712 A MANUAL OF BACTERIOLOGY 



detection of sewage or faecal pollution ; the bacterioscopic 

 analysis does not give any information as to the suitability 

 of the water for household, trade, or factory purposes. 



Number of colonies on the gelatin plates. The number of 

 colonies represents approximately the number of organisms 

 in the original sample capable of development aerobically 

 at 20 C. in gelatin. This number in a good water rarely 

 exceeds 100 or 150 ; in pure waters, particularly those 

 coming from deep chalk- wells, there may be only a few 

 5-10 per c.c. (the results are always expressed in numbers 

 per cubic centimetre of ihe original water). In waters of 

 poorer quality the number may approach 500 per c.c. 

 Anything over this casts suspicion on the water, and 

 1,000 per c.c. or more should probably condemn the 

 sample, always supposing; of course, that multiplication 

 in vitro has been excluded by the proper storage of the 

 sample bottle in ice. As a rule in water of good quality 

 liquefying organisms are scanty, while in a polluted water 

 they are numerous. 



Number of colonies on the agar plates. As mentioned 

 before (see p. 706), it is the ratio of the number of 

 organisms developing on the agar plates to the number 

 of those developing on the gelatin plates that is of 

 importance. 



Number of B. coli. The detection and enumeration of 

 B. coli are regarded by all as perhaps the most important 

 part of water examination. The number of B. coli is 

 estimated from the amounts of water that have been 

 added to the tubes of media, which, however, assumes 

 that the organism is regularly distributed throughout the 

 sample, and this must so far as possible be ensured by 

 thorough mixing. The results generally come out fairly 

 concordantly, though irregularities exceptionally occur ; 

 Greenwood and Yule a give some data by which approxi- 



1 Journ. of Hygiene, vol. xvi, 1917, p. 30. 



