THE CHOLERA VIBRIO 529 



may be almost absent. On surface agar a thick, moist, 

 shining, greyish growth quickly develops with more or less 

 crenated margins often becoming brownish when old. On 

 blood serum much the same growth occurs with slow 

 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 with more or less acid-production 

 and sometimes curdling. Acid, but not gas, is produced 

 from glucose, maltose, saccharose, lactose, mannitol 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 peptone-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 perman- 

 ganate. Some specimens of " peptone " are unsuitable 

 for preparing the peptone water used for obtaining the 

 reaction, on account of the absence either of a trypto- 

 phane 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). 



1 Wherry, Bureau of Government Laboratories, Manila, Bulls. 19 and 31, 

 1904 and 1905. 

 M.B. 34 



