84 THE GROWTH AND DEATH OF BACTERIA 



inoculum is used, when the inoculum is taken from an old culture, when the medium 

 is unsuitable, and when the culture is incubated at a temperature unfavourable to 

 maximal growth, and (6) to be short when the opposite conditions obtain. The 

 most striking factor of all, however, is the stage of growth of the parent culture 

 from which the inoculum is taken. Organisms from a culture in the lag, the 

 stationary, or the decline phase of growth fail to multiply for some time when intro- 

 duced into a fresh medium, whereas organisms from a culture in the logarithmic 

 phase show no lag, but continue to multiply at the same rate as in the parent culture 

 (Miiller 1895). During the logarithmic phase the organisms are known to be 

 multiplying at maximal rapidity, and since this phase lasts for not more than a 

 few hours, it follows that the organisms must all be comparatively young. By 

 the term " young " we mean an organism that has been generated within a com- 

 paratively recent time, from a few minutes to about an hour. The suggestion, 

 therefore, is that young organisms inoculated into a fresh medium multiply without 

 lag, and that old organisms exhibit a lag phase before beginning to grow. 



Numerous explanations were put forward to explain the phenomena of lag. 

 It was suggested by Rahn (1906) that before multipUcation could occur some 

 essential substance or " bios " had to be excreted by the organisms into the medium. 

 Penfold (1914) modified this view by supposing that certain bodies necessary for 

 the synthesis of protoplasm had to accumulate within the bacteria themselves 

 before multiplication became possible. Chesney (1916) regarded the phenomenon 

 of lag as an expression of injury which the bacterial cell had sustained in its previous 

 environment. Ledingham and Penfold (1914) postulated an inherent difference in 

 the multiplication rate of the individual cells in the inoculum, and suggested that 

 during the lag phase a selection of the more rapidly multiplying cells was occurring, 

 the result of which did not become manifest till the beginning of the logarithmic 

 stage. Experimental evidence does not support this view (Kelly and Rahn 1932, 

 Topley and Wilson 1936). 



These explanations need not be discussed further, because more recent work 

 has brought to light one supremely important fact of which the earher observers 

 were ignorant, and which goes a long way towards explaining the difference in 

 behaviour of young and old organisms. Briefly stated, the lag phase is not a phase 

 of rest, as had been previously supposed, but a phase of intense growth activity 

 during which cell enlargement but Httle or no cell division occurs. The evidence 

 for this statement must now be considered in some detail. 

 Evidence of Growth without Multiplication during the Lag Phase. 



(1) Increase in Size. — If a small number of organisms from a 24-hours' culture 

 of Bad. coli, for example, are seeded into an agar medium, which is then spread 

 between a slide and a cover glass and observed on a warm stage by dark-ground 

 illumination, the first change observed is not division, but a gradual and pro- 

 gressive enlargement affecting a certain proportion of the organisms. This en- 

 largement becomes visible shortly after the preparation has been put up and 

 continues till the organisms reach a certain size, when they finally divide. The 

 rate at which the enlargement occurs varies with the different organisms in the 

 preparation. The time elapsing between the incubation of such a preparation 

 and the first division varies from about 1 J to 3 hours ; but after the primary 

 division has occurred, subsequent generations may be produced at the rate 

 of about one in every 30 to 60 minutes. Examination of such a preparation 

 at the end of about five hours will reveal a small proportion of cells that 



