THE COLON GROUP OF BACILLI 105 



Chapter IV, furnishes one ready method for the isola- 

 tion of B. coli from water, and it was used by Sedgwick 

 and Mathews for the purpose as early as 1892 (Mathews, 

 1893). The process is based upon the fact already 

 alluded to, that B. coli readily ferments lactose with 

 the formation of acid. If, therefore, plates are made 

 with agar containing both lactose and litmus, the colon 

 colonies develop as red spots in a blue field. Since 

 organisms other than B. coli (notably the streptococci) 

 may also develop red colonies, it is necessary to examine 

 them further. This is done by fishing from isolated 

 colonies, replating and inoculating into other media 

 for identification. 



The plate method of isolation is recommended by the 

 Committee on Standard Methods of Water Analysis 

 (1912) for sewages and polluted waters, in which colon 

 bacilli are present in i c.c. or less. They recommend 

 that Petri dishes with porous covers be used and that 

 incubation be carried out at 40 instead of 37. 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, 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 per cent 

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



