494 THE CHOLERA VIBRIO 



already been laid on the variations in the virulence of the vibrios. The 

 virulence of the organism rapidly becomes attenuated in cultures but can be 

 increased by passage through susceptible animals (vide infra Toxin). 



Cholera vibrios are very susceptible to the action of micro-organisms with 

 which they may be associated (Metchnikoii). It has already been pointed 

 out that certain organisms favour the growth in culture of the cholera vibrio ; 

 there are others however which have an adverse influence, e.g. Bacillus 

 pyocyaneus, and a white coccus isolated from water. This white coccus has 

 a remarkable effect on the vibrio ; during the first few days it prevents any 

 growth at all taking place ; after a little while the organism begins to grow, 

 but the colonies are few in number and small in size and composed not of 

 " comma " bacilli but of double club-shaped involution forms (Metchnikoff). 



2. Bio-chemical reactions. 



Nitroso-indol reaction. In cultures in peptone water the cholera vibrio 

 reduces nitrates to nitrites and produces indol. If a mineral acid free from 

 nitrous compounds be added to a peptone water culture of the cholera vibrio 

 a characteristic red reaction develops, known as the cholera-red or nitroso- 

 indol reaction (reaction of Bujwid and of Salkowski). 



The reaction is most striking if a little nitrate of potassium be added to the peptone 

 water. A useful medium in which to grow the vibrio in order to obtain the cholera- 

 red reaction is composed of : 



Peptone (Chapoteaut or Witte), 1 10 grams. 



Common salt, - 5 ,, . 



Potassium nitrate, - 1 



Water - - - 1,000 



This solution is alkaline in reaction and does not require the addition of soda. 

 Sterilize at 115 C. 



Sow the peptone- water solution with the vibrio and incubate at 37 C. : 

 after 24 hours' incubation add gently 1-2 c.c. of pure sulphuric or hydro- 

 chloric acid. The peptone solution acquires a pink tint which deepens in 

 intensity during the next few hours. 



Not all cholera vibrios give the nitroso-indol reaction, while on the other 

 hand the reaction is produced by certain other micro-organisms which are not 

 cholera vibrios. 



Indol reaction. The cholera vibrio produces indol in cultures (p. 374). 



3. Toxin. 



Cholera is an acute toxaemia caused by the absorption of a toxin elaborated 

 by the cholera vibrio in the intestine. The toxin of cholera has for years 

 been a subject of investigation. 



I. Brieger and Frankel described the occurrence of an albuminoid substance 

 of unknown composition tox-albumin in cultures of the cholera vibrio ; 

 this substance they regarded as a diastase. Utchinsky showed that the 

 same toxin is elaborated in an exclusively mineral medium. 



Petri prepared a toxin which led to a fatal result in guinea-pigs when 

 inoculated intra-peritoneally in doses of 2 c.c. The organism was grown on 

 a 5 per cent, peptone solution and sterilized at 120 C. ; it follows therefore 

 that the cholera toxin, which Petri describes as toxo-peptone, is not destroyed 

 at the temperature of boiling water, and for .this reason is essentially different 

 in its nature from diphtheria toxin. 



1 A good quality peptone must be used. It is said that the presence of glucose in 

 some brands of peptone prevents the formation of indol and so leads to a negative reaction 

 Gorini). 



