DIFFERENTIATING MEDIA. 329 



typhoid bacillus. It is to be noted here that the presence of 

 lactose or dextrose in a medium prevents the production of indol 

 by the B. coli. The indol reaction thus ought to be sought for 

 in a sugar-free medium. 



(4) Agar containing Neutral-red. — The method here is to 

 take sterile tubes of this medium (see p. 42) and either make 

 stab or shake cultures and incubate for twenty-four hours at 

 37° C. In the case of the typhoid bacillus no change in the 

 colour occurs, but in the case of the B. coli there is developed 

 a beautiful canary yellow with a greenish fluorescence. The 

 value of the medium as a means of differentiating the two 

 organisms is still stib judice. Fitz Gerald and Dreyer have 

 shown that a very important factor is the reaction of agar or 

 glucose agar, and consider the difference in the effects 6\ the 

 two bacteria only one of degree. They state that the best 

 results are obtained by employing as a medium a 3 per cent 

 lactose bouillon, to which .5 per cent of a i per cent watery 

 neutral-red has been added. 



(5) The Media of Capaldi and Proskauer. — The first of these 

 is a medium free of albumin, in which B. coli grows well and 

 freely produces acid, .while the typhoid bacillus hardly grows at 

 all, and certainly will produce no change in the reaction. Its 

 composition is as follows : — 



The second medium contains albumin, and is such that the B. 

 coli produces no acid, while the typhoid bacillus grows well and 

 produces an acid reaction. It consists of : — 



Witte's peptone ..... 2.0 grms. 



Mannite o.i „ 



Water (distilled) . . . . 1 00.0 c.c. 



After the constituents of each medium are mixed and dissolved it 

 is steamed for one and a half hours and litmus solution added (the 

 tint being judged of by experience), and the medium is then 



