MICROBIAL ASSOCIATIONS 245 



The cause for cessation of growth in a culture is of great theoretical 

 and practical interest. Apparently there are various factors concerned 

 in this. Lack of food, or of one single essential food compound, may be 

 the cause. This is found sometimes in media where it would be least 

 expected. Some strains of Strep. lacHcus are supposedly limited in 

 milk by the lack of available nitrogen; they cannot attack casein readily 

 and albumin; besides these proteins, nitrogen compounds are not plenti- 

 ful. Addition of peptone increased the maximum nuinber of cells from 

 0.7 billion to 2.5 billions per c.c. More commonly, however, growth 

 is checked by the accumulation of metabolic products. Yeasts are 

 checked by the alcohol, and acid-formers by the acid, urea bacteria 

 by the alkali. In many of these cases, the removal, or neutralization, 

 of the inhibiting product will bring about new development. 



The harmful products accumulating are not always of such simple 

 nature. Some very interesting observations have been made during the 

 last ten years. Eijkmann, as the first, found that B. coli reached its 

 maximum growth in gelatin at 37° in a few days, and that this gelatin, 

 after hardening at 20°, would not support growth after streaking with 

 a young culture of the same organism; but after this gelatin had been 

 heated at 60° for half an hour, B. coli grew on it as well as on fresh 

 gelatin. Broth in which B. coli had grown became fit again for growth ~ 

 of the same bacillus after filtration through porcelain. The inhibition 

 of growth is, in this case, due to a compound which resembles a toxin 

 in many respects. The importance of such investigations to general 

 physiology is evident. 



