Effects of Physical Forces on Bacteria 159 



temperature close to that of the maximum temperature for growth. 

 It should be made clear that the growth temperature range does 

 not necessarily coincide with the resistance range of organisms, 

 particularly on the low end of the temperature scale. 



We can classify bacteria into three general groups on the basis 

 of their temperature ranges. Again, the lines between these groups 

 are not sharp, and some overlapping occurs. But for the sake 

 of convenience microbes are designated as psychrophilic, meso- 

 philic, and thermophilic. Scientists do not agree as to the tem- 

 perature limits of each group. The following summary, however, 

 represents a cross-section of accepted limits for growth tem- 

 peratures of bacteria. 



DEGREES CENTIGRADE 

 MINIMUM OPTIMUM MAXIMUM ' 



Psychrophilic 15-20 30 



Mesophilic 15-25 25-40 50 



Thermophilic 25-45 45-55 55-85 



The PSYCHROPHILIC (cold-loving) bacteria are those organisms 

 which play an important part in the spoilage of foods in your 

 refrigerator and in large cold-storage plants. 



Mesophilic (middle-loving) organisms include the bulk— that 

 great middle group— of organisms, including the pathogens for man 

 and lower animals as well as the soil and water forms. 



A group of organisms which grow well at elevated temperatures 

 lethal to most bacteria are termed thermophilic (heat-loving). 

 Proteins usually coagulate at between 55-60° C, but these organ- 

 isms thrive in such localities as hot springs where the temperature 

 may go as high as 85° C, or more. The nature of protein able 

 to withstand such elevated temperatures needs further investigation. 



The term microphilic is a designation sometimes applied to 

 those organisms having a narrow growth range. Some of the 

 highly pathogenic species, including the gonococcus and the 

 meningococcus, fall into this category. 



A great deal of trouble can be caused in the food canning in- 

 dustry and in milk processing plants by thermoduric bacteria— 



