RATE OF GROWTH OF BACTERIA 29 



difference in the duration of the lag phase. Graham-Smith has 

 observed that the frequency with which the parent strain has been 

 subcultured influences the rate of growth, the growth rate being 

 slightly higher and the final yield somewhat greater if the parent 

 culture has been rapidly transplanted for some time. 



McKendrick and Pai concluded from their mathematical analysis 

 of the growth of bacteria that the concentration of nutrients in the 

 medium was a limiting factor for growth, and that the rate of growth 

 was proportional to this concentration, and the same conclusion was 

 arrived at by Carlson for yeasts. Penfold and Norris observed the 

 effect of varying the concentration of peptone on the rate of growth, 

 taking care to eliminate the lag phase from their data. Their re- 

 sults are presented graphically in Figure 7. In reading this graph 

 it should be noted that the ordinates are the generation times, the 

 reciprocal of the actual rate of growth. With this in mind it is 

 interesting to compare their curves with that of Brown for the 

 effect of the size of seeding in yeasts. The two curves are very 

 similar. It would seem, therefore, that the amount of available 

 foodstuff bears a reciprocal relation to the number of cells in its 

 effect on the growth curve. With small concentrations, slight dif- 

 ferences have a profound influence on the rate of growth; with 

 larger concentrations the effect is much less, the critical concentra- 

 tion in this experiment being about 0.4 per cent peptone. Salter, 

 using higher concentrations of peptone, also found that increasing 

 the concentration increased the rate of growth, the effect being most 

 pronounced in the lag phase. Curran, on the other hand, found 

 very little difference in the rate of growth in cultures in 

 1 per cent and 4 per cent peptone, and this difference was mani- 

 fested in the later stages of growth, not at all in the lag phase. 

 Zikes (1919a) also found that while the final yield of yeast was 

 proportional to the concentration of wort, and the rate of growth 

 must be correspondingly proportional, the initial rate of growth was 

 actually somewhat greater with smaller concentrations of foodstuffs. 

 Montank has observed the rate of growth of yeast in media of 

 decreasing concentrations of dextrose and peptone. His results are 

 shown graphically in Figure 8, the data in Table II. There was 

 practically no effect on the duration of the lag phase, but the rate 



