HARMFUL EFFECTS OF CROWDING UPON GROWTH 105 



Rahn (1906), with B. fltiorescens liquefaciens and three other 

 species of bacteria, obtained essentially similar results except that in 

 his experiments treatment with ether killed the bacteria without 

 removing the unstable toxic material. The ether could be evaporated 

 off but no growth would occur on reinoculation. Appropriate con- 

 trols made by treating sterile broth with ether, which was then 

 evaporated, showed growth on inoculation. Heated cultures gave 

 growth when similar cultures treated with ether gave none, show- 

 ing apparently that the food value of the medium was not ex- 

 hausted. 



Chesney (19 16), working with pneumococci, found that if the 

 organisms are removed by centrifuging a rapidly growing culture, 

 those remaining will continue to grow at the same rate; but that if 

 the culture be similarly treated after the period of maximum growth 

 is past, growth is delayed and some of the cells may die off. Filtrates 

 from 24-hour cultures inhibit growth of new inoculations of similar 

 organisms, but lose this property if the filtrates are allowed to stand 

 for a time in the incubator. Chesney concludes that the cells do 

 produce an unstable, toxic, growth-inhibiting material. Some inves- 

 tigators have believed this substance inhibiting growth in pneumo- 

 cocci is fairly specific in its action; but Henrici cites later work show- 

 ing that the limitation of growth in pneumococcus cultures is due to 

 three factors: the accumulation of acid, the production of peroxide, 

 and the exhaustion of the nutrients. The first two come under the 

 general heading of "toxic products of metabolism," which are here 

 shown to' limit growth of this organism. Hajos (1922) also demon- 

 strated growth-inhibition due to the accumulation of products of 

 metabolism among the colon-typhoid type of bacteria. 



Curran (1925), again using B. coli, found a thermolabile growth- 

 arresting material readily adsorbed by bacterial filters. The first 

 50 cc. of filtrate from a 200 cc. solution which had supported bac- 

 terial growth for 3 days was found to support growth on reinocula- 

 tion much better than did the last 50 cc. of the same solution. It 

 would appear that the filter became loaded with the growth-inhibit- 

 ing material and so allowed material that was stopped at the begin- 

 ning to pass at the end of the filtering. Using a different sort of or- 



