5G The N^on-persisteuee of Bacterio-toxlns hi the Soil 



The general method of working was similar to that followed with 

 B. prodigiosus, but the experiments were confined to two soils only — 

 the Woburn and Rothamsted unmanured — the extracts of which had 

 given medium and low results respectively with B. prodigiosus. The 

 experimental results are given in Tables VIII and IX. 



Table VIII. Woburn Soil (unmanured): B. fluore.scens liquefaciens. 



Relative Number of Cells in the Extracts after 



Table IX. Rolhatnsled Soil (unmanured): B. fluorescens liquefaciens. 



Relative Number of Cells in the Extracts after 



These results are significant in that they show the vital importance 

 attaching to the choice of organisms for the test; with an extraneous 

 organism such as B. prodigiosus effects are induced which might without 

 further inquiry be interpreted as being due to the action of toxins: 

 with a soil organism such as B. fluorescens liquefaciens no such interpre- 

 tation is possible. Whilst the relative numbers of the former organism 

 in the Woburn soil extract decreased within 72 hours to 574, those of 

 B. fluorescens liquefaciens steadily increased to over 1-3 millions. Still 

 more striking results were obtained with the Rothamsted soil and having 

 regard to these facts it is difficult to advance any satisfactory explanation 

 on the basis of the occurrence of toxins. The decrease in the value of 

 the extracts on heating, although considerable, is not so pronounced as 

 m the case of B. prodigiosus; this conforms with the known difference 

 in proteolytic power of the two species. 



The experimental evidence obtained from the study of the above 

 six soils may therefore be summarised as follows: 



(a) Extracts of untreated .soils and especially of poor soils are 

 relatively unsuitable for the growth of B. prodigiosus. 



(b) The low value of such extracts is .still further decreased when 

 the extracts are subjected to heat. 



