56 BACTERIA IN WATER 



do and not by what it cannot; and in a general way it may 

 be said that there are three groups of facts contained in a 

 systematic bacteriological report of water. These findings are 

 concerned with the number of bacteria per c.c., the presence of 

 any organisms of contamination, and the presence of any specific 

 organisms of disease. 



(1) Number of Bacteria per ex. It would appear that in the past 

 a great deal too much weight has been attributed to the number of 

 bacteria per c.c. This fact is not of the first importance for two 

 obvious reasons. In the first place there is no standard as to how 

 many bacteria should be present in 1 c.c. of a potable water, and in 

 the second place there is no known means by which this number 

 can be accurately measured. In this country any number of bacteria 

 under one hundred per c.c. is generally considered low. The 

 metropolitan water supply, as consumed, usually contains less than 

 twenty bacteria per c.c. Deep - well waters and spring waters 

 frequently contain very few bacteria. Polluted or surface waters 

 contain thousands of organisms per c.c. More than this, no 

 standard exists. Nor would any numerical standard taken alone 

 be of much value, for the reason that the number of bacteria in 

 water is of comparatively little value apart from a knowledge of the 

 species, and moreover a really accurate record of the number of 

 bacteria per c.c. is not obtainable. Whether the organisms detected 

 be many or few depends upon a variety of external circumstances, 

 such as medium used for cultivation, temperature and period of 

 incubation, length of time of cultivation before counting, or the 

 use or not of a lens when counting. For these reasons it is 

 evident that great reliance cannot be placed upon the number 

 of bacteria per c.c. returned in bacteriological reports, and it is 

 well that should be understood. The only circumstances under 

 which such returns are valuable are (a) when used in a series of 

 examinations of the same water supply, when such returns, if always 

 obtained under the same conditions, are of great comparable value, 

 and (&) when used in the examination of water before and after 

 filtration. In these two circumstances the number of organisms per 

 c.c. is of great value in forming an opinion as to pollution or as to 

 failure of filtration. 



(2) Presence of Organisms, of Contamination. In the general 

 bacteriological examination of water this point is perhaps the most 

 important. Judgment must be formed on two facts, namely, the 

 presence of any of the "bacteria of indication," such as B. coli, 

 B. enteritidis sporogenes, streptococci, and the para-colon types 

 (enteritidis, Gaertner, and the chologenes type), and the relative 

 abundance of these species. The latter point is one of importance. 

 The chief organism of indication is B. coli, including under that 



