RATE OF GROWTH OF BACTERIA 21 



rate decreases after a time, and some cells may be found alive after 

 relatively long periods. 



This division into phases is more obvious in cultures which grow 

 rather slowly. Most organisms when grown on the surface of agar 

 develop much more rapidly than in liquid media (which have been 

 used almost exclusively in growth studies of bacteria) and the growth 

 curves of such agar cultures, as will be seen later, do not show such 

 a sharp separation into phases, the so-called logarithmic growth 

 phase being practically absent, the maximum rate being maintained 

 for only a brief interval, so that the logarithmic curve is nearly 

 S-shaped; this S-shaped curve is then repeated in reverse during 

 the period of death, but extends over a much longer interval of time. 

 The total growth curve thus has the appearance of a rather skewed 

 frequency distribution curve. One can then distinguish a period of 

 accelerating growth, a period of negative acceleration in growth, a 

 period of accelerating death, and a period of negative acceleration 

 in death. 



Various factors, as temperature; the size, age, and previous history 

 of the inoculum; and the composition and nutrient value of the 

 medium, influence the form of the growth curves of bacteria. It 

 is of course well known that growth is slower on either side of the 

 optimum temperature, the decrease in growth rate being greater 

 per degree with higher than with lower temperatures. It is apparent 

 that the effect of temperature is due both to a shortening of the lag 

 phase (Tenfold, Chick) and to an actual increase of growth rate 

 during the maximal growth period (Lane-Claypon, Barber). Lane- 

 Claypon interprets the effect of temperature on the growth rate 

 as an operation of the Van't Hoff law. Zikes (1919b) has made 

 the interesting observation that the growth rate-temperature curve 

 for yeast is modified by the temperature at which the strain has 

 previously been cultivated. After being continuously subcultivated 

 for some time at a low temperature (8 degrees), it was found that 

 growth was more rapid at all lower temperatures than before, and 

 that there was a tendency to develop two optima; this might be 

 interpreted as indicating that the growth-temperature curve is in 

 reality an expression of the frequency distribution of the cells with 

 regard to their optimum temperature, and that by growth at low 



