RATE OF GROWTH OF BACTERIA 41 



If the cessation of growth is due to a partial exhaustion of the 

 medium, repeated subcultures on the same medium, with removal 

 of the organisms after each culture, should behave as cultures in 

 media decreasing in concentration of nutrients, and one should ob- 

 tain a series of growth curves similar to those in Figure 8. While 

 there is much evidence in the literature that such repeated sub- 

 cultures do yield steadily diminishing amounts of bacteria, there is 

 very little quantitative data available. Rahn measured the rate 

 of growth in successive cultures of B. fluorescens in parallel series 

 heated to 100 degrees and 68 degrees, but the various cultures were 

 not equally seeded, and growth was not continued to the maximum 

 in each case, so that the figures are not directly comparable. The 

 68 degree series gave the following mean generation times for the 

 first twenty- four hours of growth in five successive crops: 98, 96, 108, 

 108, and 103 minutes, certainly not enough difference to be signifi- 

 cant. In these cultures, of course, the dead cells remained in the 

 medium, and it is probable that on heating some nutrient was 

 liberated from them and returned to the medium, and that during 

 growth further food was made available by the action of enzymes 

 secreted by the living cells upon the dead ones. The amount of 

 nutrient thus made available, however, must be small. Similar 

 studies with filtered or centrifuged cultures would be more signifi- 

 cant. 



Curran made an experiment similar to the above, using B. coU, 

 the medium being autoclaved and reseeded every four days. The 

 first culture yielded 500,000,000 cells per c.c; the four subsequent 

 crops yielded about 100,000,000 each time. He also measured the 

 rate of growth hour by hour in a culture which had been heated, with 

 a control in fresh broth, and another culture from which the bacteria 

 had been removed by filtration with a similar control. The results 

 of the two experiments were very similar; in neither case was there 

 any difference in the duratian of the lag phase between the used 

 broth and the fresh medium, while in both cases there w^as a decrease 

 in the rate of growth during the late logarithmic phase in the used 

 broth. 



The apparent contradiction in the findings of various authors 

 with regard to the limitation of growth in cultures may be explained 



