ISOLATION OF THE B. COLI 365 



D. Reduction of Nitrates. The b. coli is frequently capable 

 of reducing nitrates to nitrites. For this test, Savage recom- 

 mends the use of a medium made by dissolving 10 grms. of 

 peptone in 1 litre of ammonia-free distilled water, and adding 

 2 grms. of nitrite-free potassium nitrate. The medium is filtered, 

 tubed, and sterilised for half an hour on three days. Tubes 

 are infected and incubated for forty-eight hours, the forma- 

 tion of nitrites being now tested for by Ilosvay's method. 

 The following solutions are required : (a) sulphanilic acid, '5 

 grm. dissolved in 150 c.c. dilute acetic acid (s.g. 1*04); (b) 

 1 grm. a-naphthylamine is dissolved in 22 c.c. of water, the 

 solution filtered, and 180 c.c. dilute acetic acid added. In 

 using the test, 2 c.c. of each of these solutions is added to 10 c.c. 

 of culture. If reduction of the nitrates has occurred, a rose- 

 pink colour should develop almost immediately. It is' to be 

 noted that the pink colour first produced sometimes disappears 

 as it is formed or on shaking ; in such a case further portions 

 of the two reagents in equal quantities should be added. 



Agglutination Eeactions of the B. coli. When the b. coli has 

 produced a pathological condition in an animal, the serum of 

 the infected animal frequently manifests specific agglutinative 

 characters, especially towards the strain of the organism isolated 

 from the lesions. Under certain circumstances, also, the serum 

 of an animal infected by some other member of the b. coli group 

 may also agglutinate strains of this organism. This subject will 

 be treated of when we consider the differentiation of the 

 members of the group one from another. 



Isolation of the B. coli. In the case of abscesses or coli 

 infection of the kidney or bladder, etc. (p. 367), the isolation of 

 the organism is usually easy, the use of agar plates being here 

 sufficient. When, however, the organism is present along with 

 other bacteria, as in the case of water, sewage, etc., special means 

 must be adopted, the object of which usually is to inhibit the 

 growth of organisms except those belonging to the coli group. 

 Formerly media containing small quantities of carbolic acid 

 were used for this purpose, but now the inhibition is usually 

 effected by the use of certain aniline dyes, by picric acid, or 

 by bile salts. The media of Conradi-Drigalski, Conradi, Endo, 

 Fawcus, and of MacConkey (pp. 48-51) are examples. All 

 these media have their uses, and it is best to select that with 

 which the worker has had most experience. In this country 

 MacConkey's bile-salt lactose agar is perhaps most widely used. 

 The methods of the application of these media and the appear- 

 ances of b. coli have already been described (pp. 47-51). 



