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C. B, VAN NIEL 



Studies. By relating the rate of the process to a number of variables, 

 preferably one at a time, regularities and irregularities are often dis- 

 closed which serve as useful indicators of component steps. It therefore 

 seems appropriate to examine what kinetic studies on the growth of 

 microorganisms have contributed and may contribute to a deeper pene- 

 tration into the problem of growth in general. 



Nearly all that is known about the kinetics of growth of micro- 

 organisms has been learned from studies of so-called growth curves. 

 By inoculating a specified culture medium with a suitable organism and 

 determining, at the start and after various time intervals, the number 

 of individuals present — frequently restricted to a determination of the 

 number of viable cells — the requisite data are collected for constructing 

 a graph which represents the relationship between the microbial popula- 

 tion (or density) and the time elapsed since the initiation of the culture. 

 The most convenient curves are those in which the number of individuals 

 per unit volume is plotted on a logarithmic scale against time. Figure i 

 demonstrates the commonly encountered shape of such growth curves. 



Figure i. A representative growth curve. 



Much attention has in the past been paid to an interpretation of the 

 various phases of these curves. It will be clear that the first phase [i] 

 denotes a period in which a gradual transition takes place from a condi- 

 tion where an increase in numbers does not occur to one where the in- 

 crease is exponential [phase 2]. This second phase is easily accounted 

 for ; it means that during this period all organisms divide regularly, with 

 the daughter cells behaving in an identical fashion. Towards the end of 



