30 GROWTH OF BACTERIA 



2. Heat. Like other living organisms, bacteria can 

 only grow and carry on their functions between certain 

 temperatures. The minimum or lowest temperature is 

 that below which growth and multiplication ceases, the 

 maximum or highest being the point on the thermometric 

 scale above which growth is checked. Between these 

 extremes is the optimum or best temperature, at which 

 growth and division is most active; it is usually some- 

 what nearer the maximum than the minimum point. 

 The minimum for many common species of bacteria lies 

 between I to 10 C., the optimum 25 to 35 C, and the 

 maximum between 42 to 50 C, Certain kinds fre- 

 quently met with in soil, well water, and in the sea, 

 tolerate low temperatures and grow freely at a few degrees 

 above freezing-point On the other hand, the species 

 Bacillus thennophilus, sometimes present in sewage and in 

 the intestinal tract of animals, can develop and multiply 

 at 70 C., a temperature sufficient to kill all animal cells. 

 Many kinds which grow well at 60 C. are also met with 

 in garden soil, manure heaps, and milk, and on the 

 surface of potatoes, straw, hay, and various cereal grains. 

 Although the vital activities of bacteria are reduced or 

 checked altogether by cold, these organisms are not 

 killed even when subjected to temperatures much below 

 the freezing-point of water. Certain forms have, indeed, 

 been immersed in liquid air for some time without show- 

 ing any loss of vitality. Temperatures of 5 or 10 degrees 

 above the optimum, if continued for a time, act dele- 

 teriously on bacteria in a vegetative state of growth ; 

 their physiological powers are diminished, and at a few 

 degrees above their maximum death takes place in 

 actively growing cells of most species when they are kept 

 moist. The spores of bacteria, however, resist high 



