20 ISOLATION OF INDICATOR ORGANISMS 



for the first stage this is already carried out, but if liquid differ- 

 entiating tubes are employed the bacillus has to be isolated from 

 the tubes selected. 



The principles enunciated above apply equally here and it is 

 a great advantage to employ media containing both a differen- 

 tiating agent and a substance which retards the growth of 

 bacteria other than those to be isolated. 



On these grounds the use of ordinary gelatine and agar have 

 been given up and coloured media employed. 



Of such media lactose bile salt neutral red agar (L.B.A.) } 

 nutrose agar (Drigalski-Conradi agar), and fuchsin agar may be 

 specially mentioned. 



The ingenious aesculin agar may also be mentioned here. 

 This contains the glucoside aesculin and ferric citrate. The action 

 of B. coli causes the aesculin to combine with the iron citrate 

 and form a dark brown salt, the B. coli colonies being black. 



Fig. 2. Bent glass distributing rod. 



'In brushing these plates of coloured media it is important 

 that the media should be thoroughly dried. A convenient 

 general procedure is to add one loopful of the tube selected for 

 examination to a tube of sterile water. After mixing well place 

 two loopfuls of the latter upon the surface of the L.B. A. or other 

 material selected and distribute uniformly over the plate with a 

 sterile bent glass rod. The medium in the plate will be wet from 

 condensed water on the surface. To dry incubate it for \\ to 

 2 hours uncovered in the 37 C. incubator. Then cover and 



