THE CHOLERA-RED" REACTION. 413 



given by a true cholera spirillum, and though the reaction is not 

 peculiar to it, the number of organisms which give the reaction 

 under the conditions mentioned are comparatively few. The test 

 is made by adding one or two drops of pure sulphuric acid to a 

 culture in peptone bouillon or in peptone solution (i per cent) 

 which has been incubated for twenty-four hours at 37" C. ; in 

 the case of the cholera spirillum a reddish-pink colour is pro- 

 duced. This is due to the fact that both indol and a nitrite are 

 formed by the spirillum in the medium. The addition of sul- 

 phuric acid causes a nitroso-indol body to be produced from these, 

 and this gives the red colour. Here, as in testing for the pro- 

 duction of indol by other bacteria, it is found that not every 

 specimen of peptone is suitable, and it is advisable to select a 

 peptone which gives the characteristic reaction with a known 

 cholera organism, and to use it for further tests. It is also 

 essential that the sulphuric acid should be pure, for if traces of 

 nitrites are present the reaction might be given by an organism 

 which had not the power of forming nitrites. 



The cholera organism is one which grows much more rapidly 

 in the presence of oxygen than in anaerobic conditions. Koch, 

 in his earlier work, believed that no growth took place in the 

 absence of oxygen, and it has been recently stated that this . is 

 the case in absolutely anaerobic conditions. Growth, however, 

 takes place in the ordinary anaerobicconditions, usually employed 

 in the culture of anaerobic organisms, such as those of tetanus 

 and malignant oedema, though it occurs more slowly than in the 

 presence of oxygen. In the intestines the oxygen supply, though 

 small in amount, is yet sufficient for the growth of the spirilla. 



Powers of Resistance. — In their resistance against moist 

 heat cholera spirilla correspond with most spore-free organisms, 

 and are killed in five minutes by a temperature of 65° C, and 

 much more rapidly at higher temperatures. They have com- 

 paratively high powers of resistance against great cold, and have 

 been found alive after being exposed for several hours to the 

 temperature of — 10° C. They are, however, killed by being 

 kept in ice for a few days. Against the ordinary antiseptics 

 they have comparatively low powers of resistance, and Pfuhl 

 found that the addition of lime, in the proportion of i per cent, 

 to water containing the cholera organisms, was sufficient to kill 

 them in the course of an hour. 



