INFLUENCE OF TEMPERATURE 215 



• 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,ooogerms. 



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 the 

 maximum temperature is considerably lower. Bact. phosphoreum dies 

 if exposed for a few hours at 30°; others may require still lower tem- 

 peratures. 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 hes with most of these §pecies above 30°. These or- 

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

 They find the temperature suitable for their Ufe 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. 



