POWERS OF RESISTANCE 465 



cholera spirillum a reddish-pink colour is produced. This is due 

 to the fact that both indol and a nitrite are formed by the 

 spirillum in the medium, and hence, in applying the test for 

 indol, the addition of a nitrite is not necessary. Here, as in 

 testing for the production of indol by other bacteria, it is found 

 that not every specimen of peptone is suitable, and it is ad- 

 visable 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 may 

 be given by an organism which has not the power of forming 

 nitrites. 



Hcemolytic Test. This method, introduced by Kraus, is performed by 

 means of agar plates, a small quantity of sterile denbrinated blood being 

 added to the agar and thoroughly diffused ; if any organism has hsemo- 

 lytic properties, a clear zone or areola forms around each colony by the 

 diffusion of haemoglobin. As a rule the cholera organism does not 

 produce haemolysis, but some strains recently investigated are found to 

 do so. The test has thus only a comparative value. 



Powers of Resistance. In their resistance against heat, 

 cholera spirilla correspond with most spore-free organisms, and 

 are killed in an hour by a temperature of 55 C., and much 

 more rapidly at higher temperatures. They have comparatively 

 high powers of resistance against great cold, and have been 

 found alive after being exposed for several hours to the tempera- 

 ture 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 1 per cent., to water 

 containing the cholera organisms was sufficient to kill them in 

 the course of an hour. 



As regards the powers of resistance in ordinary conditions, 

 the following facts may be stated : In cholera stools kept at the 

 ordinary room temperature, the cholera organisms are rapidly 

 outgrown by putrefactive bacteria, but in exceptional cases they 

 have been found alive even after two or three months. In most 

 experiments, however, attempts to cultivate them even after a 

 much shorter time have failed. The general conclusion may be 

 drawn from the work of various observers, that the spirilla do 

 not multiply freely in ordinary sewage water, although they may 

 remain alive for a considerable period of time. On moist linen, 

 as Koch showed, they can nourish very rapidly. Though we 

 can state generally that the conditions favourable for the growth 

 of the cholera spirillum are a warm temperature, moisture, a 



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