ISOLATION OF INDICATOR ORGANISMS Ip 



writer decidedly prefers liquid media for primary enumeration. 

 It should, however, be said that a number of authorities prefer 

 the solid plate method 1 . 



A second object to be aimed at in the detection of B. coli and 

 other indicator organisms is to use means whereby the growth 

 of these organisms is favoured while that of other bacteria is 

 retarded. A simple way of doing this is to employ a temperature 

 which selectively favours the growth of the bacilli required. 

 Vincent suggested a temperature of 42 C, MacConkey used 

 at first a temperature of 43 C., while in Eijkmann's method a 

 temperature of 46 C. was employed. B. coli organisms grow well 

 at 37 C. and the advantage of these higher temperatures is not 

 commensurate with the trouble of using special incubators. 



Growth under anaerobic conditions was for similar reasons 

 advocated by Fakes, but the advantages are not great enough 

 to balance the extra trouble and other methods are better. 



Selective growth may also be favoured by the addition of 

 certain chemicals. Studies along this line have been very valu- 

 able and fruitful. Phenol was at one time extensively employed 

 but is now largely given up as it does not satisfactorily differ- 

 entiate. 



We owe to MacConkey the most valuable substance for 

 selective differentiation in the bile salts, usually employed as 

 sodium taurocholate. This substance is a satisfactory inhibitory 

 substance, while in the amounts used it does not interfere with 

 the growth of B. coli. 



The inhibitory action of certain colouring media, such as 

 crystal violet, have also been successfully employed. 



Of all these agents it may be said that the use of lactose 

 bile salt broth in double tubes is the most valuable when the 

 liquid enumeration method is employed, while for solid media 

 any of those mentioned in the next stage may be employed. 

 The composition of these media is given in the Appendix. 



The second stage of the examination consists in the isolation 

 of the bacillus in pure culture. If solid media were employed 



1 For a good discussion of this question and in favour of solid media see Gartner, 

 " Das Bacterium coli als Indikator fiir fakale Verunreinigung eines Wassers," Zeit.f. 

 Hyg. 1910, vol. 67, p. 55. 



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