36 MORPHOLOGIC VARIATION 



growth, or the relative rates of growth, depend not on the absolute 

 concentration of nutrient, but on the amount of food available per 

 cell. The occurrence of growth in a medium in which organisms 

 have grown and from which they have been removed is therefore 

 no proof that their cessation of growth in that medium was not 

 due to a partial exhaustion of the foodstuff present. 



Eijkman's filtration experiment indicated that if a toxic substance 

 were active it would not pass a porcelain filter, even if the latter 

 were paper thin. Further experiments showed that the growth in- 

 hibiting substance was destroyed by any temperature continued long 

 enough to kill the organisms, and by volatile antiseptic substances 

 — ether and ammonium sulphide — which could be evaporated from 

 the medium after they had acted. Similar results were obtained 

 with a number of other species of bacteria. 



A number of further interesting experiments were made with the 

 gelatine plate technique. If the "coli-gelatine" (gelatine in which 

 coli had grown, resolidified in a plate) were inoculated with coli and 

 covered with a layer of fresh gelatine, no growth occurred. If a 

 fresh gelatine plate was streaked with coli and covered in part with 

 fresh gelatine, in part with coli-gelatine, the former part grew, 

 the latter did not. This was interpreted as indicating that a diffusible 

 inhibiting substance passed out of the coli-gelatine into the fresh 

 medium. Similar experiments were made, covering the coli-gelatine 

 with paper dipped in agar; inoculation of the latter failed to give a 

 growth unless the coli-gelatine had been heated. This experiment 

 was made to prevent the possibility of organisms migrating from coli- 

 gelatine to the upper layer of medium. 



From these experiments Eijkman concludes that growth inhibi- 

 tion is due to a diffusible, very thermolabile substance formed by 

 the bacteria themselves. Further experiments showed that it pos- 

 sessed a certain degree of specificity, inhibiting some closely related 

 organisms but not other species. He failed to explain its failure 

 to pass through porcelain filters in spite of the fact it would diffuse 

 through agar and gelatine, and also found that it could not be 

 separated from the bacteria by centrifuging. 



It will be noted that in none of the above experiments was the 

 hypothetical inhibiting substance separated from the living bacteria 



