158 Microbes and You 



HEATING 

 The destructive action of heat was discussed under media 

 making in an earHer chapter. Suffice it to say here that dry heat is 

 less destructive than moist heat at the same temperature, since 

 coagulation of protein is readily induced when moisture is present 

 to aid in the process. Hence, sterilization in the autoclave 

 ordinarily requires only 121° C. for 15 minutes, while dry oven 

 sterilization is accomplished only after subjecting the material to 

 about 170° C. for a minimum of one hour. 



GROWTH RANGE OF BACTERIA 

 Every organism has three cardinal points with respect to temper- 

 ature—a maximum, an optimum, and a minimum. The maximum 

 growth temperature may be considered as the highest temperature 

 at which an organism may live and carry on any of its life processes. 

 The maximum for growth may be different from the maximum for 

 fermentation, but in general we think in terms of growth. Too 

 high an environmental temperature results in "malaise" of the 

 bacteria, if we may use that expression for microbes. Prolonged 

 exposure of an organism to a temperature exceeding its maximum 

 growth temperature will eventually cause the death of the organism. 

 Naturally, the higher the temperature, the more rapidly will death 

 occur. For most rapid growth, other factors being equal, an 

 organism should be kept at the optimum temperature. This is the 

 point where organisms do their best work. We humans also have 

 our likes and dislikes with respect to temperature. In general the 

 temperate zone encourages more efficient use of our bodies, both 

 mental and physical, than we find true in either the tropics or the 

 arctic zones. 



The minimum growth temperature is the lowest point on the 

 thermometer at which growth occurs to a measurable degree, and 

 this is above the freezing point. There are indications that minimal 

 growth and metabolism are possible below freezing, which makes 

 an exact demarcation difficult for some organisms. Some pathogenic 

 bacteria have an extremely narrow growth range, with the minimum 



