Bacteria and Their Relations to Life 387 



follows. However, some bacteria, especially those found in 

 soils, may be dried for days, months, and even years and remain 

 alive. Most disease-producing bacteria cannot withstand desic- 

 cation, so that there is little danger of their being spread by dust. 



Temperature. Bacteria have the temperature of the medium 

 in which they live. Low temperatures retard the life processes, 

 and high temperatures accelerate them. Likewise at low tem- 

 peratures less amounts of food are consumed ; hence they may 

 live longer on a limited supply. 



Between the highest and lowest temperatures at which an 

 organism can live is a point at which it develops most rapidly, 

 called the optimum, or best, temperature. Most bacteria grow 

 best in temperatures between 70° and 100° F. 



Very few bacteria grow well above 115° F. There are some, 

 however, that live in rapidly decaying organic matter (e.g., in 

 silos and self-heating hay) and in hot springs at temperatures as 

 high as 175° F. — a most remarkable fact, when we consider that 

 proteins which make up so much of the protoplasm commonly 

 begin to coagulate at 145° F. 



At the freezing point most bacteria grow very slowly. When 

 freezing occurs and the medium becomes solid, diffusion of nu- 

 trients no longer takes place and all activities are checked. The ■ 

 bacteria may remain alive, however, for weeks and months in 

 this condition. 



Light. Bacteria living in nature in the soil, in decaying matter, 

 in foods, and inside plants and animals are only temporarily 

 exposed to the light. Most of them cannot withstand exposure 

 to full sunlight for even a few hours. This action of sunlight is 

 of great importance in the purification of rivers and in the destruc- 

 tion of bacteria on streets and sidewalks. Death is brought about 

 either by chemical processes initiated by light within the cells 

 or in the medium. 



Oxygen. Bacteria are very sensitive to oxygen. Although it 

 makes up 20 per cent of the atmosphere, bacteria are exposed only 



