GROWTH PHASES 97 



in 20 minutes the cell has to re-synthesise itself entirely — all the cell structure and all 

 the enzymes. In view of the complex of proteins, polysaccharides, nucleic acids and 

 lipins this seems an incredible feat. At the end of the first twenty minutes 1 cell 

 becomes 2, another twenty minutes and the 2 become 4. So that after n periods of 

 20 minutes the original lone bacterial cell will be 2n; in 6 hours one bacterium becomes 

 1,000,000. (If all the bacteria in the world started multiplying at this rate the earth's 

 crust would be covered by layers of bacteria a mile deep in 24 hours.) If the number 

 of bacteria is plotted against time a sharply rising curve soon runs off the graph paper ; 

 but if the logarithm of the number of organisms is plotted against time a straight 

 line is obtained, hence the term " logarithmic phase " of multiplication. 



The logarithmic growth phase is characterised by intense enzymic and synthetic 

 activities and, presumably, this phase could be extended indefinitely if the constituents 

 of the medium could be suitably adjusted. In practice, however, this phase of intense 

 activity is comparatively short-lived, lasting generally for only a few hours and hence 

 covering a period of only 10 to 20 generation periods. 



STATIONARY AND DECLINING PERIODS 



Following the intense activities of the logarithmic growth phase things begin to 

 slow down, multipHcation occurs less frequently, cells begin to die : soon the death 

 rate equals the multiplication rate and the population is stationary, then the death 

 rate overtakes the generation rate and the count of viable organisms decreases. It is 

 simple to measure what is happening in the culture as a whole, but this is a mere 

 statistical figure ; it gives no information as to what is happening to each individual 

 cell. Assuming the logarithmic growth phase of a culture has ceased after 10 hours 

 incubation what is the history of the individual cells still present after 24 hours 

 incubation ? What are the ages of cells in an old culture, i.e., how long is it since the 

 individual cells have undergone sub-division ? Can a cell survive long after it has 

 ceased to sub-divide and are the surviving cells in an old culture still undergoing the 

 process of sub-division, albeit very slowly. It is possible that bacteria cannot survive 

 long after the processes associated with multiplication have ceased. Unlike the 

 tissues of higher organisms it is possible that bacterial proteins are denatured and 

 enzymes decay unless they are constantly renewed and take their place in new cells. 

 It is known, however, that some bacteria which have been rapidly dehydrated, e.g., 

 by freeze-drying, survive for many years in the dry (lyophilised) state when cell 

 multipHcation is impossible, but, on the other hand, protein denaturation is also 

 slowed down to an almost zero value. Recent observations by Fry and others suggest 

 that there is an " optimal dryness " for the preservation of live bacteria. It would 

 seem that if they are not dry enough protein denaturation might occur, whilst if they 

 are too dry the bacteria are killed by the desiccation. It is necessary, of course, to 

 emphasise at this stage the danger of generalising too much about " bacteria " as a 

 class — the individual variations are as great as the differences of diet of lions and 

 giraffes. 



It may be a mistake to class the post-logarithmic growth phase of bacteria with 

 the decline of the Eoman Empire as a mere slow decay of former glories. Many func- 

 tions of interest to us happen to occur in this phase — for example, the occurrence of 

 toxin in cultures of C. diphthericB and the production of penicilhn in stationary cultures 

 of PeniciUia. The chemical composition of the filtrate changes greatly in this period. 



