174 BACTERIA. 



Mention has been made of the conditions as regards 

 temperature that are necessary for the growth of the cholera 

 bacillus when other conditions are slightly adverse. As the 

 result of an extensive series of experiments, first carried on by 

 Koch, and afterwards repeated by other observers, it has been 

 found that the comma bacillus flourishes most luxuriantly, 

 and is most productive, at a temperature ranging between 

 30 and 40 C., although it can grow at a much lower tem- 

 perature, and even at several degrees above the higher point 

 mentioned. At 20 C. it flourishes luxuriantly upon a 

 peptonized gelatine medium. 



As early as 1884 Koch described the growth of the 

 cholera bacillus at a temperature of 17 C., but as might 

 be expected the growth is not nearly so prolific, and it is 

 certainly considerably less rapid than at a slightly higher 

 temperature. A temperature below 17 C. seems to be 

 inimical to the growth of the bacillus, as at 16 C. develop- 

 ment may be said to have almost ceased. It is, however, 

 remarkable that an intense degree of cold does not deprive 

 the organism of its power of growing, for, if after being ex- 

 posed to great cold for some time, it is again placed under 

 favourable conditions, it appears to regain its powers of 

 rapid multiplication. Koch, to test this, submitted a culti- 

 vation of the bacillus for one hour to a temperature of 10 

 C. (10 C. below freezing point), with the result that the 

 nutrient medium was completely frozen. This was now 

 thawed, and a cultivation made at a higher temperature and 

 under favourable conditions ; the bacillus began to grow 

 again almost immediately, and appeared, indeed, to have lost 

 none of its vitality. These experiments accord well with, 

 and verify the observations that have been made on, the 

 appearance and spread of cholera epidemics in the hot and 

 cold seasons. Most of the cholera epidemics seem to have 

 attained their maximum virulence during or at the end 

 of the hot season of the year. In those regions that are 

 periodically visited it usually breaks out in the autumn, when 

 the external temperature is most favourable to the growth of 

 the bacillus as a saprophyte, and when in consequence the 

 micro-organism is enabled to live for a longer period outside 

 the body, to give rise to numerous progeny and thus to 

 multiply the possible sources of infection. On the other 

 hand epidemics are of frequent occurrence, even in the 



