154 



PHYSICAL INFLUENCES. 



The great majority of bacteria and related organisms, in fact of all 

 living organisms, has its optimum temperature between 20 and 40. 

 The optimum temperature of an organism is generally somewhat higher 

 than the average temperature of its natural habitat. This will be in most 

 instances between 10 and 38. A few examples will illustrate this. 

 The bacteria of animal diseases are accustomed to body temperature and 

 grow best at 37 to 40. The organisms found in soil are of various 

 natures. Since good soil in summer under the direct radiation of the sun 

 quite often reaches high temperatures, a great number of soil bacteria will 

 have their optimum nearer 40 than 30. The ordinary lactic bacteria 

 of milk and many of the water bacteria have their optimum near 30. 

 Most of the common molds grow best between 20 and 30. Only very 

 few can grow at body temperature. The optimum temperature of some 

 water bacteria is quite low, the best-known example being perhaps the 

 photogenic bacteria (page 129). 



The following table shows the data obtained for a few microorganisms. 



THE MINIMUM TEMPERATURE or the lowest limit of growth is usually 

 farther from the optimum than the maximum temperature. It will vary 

 with the organisms just as do the other cardinal points. But there is a 

 natural limit drawn by the freezing-point of the nutrient liquid. Not all 

 organisms can grow at such low temperatures, in fact the greater number 

 does not develop below 6 to 10. Those that can grow near the freezing- 

 point will be inhibited by the, solidification of the w^ater in the nutrient 

 medium, for if the water is frozen, food cannot diffuse into the cells and 



