ISOLATION OF INDICATOR ORGANISMS I? 



The examination and isolation of these organisms involve 

 three considerations : 



(a) A preliminary determination of their probable presence. 



(b} Their isolation in pure culture. 



(c) The application of determining tests. 



Sometimes a and b are combined. 



Bacillus colt Group. 



To prevent the separate examination and plating of all the 

 different quantities of water, milk, or other substance selected 

 for examination it is necessary to have some means by which 

 those amounts which contain B. coli organisms can, with 

 some measure of probability, be distinguished from those which 

 are free from this organism. Authorities are not in complete 

 agreement as to the respective value of glucose and lactose 

 fermentation, but the great majority of workers select the fer- 

 mentation of lactose as the essential primary means of differentia- 

 tion since this character is a convenient and satisfactory one to 

 employ. 



By the employment of some agent such as litmus or neutral 

 red in conjunction with lactose to indicate when the acid fer- 

 mentation of this sugar has taken place, or by the use of double 

 tubes so that the formation of gas can be seen, it is possible to 

 differentiate between liquid media tubes (or individual colonies 

 when solid media are employed) which respectively do not or 

 which do contain lactose-fermenting bacilli. Such means of 

 differentiation are of immense importance in routine work as 

 they enable tubes or plates not containing lactose-fermenting 

 bacilli to be at once discarded. All modern methods make use 

 of some such means of differentiation. 



For liquid media the fermentation of lactose with production 

 of gas as well as acid is by far the best means of differentiation, 

 with solid media the production of acid alone has to be relied upon. 



In regard to the comparative value of liquid and solid media, 

 for this primary differentiation there are considerable differences 

 of opinion. 



S. W. 2 



