CHAPTER IX • 



EFFECT OF PHYSICAL ENVIRONMENT UPON 

 MICEOOEGANISMS 



Bacteria as well as all other microorganisms are con. 

 stantly being subjected to physical and chemical environ- 

 ment which bring about increases or decreases- in the rates 

 of growth, or increases and decreases in the rates of death, 

 and various alterations in morphology, physiology and cul- 

 tural characters. Before discussing these effects it is neces- 

 sary to note the laws which govern rates of bacterial 

 multiplication and rates of death. 



Rates of Growth and Death 



"With large plants and animals it is comparatively easy 

 to determine rates of growth by weighing at suitable inter- 

 vals. With organisms like bacteria and yeasts, however, it 

 is usually best to count the number of living cells present 

 after varying lengths of time. It has already been noted 

 that bacteria multiply by fission, that is, each mother cell 

 divides into two daughter cells. The length of time which 

 elapses between consecutive cell divisions, that is, the length 

 of time that is required for a single cell to grow to its full 

 size, and divide to form two individuals, is called the gen- 

 eration time. It is apparent that the shorter the genera- 

 tion time the more rapidly are the bacteria multiplying. 

 "We can, therefore, judge of the effect of various physic£il 

 or chemical influences upon the growth of an organism by 

 noting any variations in the length of the generation time 

 which are produced. It is usually not convenient actually 

 to watch the microorganisms under the microscope and to 



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