INFLUENCE OF TEMPERATURE 271 



x 



The number of microorganisms that developed at the freezing- 

 point was found to be: 



In i c.c. of market milk, up to 1,000 germs. 

 In i c.c. of sewage, up to 2,000 germs. 



In i g. of garden soil, up to 14,000 germs. 



THE MAXIMUM TEMPERATURE is usually about 10 to 15 higher 

 than the optimum. The development of microorganisms above the 

 optimum temperature is not quite normal; there is a great tendency 

 toward involution forms. The mycelium of molds grown near the 

 maximum temperature appears unhealthy and pathogenic bacteria 

 lose part of their virulence. This loss of virulence is made use of in 

 the preparation of attenuated cultures for vaccines. 



The maximum temperature varies with different species of bac- 

 teria. Most bacteria do not grow above 45, but with some of them 

 the maximum temperature ,is considerably lower. Bad. phosphor eum 

 dies if exposed for a few hours at 30; others may require still lower 

 temperatures. The average organisms found in water, soil, milk, and 

 the body, which have their optimum near 30 to 38, do not grow higher 

 than about 45. There are very noticeable exceptions to these, such 

 as the physiological group known as thermophilic bacteria. 



These extraordinary organisms have their maximum between 70 

 and 80, a temperature which coagulates albumin. Corresponding to 

 the high maximum the thermophiles have a very high optimum, and 

 the minimum lies with most of these species above 30. These or- 

 ganisms are found in soil, sewage, ensilage and occasionally in milk. 

 They find the temperature suitable for their life only under extra- 

 ordinary circumstances, as in fermenting manure piles, in silos, in 

 self-heating hay and similar organic material that develops a high 

 temperature by fermentation. Some hot springs have a very remark- 

 able flora of thermophilic bacteria. 



The range of temperature within which growth is possible, is very 

 uniformly 35 to 45; the starting points and end-points of this range 

 vary greatly, while the total range is quite constant, except for some 

 bacteria adapted to special conditions, such as some pathogenic bac- 

 teria. The temperature relations of bacteria can be shown graphically 

 by using as ordinate the rate of growth; as abscissa the temperature 



