CHAPTER VII 



A NOTE ON SPORE FORMATION 



It is frequently stated that bacteria form spores when condi- 

 tions become unfavorable for growth. This is, however, only true 

 in a narrow sense. Certain factors which tend to restrict the growth 

 rate also tend to delay rather than to accelerate spore formation. 

 Thus Migula has shown that spore formation is less rapid and 

 complete on either side of the optimum temperature for growth. 

 It is well known, however, that bacteria do not form spores until 

 the culture has practically stopped growing; that spores are struc- 

 tures peculiar to the resting cells rather than to the actively growing 

 cells. There is, however, as far as I can learn, no very definite 

 evidence in the literature concerning the actual relationships of 

 spore formation to phases of the growth curve. 



This, however, is apparent in the data obtained on the growth 

 of Bacillus megatherium, presented in Chapter IV. During the 

 course of the investigation of the sizes of the cells in the first 

 culture reported in that chapter, the number of free spores was 

 incidentally determined, and is recorded in Table VI. The relation- 

 ship of spore formation to the rate of growth in the culture is indi- 

 cated in Figure 31, where the logarithms of the number of vegetative 

 cells and the number of free spores are plotted on the same scale 

 (sporangia being counted as vegetative cells, with only free spores 

 being considered as spores). It wilt be noted from this graph that 

 spore formation commenced practically at the point of inflection 

 between the logarithmic growth phase and the resting phase; that 

 after spore formation commenced it proceeded at a practically con- 

 stant rate for some time, the logarithms of the number of spores 

 falling on a straight line; but that spore formation also began to 

 slow up after some hours, the rate continually decreasing during 

 the period of observation. Not all of the cells, therefore, developed 

 into spores, at least within the period of time that the culture was 

 studied, 



111 



