Isolation of the Colon Bacillus. 89 



is done by fishing from isolated colonies, replating and 

 inoculating into the usual media for identification. 



The plate method of isolation is recommended by the 

 Committee on Standard Methods of Water Analysis 

 (1905) for sewages and polluted waters, and with such 

 sources it yields good results. For success in the use of 

 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, 

 combining 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 per cent solu- 

 tion 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 I part gave very ' satisfactory results, the plates 

 showing pure cultures of B. coli. The addition of anti- 

 septics in this way is always open to the objection that 

 weaker strains may be killed and lost. 



The test for the colon bacillus in less heavily polluted 

 waters may be made more delicate by a preliminary 

 enrichment of lie sample by growth in a liquid medium 

 for twenty-four hours at 37 degrees, thus greatly increasing 

 the proportion of these organs present before plating. As 



