RATE OF GROWTH OF BACTERIA 35 



free in the broth. Finally Rogers and Whittier have found in the 

 case of Streptococcus lactis that if the effect of crowding is eliminated 

 by repeatedly filtering off the organisms during growth and return- 

 ing the filtrate to the parent culture, the period of multiplication 

 and fermentation is not prolonged. 



The idea that growth is limited by the accumulation of some 

 toxic substance is the one that seems to be most generally accepted, 

 though the evidence for it is far from being convincing. It is diffi- 

 cult to say who first suggested this theory, but Eijkman was one 

 of the first to clearly formulate it. He grew the colon organism 

 in gelatine for fourteen days at 37 degrees, divided the gelatine into 

 two parts, one of which was heated to boiling, the other left un- 

 heated, and allowed both to solidify in culture dishes, after which 

 they were inoculated on the surface with the same strain. No growth 

 occurred in the unheated culture; a good growth occurred on the 

 heated gelatine. Three possible explanations are offered: (1) That 

 the medium was exhausted of nutrients which were set free again 

 in the medium by heating; (2) that during growth antagonistic 

 products of metabolism accumulated which were destroyed by heat; 

 (3) that a diffusible thermolabile inhibitory substance was formed 

 during growth. 



The first possibility was excluded because after removing the 

 bacteria from the unheated gelatine culture by filtration, it. again 

 yielded a good growth; therefore the nutrients were not exhausted. 

 Herein lies a fallacy which has been repeated by almost all of the 

 workers on this problem. It is assumed that growth should continue 

 as long as any nutrient remains, taking no account of the amount 

 necessary to merely sustain existence. The amount of food utilized 

 by a cell during any given period of time will depend not only upon 

 the amount of food present, but also on the number of cells present, 

 and a culture which contains only sufficient food to maintain exist- 

 ence without growth for a population of say a million cells: per c.c. 

 will have enough to permit a very active growth of a population 

 starting with say ten cells per c.c; and similarly a medium might 

 yet contain enough nutrient to support an active growth of a small 

 number of cells, when with a very large number they would be 

 actually starving and dying. That is, the occurrence or absence of 



