54 Tlie Non-persistence of Bacterlo-toxlns In the Soil 



Reference to the tables already given (I-VI) will show that in the 

 extracts of three toluened soils the relative numbers of B. prodigiosus 

 (i.e. on basis of inoculation with 1000 cells) were found to amount to 

 4-8, 4-0 and '2-8 millions while the actual contents per cubic centimetre 

 of the extracts were 38, 34, and 20 millions respectively. As these soils 

 in the untreated state normally contain per gram from 8-12 million 

 bacteria (i.e. capable of growth on nitrogenous media) it is evident that 

 the organic substances spontaneously liberated on treatment with 

 antiseptics must in part at least determine the bacterial increases which 

 occur. In fact, some relation appears to exist between the nitrogen 

 thus liberated and the relativeh' high numbers of bacteria observed in 

 partially sterilised soils^. 



The Effect of Food Substances on Growth in Soil Extracts. 



As it appeared possible that the unsuitability of the extracts of un- 

 treated soil — and i:)articularly those which had been subjected to heat — 

 might be due largely to the lack of food substances, further determina- 

 tions were made with extracts of untreated soil which, after being heated 



for the usual period, received small quantities of peptone solution suffi- 

 cient to bring the nitrogen content up to about six parts per million of 

 extract. This amount is somewhat higher than that by which the extracts 

 of toluened soils are normally enriched, and was not only sufficient 

 in almost all cases to prevent any decrease in numbers of B. produ/iosus 

 in the early stages of incubation, but also resulted in very considerable 

 bacterial growth. In the case of the Rothamsted manured soil a marked 

 decrease occurred, but this was eventually followed by growth after 



^ It is probable that this nitrogen is derived from the organisms (bacteria, algae, 

 protozoa, etc.) which succumb during treatment. // can be sliown, for example, that 

 ciliated protozoa in the trophic state disrupt after 60-90 secoinli' exposure to the vapour 

 of mild antiseptics. 



