Automatic Adjustment in Bacterial Cells 318 



on which it is based is the tendency for division to occur when 

 a more or less standard amount of some key substance (or 

 substances) has been formed. 



Mutation 



Another mode of adjustment relevant to populations 

 rather than individual cells, and of significance in major 

 changes to quite new environments rather than to the con- 

 tinuous variations accompanying the growth cycle, is that of 

 mutation and selection. The commonly postulated prelimin- 

 ary to the operation of a selective mechanism is a discontin- 

 uous change in some structural element determining the 

 heredity of the cell. Adjustments in individual cells on the 

 one hand and selective population shifts on the other can 

 of course be superposed. They are not mutually exclusive 

 mechanisms. 



If the change is rapidly and easily reversible it is normally 

 ascribed to an adjustment in the individual cell activity, if 

 irreversible it is commonly ascribed to a mutation followed 

 by selection. But, as has been shown, this is not a significant 

 distinction, since all degrees of reversibility are observed. 



Types of response 



In the first part of what follows, however, very easily 

 reversible changes will first be considered. In section 2 an 

 account will be given of the way in which certain enzyme 

 activities vary during the growth cycle, and the significance 

 of these variations will be considered in relation to the cell 

 function. Section 3 deals with responses to changed conditions 

 of aeration, and section 4 with the adjustments to changing 

 pH. The part played by the division mechanism in regulating 

 the cell economy is considered in section 5. 



The discussion of these various adjustments leads on to the 

 consideration of the way in which the cells respond to changes 

 in their nutrient supply, and to the development of drug 

 resistance. These topics involve a more detailed assessment 



