POWERS OF RESISTANCE 405 



the red colour. 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 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. 



Hcemolytic Test. This method introduced by Kraus is 

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

 defibrinated blood being added to the agar and thoroughly 

 diffused ; if any organism has hsemolytic 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 haemolytic 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 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 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. When the 

 cholera organisms are grown along with other organisms 

 in fluids at a warm temperature, it is found that at first they 

 may multiply more rapidly than the others, but that after a 

 certain time they are outgrown by some of the organisms present, 



