Effects of Physical Forces on Bacteria 155 



TEMPERATURE 



FREEZING 



Higher plants and animals are adversely affected when their 

 temperature is reduced, and warm-blooded animals will perish if 

 their heat-producing devices are unable to overcome falling tem- 

 perature. Because of efficient temperature-control mechanisms, 

 however, extremes in the environmental temperature can be com- 

 pensated for by physiological adjustments and by the wearing of 

 clothes by man. Fur-bearing animals alter the thickness of their 

 coat with temperature cycles. The tolerance of cold-blooded 

 species varies considerably. 



The unique resistance of bacteria to low temperatures with no 

 pronounced change in their survival power when they are once 

 more returned to normal temperatures, may well explain their 

 survival during glacial periods in our earth's history. "Brother and 

 sister" bacteria, as well as those relatives many times removed, 

 vary in their temperature tolerance, however. Some of the more 

 fastidious pathogens, including members of the Neisseria genus, 

 fail to survive low temperatures, particularly in the moist state. 

 Most microorganisms, even including pathogens, can be stored at 

 refrigerator temperature (4° C.) for many months, and this is a 

 common laboratory procedure for maintaining a stock culture col- 

 lection for research and for teaching purposes. The life of a bac- 

 teriologist would be considerably more involved if he were obliged 

 to isolate fresh cultures every time he needed specific organisms. 



The effect of freezing upon bacteria depends upon the speed 

 with which the organisms are frozen. Rapid freezing is much less 

 harmful to microorganisms than is slow freezing. When slow- 

 frozen, ice crystals are larger and their shearing action on the cell 

 substance is materially greater than the cutting, if any, of the small 

 crystals formed during fast freezing. This principle is vital to the 

 frozen food industry. Slow-frozen strawberries, for example, 

 would thaw into shapeless mush because of the breakdown of tis- 

 sue cells caused by large, sharp ice crystals. Freezing does not 

 sterilize food. You can't take a product with a high bacterial 



