larger amounts, down to 10 c.c., 

 be prepared of double strength. The inoculated media 

 should be incubated at 37-40 C. for not less than forty- 

 eight hours. 



AYhile a lactose medium has the advantage of excluding Proteus 

 and other forms, which, though fermenting glucose, do not ferment 



