CHAPTER VI 

 THE RISE AND FALL OF BACTERIAL POPULATIONS 



C.-E. A. WINSLOW 

 Yale School of Medicine 



I. THE LIFE-CURVE OF A BACTERIAL POPULATION 



In a study of the distribution of bacteria in their various natural habitats the bac- 

 teriologist is inevitably impressed with a sense of a relatively stable adjustment be- 

 tween a specific environment and the numbers and kinds of bacteria which will gener- 

 ally be found therein. The botanist knows that on a certain kind of soil in a certain 

 climate such-and-such trees and shrubs will be present, about so many to the acre. So, 

 in our microscopical realm, we find that uncultivated sandy soils may yield 100,000 

 bacteria per gram while garden soils show 1,500,000. A given river in a dry summer 

 will contain from 1,000-2,000 bacteria per cubic centimeter while a lake will contain 

 only 50-150, and the deep waters of the ocean or those of a driven well will show only 

 5-10. From day to day, and from year to year, both the numbers and kinds of mi- 

 crobes in a given habitat will remain extraordinarily stable — provided that the en- 

 vironmental conditions themselves remain approximately constant. 



If, on the other hand, the conditions of the habitat change, or if a section of the 

 bacterial population be transferred to a new environment, the balance is upset. A 

 new and active struggle for existence is initiated, such as has occurred among the 

 higher forms of life when a glacial epoch has changed the climate of a continent. With 

 our short-lived forms of life, capable of completing a whole cycle of evolution in 

 twenty-four hours, we can trace the course of such a struggle in a fashion which 

 should be the envy of the ecologist; and as we do so, we find a remarkable degree of 

 constancy underlying even the phenomena of change which characterize such a period 

 of adaptation. 



The curve which marks the rise and fall of a bacterial population in a new environ- 

 ment is illustrated graphically and schematically in Figure i ; and it may be claimed 

 that this curve is a widely representative one for all conditions, with the limitation 

 that according as the environment is more or less favorable the subdivisions of the 

 curve may be relatively increased or decreased or even suppressed entirely. 



2. THE PHASE OF ADJUSTMENT 



The first phase in the cycle of a bacterial population (if the environment be not 

 too severe) is what may be called the "phase of adjustment." In a medium which is 

 highly favorable this phase will be indicated, as in the solid line AB of Figure i, by 

 a relatively slow increase, the period of lag or dormancy, as described by Rahn (1906), 

 Barber (1908), Lane-Claypon (1909), Coplans (1909), Penfold (1914), Chesney (1916), 

 and Sherman and Albus (1924). 



The first careful studies of the rate of bacterial multiplication and the first de- 



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