60 ELEMENTS OF WATER BACTERIOLOGY. 



This is done by fishing from isolated suspicious-looking 

 colonies, replating and inoculating into the usual media 

 for diagnostic work. 



For success in examining polluted waters by this method 

 it is necessary to get a sufficient dilution so that colonies 

 may be well isolated, and to this end it is advisable that a 

 number of different dilutions be employed, a series of 

 plates being prepared from each. Under any conditions 

 the detection of the colon bacillus is seriously hampered 

 by the development of other forms. Certain observers 

 have therefore added phenol to the agar medium, com- 

 bining the effect of high temperature and an antiseptic 

 to check the growth of water-bacteria. Copeland for 

 this purpose added to his tubes .2 c.c. of a 2% solution 

 of phenol (Copeland, 1901). Chick (Chick, 1900) 

 found that 1.33 parts of phenol in 1000 materially de- 

 creased the number of colon bacilli which would develop, 

 while 1 part gave very satisfactory results, the plates 

 showing pure cultures of B. coli. 



The test for the colon bacillus may, however, be made 

 still more delicate by a preliminary enrichment of the 

 sample by growth in a liquid medium for twenty-four 

 hours at 37 , thus greatly increasing the proportion of 

 these organisms present before plating. As suggested 

 in the classic researches of Theobald Smith (Smith, 1892), 

 this method may be made approximately quantitative by 

 the inoculation of a series of tubes with measured por- 

 tions of the water. If, for example, of ten tubes inocu- 

 lated each with j-j^ of a cubic centimeter, four show 



