44 Tlie Non-persistence of Bacterio-toxins in the Soil 



degradation products, bases, alcohols and organic acids — but it cannot 

 be assumed without definite experimental evidence that they act as 

 toxins to any considerable section of the bacterial population. 



During the past seven years several experimenters claim to have 

 obtained evidence of the existence of bacterio-toxins in the soil. Although 

 the possibility of their action in partial sterilisation work was considered 

 by Russell and one of us(i) the first claim to have obtained positive 

 evidence was advanced by Greig Smith (2) who studied the behaviour of 

 a pure culture of Bacillus prodigiosus when inoculated into an extract 

 prepared by treating soil with saline solutions. For this purpose the 

 soil was digested with a dilute solution of sodium chloride and the 

 resulting extract after being rendered germ-free by passage through a 

 sterile porcelain filter was divided into two portions, one of which was 

 allowed to remain untreated while the other was heated to 94° or 

 100° C. for a definite period. The heated extract was then cooled and 

 the two portions were inoculated with a pure culture of the test organism; 

 enumerations of the organisms were made immediately and also after 

 an incubation period — generally 24 or 30 hours. 



The results obtained from such experiments showed that when a 

 culture of B. prodigiosus was brought into an untreated saline extract 

 . of soil, there ensued a marked reduction in the number of cells remaining 

 ahve after 24 hours, while a similar culture was capable of appreciable 

 growth when carried into a like extract but which had been previously 

 subjected to heat. Treatment of a soil with chloroform prior to extraction 

 was found to increase the nutritive value of the extract considerably. 

 The extent to which treatment by heat served to improve the suita- 

 bility of the extract for the life of the organism apparently varied very 

 greatly: in some cases it sufficed to bring the heated extract up to the 

 level of the saline control solution whilst in others the improvement 

 was comparatively small. 



The two main objections to which the method is open are, firstly, 

 that the test organism is not a common inhabitant of the soil and the 

 results obtained by its use do not necessarily apply to the typical soil 

 flora ; secondly, the assumption is made that all substances in the extract 

 which exert an unfavourable influence on the test organism must 

 necessarily be bacterio-toxins. An equilibrium is said to exist under 

 normal conditions between toxins and nutrients in the soil. Volatile 

 antiseptics are regarded by Greig Smith as effecting a reduction of the 

 water-proofing action of the soil wax, or "agricere" which is assumed 

 to surround the soil particles, and the observed high value, for bacterial 



