146 BACTERIA 



in the part of the world where they are. In temperate 

 climates few species will grow at temperatures above 

 30° C, though some occur in manure heaps, etc., whose 

 optimum' is as much as 70° C. Species which are para- 

 sitic on warm-blooded animals live best at 37° C, and 

 some of them die at temperatures less than 20° C. 



Sterilisation. — If all bacteria and their spores are 

 to be destroyed in a given enclosed space, the air in 

 that space may be raised to a temperature which they 

 cannot survive. This can usually be effected by dry 

 heating to 160° C. for half an hour or to 180° C. for ten 

 minutes. If a liquid is to be similarly " sterilised," 

 it may be boiled in a flask for several minutes, with a 

 cotton wool plug inserted in the neck of the flask. 

 The contents of the flask wUl then be thoroughly cleared 

 of active cells. The plug acts as a filter to prevent the 

 entrance of cells or spores during cooling when air is 

 drawn into the flask through contraction of the cooling 

 air inside. Spores in the liquid may, however, survive 

 the boiling. Heating with steam at high pressure 

 (which raises the temperature of the steam) at 115° C. 

 for ten minutes, or 120" for five minutes, will kill any- 

 thing, even the most resistant spores. 



A flask containing a fermentable or putrescible 

 liquid, i.e. a liquid in which bacteria can grow and 

 bring about changes in the chemical composition, 

 thus plugged and sterilised, can be kept for any length 

 of time without change. If the plug is removed at 

 any time fermentation will at once begin and the charac- 

 teristic bacteria will be found in the liquid. This is 

 a clinching proof, originally due to the great French 

 chemist and biologist Pasteur, that all such changes 

 are due to the. activity of micro-organisms. 



' Most favourable temperature. 



