"age of culture" effects 77 



the cells themselves are, ou the average, larger in phases 

 1 and 2 than they are in phases 3 and 4, and it is this difference 

 in cell-size during various growth phases that gives rise to 

 the different shapes of the growth curves. If the inoculum- 

 cells have to undergo any form of adaptation before growth 

 can take place in the new medium, then stationary and lag 

 phases may be shown by the cell-mass curves as well as by 

 the cell-number curves. 



If we wish to investigate the development of an enzyme 

 system with the growth of a culture, the results we shall obtain 

 if we correlate enzyme activity with cell-mass will obviously 

 differ from those we shall obtain if we correlate activity with 

 cell-numbers. Many of the early investigations of this problem 

 were calculated on a basis of enzyme units per cell, and curves 

 were obtained which showed very high enzymatic activities of 

 cells during the early phases of growth. Since the cells are 

 larger during these phases than in the later stages of growth, 

 it follows that they will contain more protoplasm than older 

 cells and may well therefore contain more enzyme. We can 

 only follow the development of the enzyme if we relate it to 

 the amount of cell-substance present without reference to 

 cell-numbers. Of more recent years the estimation of cell- 

 mass has become a relatively simple matter owing to the 

 development of photo-electric and turbidimetric methods, 

 and nowadays the enzymatic activity of bacteria is usually 

 expressed as enzyme units per mgrm. dry weight of organism 

 or per mgrm. nitrogen content. Whichever form of expression 

 is used we find marked variations of enzyme content with the 

 age of the culture. Since the culture is formed by continued 

 binary fission of cells, each division apparently similar to the 

 last, it is not immediately obvious why the enzyme content 

 of the cells should vary with the time elapsing since inoculation, 

 but we must remember that the physico-chemical nature of 

 the environment is changing throughout the growth period as a 

 result of the metabolic activities of the growing cells, and it has 

 already been shown that the enzyme content of a cell is largely 

 dependent upon the environment at the time of its formation. 



