452 CHOLERA 



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. 



Htetnolytic Test. This method, introduced by Kraus, is per- 

 formed by means of agar plates, a small quantity of sterile 

 defibrinated blood being added to the agar and thoroughly 

 diff used ; if any organism has haemolytic properties, a clear zone 

 or areola forms around each colony by the diffusion of haemo- 

 globin. In no instance has an undoubted cholera organism 

 been found to produce haemolysis, whereas many species of 

 spirilla closely resembling it possess marked hiemolytic action. 

 This test may accordingly be applied along with the others in 

 determining the identity of a supposed cholera organism. 



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 G. 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 

 good supply of oxygen, and a considerable proportion of organic 

 material, we do not know the exact circumstances under which 

 it can nourish for an indefinite period of time as a saprophyte. 

 The fact that the area in which cholera is an endemic disease is 



