CHAPTER IV. 



EFFECT OF TEMPERATURE UPON THE GROWTH OF BACTERIA. 



Ix judging the effect on bacteria of heat as well as other agents we 

 have to note the important fact that different species are differently 

 influenced by the same substance. Some bacteria live under conditions 

 which would destroy others, and they vary among themselves in their 

 powers of resistance to influences which are deleterious to all. 



Further, any species of bacteria will resist better when under favor- 

 able conditions than under unfavorable ones. Bacteria also in recent 

 cultures withstand injury better than those in old cultures, so long as 

 they have not entered into the spore form. According to the amount of 

 injury they have suffered, bacteria may be inhibited in some of their 

 functions or they may be totally destroyed. 



Bacteria Divided According to the Temperatures at which they Grow 

 Best. Some form of bacterial life is possible within the limits of and 

 70 C. There are some species, however, which grow at the lower and 

 others at the upper limit of these temperatures. The maximum and 

 minimum temperature for each individual species lies from 10 to 30 C. 

 apart. Bacteria have been classified according to the temperatures at 

 which they develop, as follows : 



PSYCHKOPHILIC BACTERIA. Minimum at C., optimum at 15 to 

 20 C., maximum at about 30 C. To this class belong many of the 

 water bacteria, such as the phosphorescent bacteria in sea-water. 



MESOPHILIC BACTERIA. Minimum at 5 to 25 C., optimum at 

 37 C., maximum at about 43 C. To this class belong all patho- 

 genic bacteria. 



THERMOPHILIC BACTERIA. Minimum at 25 to 45 C., optimum at 

 50 to 55 C., maximum at 60 to 70 C. This class includes a number 

 of soil bacteria which are almost exclusively spore-bearing bacilli. They 

 are also found widely distributed in feces. 



By carefully elevating or reducing the temperature the limits within 

 which a variety of bacteria will grow can be altered. Thus, the anthrax 

 bacillus was gradually made to accommodate itself 'to a temperature of 

 42 C., and pigeons, which are comparatively immune to anthrax, partly 

 on account of their high body temperature (42 C.), when inoculated with 

 this anthrax succumbed to the infection. Another culture accustomed 

 to a temperature of 12 C. killed frogs kept at 12 C. We have culti- 

 vated a very virulent diphtheria bacillus so that it will grow at 43 C. 

 and produce strong toxin. 



Effect of Low Temperature. Bacterial growth is retarded by tempera- 

 tures lower than their optimum, although the bacteria are not otherwise 



