H. B. Hutchinson and A. C. Thaysen 55 



48-72 hours. On the basis of these results it is difficult to avoid the 

 conclusion that decreasing bacterial numbers and paucity of the extracts 

 in food materials are intimately- connected: this view receives further 

 support from a study of the behaviour of the test organism in saline 

 peptone solutions (Table VII). 



From these data several interesting facts emerge. Within the first 

 24 hours the numbers of living bacteria in the saline control diminished 

 very rapidly, and after 72 hours only about one-fiftieth of the original 

 number continued to exist. Similar behaviour was evident in the most 

 dilute peptone solution for the first 48 hours, after which a slight re- 

 covery took place. It is evident, therefore, that this is about the critical 

 dilution which will permit of growth. Higher concentrations of peptone 

 have a pronounced effect in shortening the period during which the 

 organisms tend to diminish, until the solution containing 12 parts of 

 peptone nitrogen per million of saline shows not only no decrease, but a 

 twofold increase within the first 8 hours. iVIa.ximum nundjers of bacteria 

 were obtained earlier with the stronger solutions than with the weaker, 

 but the final numbers after 72 hours are in most cases roughly pro- 

 portional to the food supply and the growth curve is therefore practically 

 linear in character. Moreover the results closely resemble those obtained 

 with the various extracts of untreated and toluened soil given above and 

 the following comparison of the growth in the two poorest untreated soil 

 extracts and the saline control shows that the former results might 

 quite well be explained on the basis of food supply. 



Relative Number of Cells after 



In any case the results accord sufficiently well with the prevailing 

 conceptions as to the requisite conditions for cell growth; they differ 

 radically from the high numbers (70,000 and 170,000) which Greig 

 Smith records for some of his saline controls. 



T/ip Growth of B. fluorescens liquefaciens in Soil Extracts. 



As the routine experiments were open to the objection that the test 

 organism B. prodigiosus was of comparatively rare occurrence in the 

 soil, further work was undertaken to ascertain the behaviour of a 

 common soil organism under similar conditions, and for this purpose 

 B. jiuorescens liquefaciens is eminently suitable. 



