130 ELEMENTS OF WATER BACTERIOLOGY 



worth while to use aesculin to exclude this form, but the 

 more general tendency at present seems to be to eliminate 

 the anaerobic spore formers by subsequent subcultures. 

 The Use of Sjmthetic Media for the Isolation of the 

 Colon Group. Dolt (1908), working in Prof. Gorham's 

 laboratory at Brown University, has attempted with 

 success to substitute synthetic media of simple and 

 known composition for the usual meat-infusion-peptone 

 media used in the isolation of B. coli. He first found 

 that colon bacilli will grow readily on a medium con- 

 taining asparagin and sodium or ammonium phosphate. 

 He then attempted to substitute for asparagin various 

 simple organic substances similar in their structure 

 to the cholic acid of the bile which exerts a selective 

 action in favor of the colon group. He finally succeeded 

 in preparing two media which promise to be of con- 

 siderable value in permitting the growth of B. coli 

 while checking other forms. The first of these media 

 is made up as follows: 500 c.c. of a 3 per cent solution 

 of purified agar is mixed with an equal portion of a 

 solution of I per cent glycerin and 0.2 per cent 

 (NH4)2HP04. It is neutralized with sodium hydroxide 

 and I per cent of lactose is added before sterilization. 

 In the second medium 5 gm. of ammonium lactate is 

 substituted for the glycerin and i gm. of Na2HP04 

 for the ammonium phosphate. These media proved 

 to have a considerable selective value, cutting out most 

 of the water bacteria; but like all such selective media 

 they cut out a good many of the colon bacilli too. The 

 results of a single test are shown in the table below. 

 The procedure well merits further study, however. 



