62 THE RISE AND FALL OF BACTERIAL POPULATIONS 



hibits no such phenomenon suggests that the disinfectants were perhaps carried over 

 to the plates in antiseptic concentration). Allen (1923) and others have ascribed sim- 

 ilar slow growths of bacteria in milk after pasteurization as due to attenuation by 

 "temperature shock." 



Chesney (191 6) believed this to be the main factor in the lag phenomenon in gen- 

 eral, and he cites a striking case in which the generation time of pneumococci was 

 slowed down in a filtrate from an old broth culture of the same organism. That the 

 shock theory cannot be of general application, however, is made clear by the fact that 

 similar phenomena occur in bottled-water samples. 



The second possibility which suggests itself is in a sense the converse of the idea 

 that lag is due to a state of injury produced by an earlier environment. It involves the 

 conception that, among the cells carried over from any environment A to another en- 

 vironment B, some find themselves ill adapted to the latter and that a process of natu- 

 ral selection must ensue until the less adapted cells are weeded out. Such a condition 

 must apparently be assumed when the lag period is characterized by an actual de- 

 crease in number such as occurs in samples of water, milk, or sewage, or in soU to which 

 an antiseptic has been added. 



When a pure culture of a single species is transferred to a medium identical with 

 that in which it is already living there must be something still more fundamental in- 

 volved. We may conclude from the work of Penfold and Barber that an initial period 

 of slow development is an essential necessity whenever bacteria pass to a new en- 

 vironment from one in which they are not multiplying rapidly. Bacteria in active 

 multiplication appear to be in a different biological state from bacteria in other phases 

 of the population cycle, and it takes time to effect this change of state. 



We can, however, perhaps go a little further and visualize certain more concrete 

 conceptions of what this difference in state may mean. The work of Sherman and Al- 

 bus (1922 and 1924) indicates that cells during the early lag period are less sensitive 

 to slightly toxic salts than are cells in the late lag and logarithmic phase, and they find 

 that the sensitiveness to salts appears slightly before multiplication sets in at its most 

 rapid rate. This suggests the possibility that permeability phenomena may play a 

 part. On the whole, however, among the numerous possibilities Penfold's explanation 

 that maximum growth presupposes the existence in the cells of intermediate bodies in 

 the synthesis of protoplasm — intermediate bodies which diffuse out and are lost when 

 growth is checked — seems on the whole most plausible. On such a hypothesis cells in 

 the "resting stage" of an old culture or in a lake water or in any stable environment 

 would be cells lacking these "intermediate bodies" while the "rejuvenated" cells of 

 the logarithmic growth phase would be rich in them. 



It is also possible that such "intermediate bodies" may be transferred in the form 

 of dead as well as of living bacterial cells, or in solution in the surrounding menstruum. 

 This would explain the finding of Chesney that cells transferred from a mother-cul- 

 ture during the phase of increase and washed free from the medium by centrifugation 

 show a lag when placed in a new culture, while the cells left in the mother-culture still 

 continue to grow at a logarithmic rate. Such a supposition would also be in accord 

 with the conclusion of Rahn (1906) that maximal multiplication coincides with the 

 presence of heat-stable, non-filterable substances "formed by the bacteria," and with 



