PHYSIOLOGY OF MICROORGANISMS 49 



the temperature at which incipient coagulation of the albuminous 

 protoplasm occurs, resulting in an inability to function. Water 

 is necessary for this coagulation. The following table from Frost 

 and McCampbell illustrates this point: 



Egg albumen + 50 per cent, water coagulates at 56 C. 

 Egg albumen + 25 " 74-80 C. 



Egg albumen + 18 " " 80-90 C. 



Egg albumen + 6 " 145 C. 



Egg albumen + no water " 160-170 C. 



This fact is emphasized by the laboratory methods of sterilization. 

 The autoclave, with live steam at temperature of 120, will destroy 

 in ten minutes the most resistant spores, while in the hot-air 

 oven a temperature of 150 to 170 for an hour is necessary. 



3. Reaction and Composition of Medium. The reaction and 

 composition of the medium has been found to exert a marked influ- 

 ence on the thermal death point. In comparative work, care 

 must be exercised to use media of uniform reaction and com- 

 position. 



4. Time of Exposure. In general, the higher the temperature 

 the shorter the period required to destroy life in the cells. Math- 

 ematic formulas have been developed giving the time as a func- 

 tion of temperature for some forms. 



5. Specific Character of Organism. Intrinsic variations in 

 the character of protoplasm of different species make it necessary 

 to determine the thermal death point for each species. 



LIGHT RELATIONSHIPS OF MICROORGANISMS 



A few bacteria possessing bacteriopurpurin require light 

 for their development. For most other microorganisms, particu- 

 larly the bacteria and the pathogenic protozoa, darkness is the 

 optimum light condition. Light, particularly the direct rays of the 

 sun, will destroy all but the most resistant bacteria if .exposed 

 for a sufficient length of time. Sunlight when passed through 

 a prism is readily broken up into its constituent colors, the least 

 refracted rays, or the reds and yellows, at one end, and the more 

 highly refracted rays, the blues and the violets, at the other. 

 Exposure of bacteria to these various colored rays has shown 



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