ESTIMATION OF B. COLI 143 



Plate Methods. Quite a number of solid media have been 

 suggested for the isolation and enumeration of B. coli and allied 

 organisms and of these the most useful are Endo's medium 

 (fuchsin sulphite agar), Drigalski and Conradi's medium (nut- 

 rose agar), sesculin bile salt agar, and rebipelagar (neutral 

 red bile salt agar). On account of the difficulties connected 

 with the preparation and use of the first two media the 

 author prefers the latter two. These are easy to prepare (see 

 appendix p. 207) and may be used in exactly the same manner 

 as ordinary nutrient agar or gelatine. The Committee on 

 Standard Methods of Milk Analysis of the American Public 

 Health Association investigated the latter two media and 

 reported in favour of the sesculin medium. They found more 

 bacteria of the B. coli group on rebipelagar in nearly every 

 instance but this was due to the difficulty in deciding which 

 were the coliform colonies on the sesculin medium. Of more 

 than fifty colonies subcultured from the neutral red medium 

 only 67 per cent were found to be B. coli or B. serogenes (B. 

 lactis serogenes) whereas all the dark colonies from the sesculin 

 medium were of the B. coli family. Savage 9 , from his expe- 

 rience with sesculin agar and rebipelagar, as compared with 

 lactose bile salt broth, has expressed the opinion that both 

 media are equally useful but inferior to L. B. B. tubes on 

 account of the difficulty in arriving at accurate estimations 

 of the numbers by direct plating. The author has had very 

 little experience with sesculin agar, but the extended observa- 

 tions that he has made with rebipelagar do not entirely agree 

 with the above results. A series of comparative experiments 

 on 100 samples with rebipelagar and lactose bile salt broth 

 gave the following results, gas formation being regarded as 

 evidence of the presence of B. coli in the tube series without 

 confirmation. 



Medium. B. coli per C.cm. 



Rebipelagar 15,326 



Lactose broth 10,182 



