PHYSICAL AGENTS AND BACTERIAL LIFE. 245 



Organisms may be gradually acclimatized to temperatures that 

 prove unsuited to them under ordinary conditions. Thus the anthrax 

 bacillus, with an optimum temperature for its development of 37° C, 

 may be made to grow at 12° C, and at 42° C. Such anthrax bacilli 

 proved pathogenic for the frog with a temperature of 12° C, and for 

 the pigeon with a temperature of 42° C. 



Let us, in a very few words, consider the inimical action of tem- 

 perature on bacterial life. An organism placed below its minimum 

 temperature ceases to develop, and if grown above its optimum tem- 

 perature becomes attenuated as regards its virulence, etc., and 

 may eventually die. The boiling point is fatal for non-sporing organ- 

 isms in a few minutes. The exact thermal death-point varies accord- 

 ing to the optimum and maximum temperature for the growth 

 of the organism in question. Thus, for water bacteria with a low 

 optimum temperature, blood heat may be fatal; for pathogenic bacteria 

 developing best at blood heat, a thermophilic temperature may be 

 fatal (60° C); and for thermophilic bacilli any temperature above 

 75° C. These remarks apply to the bacteria during their multiply- 

 ing and vegetating phase of life. In their resting or spore stage 

 the organisms are much more resistant to heat. Thus the anthrax 

 organism in its bacillary phase is killed in one minute at 70° C; in 

 its spore stage it resists this temperature for hours, and is only killed 

 after some minutes by boiling. In the soil there are spores of bacteria 

 which require boiling for sixteen hours to ensure their death. 

 These are important points to be remembered in sterilization or dis- 

 infection experiments, viz., whether an organism does or does not pro- 

 duce these resistant spores. Most non-sporing forms are killed at 60° 

 C. in a few minutes, but in an air-dry condition a longer time is neces- 

 sary. Dry heat requires a longer time to act than moist heat: it re- 

 quires 140° C. for three hours to kill anthrax spores. Dry heat can- 

 not, therefore, be used for ordinary disinfection on account of its 

 destructive action. Moist heat in the form of steam is the most ef- 

 fectual disinfectant, killing anthrax spores at boiling point in a few 

 minutes, whilst a still quicker action is obtained if saturated steam 

 under pressure be used. No spore, however resistant, remains alive 

 after one minute's exposure to steam at 140° C. The varying thermal 

 death-point of organisms and the problems of sterilization cannot be 

 better illustrated than in the case of milk, which is an admirable soil 

 for the growth of a large number of bacteria. The most obvious ex- 

 ample of this is the souring and curdling of milk that occurs after 

 it has been standing for some time. This change is mainly due to the 

 lactic acid bacteria, which ferment the milk sugar with the production 

 of acidity. 



Another class of bacteria may curdle the milk without souring 



