THE COMMA BACILLUS 435 



liquefaction. A thin brownish layer is formed on potato 

 at 37 C. ; and broth becomes turbid, a delicate film 

 forming on the surface. Peptone water, or Dunham's 

 modification of it (1 per cent. NaCl), is a good cultivating 

 medium, and a delicate film forms on the surface. In 

 milk it multiplies rapidly without curdling ; neutral 

 litmus glucose- agar is reddened from the development of 

 acid, but no gas is produced under cultivation. Acid, 

 but not gas, is produced from glucose, maltose, saccharose, 

 lactose, and starch. 



An important characteristic of the cholera vibrio is 

 the rapid formation of indole in considerable quantity, 

 and the reduction of nitrates to nitrites, especially in 

 peptone water. This forms the basis of the important 

 cholera-red reaction ; a few drops of pure sulphuric or 

 hydrochloric acid added to a pep tone- water culture, eight 

 to twelve hours old, give a pink colour, and the colour is 

 intense when the culture is two to three days old, and of 

 a purplish-red colour, like that of potassium permanganate. 

 Some specimens of " peptone " are unsuitable for pre- 

 paring the peptone water used for obtaining the reaction, 

 either on account of the absence of a tryptophane nucleus, 

 or of nitrates and nitrites. The medium should be sugar- 

 free, and the addition of 0-01 per cent, potassium nitrate 

 to it is an advantage. Some believe that two pigments 

 are formed in the reaction, a cholera- red and the nitroso- 

 indole pigment. 1 The reducing action of the cholera 

 vibrio can also be shown by growing in litmus broth, which 

 becomes decolorised (Cahen's test). 



Kraus and PrantschofI 2 noticed that certain vibrios 

 dissolved red blood- corpuscles, but came to the conclusion 

 that no true recently isolated cholera vibrio is hsemolytic 

 (see also p. 441). 



1 Wherry, Bureau of Government Laboratories, Manila, Bulls. 19 

 and 31, 1904 and 1905. 



2 Wien, klin. Woch., 1906, p. 299. 



