86 



THE GROWTH AND DEATH OF BACTERIA 



same time 1 ml. portions of the culture were transferred to 100 ml. of 5 per cent, 

 sodium chloride solution ; the mixture was allowed to stand for 1 hour at 20° C, 

 after which a count was performed on it, the numbers being calculated in 

 terms of the 1 ml. of added broth culture. The results are given in Table 1. 



TABLE 1 



This table shows that the young bacilli, formed after the culture has entered 

 on the logarithmic phase of growth, are very much more susceptible to the action 

 of 5 per cent. NaCl solution than are the old bacilli inoculated at the start — a 

 mortality of 93-54 per cent, as against 14-06 per cent. In addition it will be noticed 

 that during the lag phase, lasting from to 1| hours, though there is no increase 

 in the number of bacteria, there is a progressive increase in their susceptibility to 

 salt solution. Sherman and Albus interpret this as indicating that, during the 

 lag phase, the old bacteria which have been inoculated are undergoing a process 

 of rejuvenescence, which fits them for reproduction. Numerous instances are 

 quoted from other fields of biology, such as the rejuvenescence of Paramwcium, 

 the growth curves of colonies of fruit flies, etc., that may be regarded as exempli- 

 fying the same biological law of growth. Prior to active growth, there is a latent 

 phase of preparation, during which the old cells are being rejuvenated, and fitted 

 for reproduction. 



Sherman and Naylor (1942) made the interesting observation that cells of Bad. coli 

 taken during the logarithmic phase, cooled gradually to 1° C. and stored at this temperature, 

 retained the characteristics of voung bacteria — namely, ability to multijily without lag 

 when transferred to fresh medium at 37° C. and greater susceptibility to inimical agents 

 (in this case cold shock) — for as long as 36 days. Cells of Str. lacfis, on the other hand, 

 rapidly acquired at 1° C. the characters of senescence. What this difference was due to, 

 it is impossible to say ; but the observations of Sherman and Naylor may be taken in 

 conjunction with those of Anderson and Meanwell (1936), who found that a thermoduric 

 milk streptococcus {i.e. one capable of resisting heat at 60° C. for 30 minutes) with which 

 they were working became less rather than more susceptible to heat during the lag phase. 

 Generahzations, therefore, based on the behav^iour of coUform baciUi alone, must be 

 accepted with reserve. 



(4) Diminution in the Electrophoretic Charge. — Working with Bad. coli, Moyer 

 (1936) found that when the organisms were introduced into a fresh medium their 

 rate of migration in a cataphoretic cell decreased during the first two hours, remained 

 low during the third and fourth hours, and then rose towards the end of the 

 logarithmic phase. He concluded that these changes were probably due to an 

 alteration in the electro-chemical state of the bacterial surface during the growth 

 cycle. 



(5) Decreased Susceptibility to Non-specific Agglutination. — Though few observa- 

 tions have so far been made, the results recorded by Macgregor (1910) on agglutina- 



