00 The Xoii-persisteace of Bacterio-toxins in the Soil 



2300 cells per c.c. respectively and again incubated. The results of the 

 analyses subsequently made are subjoined: 

 At 



Seconduiry Culture Beginning 1 day - days 4 days 7 days 8 days 10 days 



A. Pasteurised Extract 1000 37,500 -____ — 



B. Filtered Extract 1000 3S1 571 2140 7S50 ll,fi70 3330 



The superiority of the pasteurised over the filtered extract might, 

 at the first glance, be assumed to be due to the destruction of some labile 

 toxic substance in the former extract, but further investigation requires 

 a modification of this view. On the incidence of decreasing numbers in 

 portion B after 10 days, this extract (or culture) was again filtered and 

 divided into three fresh portions, one of which remained untreated, a 

 second was pasteurised at 70° for 30 minutes, and the third received the 

 usual addition of peptone. Counts which were made on commencing the 

 experiment, and also after 38 hours, gave the following results: 



In this instance pasteurisation failed to improve the value of the 

 filtered extract, and we must therefore conclude that the difference 

 which was found to exist between A and V> above, was not determined 

 by the destruction of a toxic body by pasteurisation but by the food 

 substances of the preceding generation of cells which were retained in 

 the liquid. Repeated growth and removal of the cells of two generations 

 apparently suffices to reduce the extract of toluened soil to a condition 

 unsuitable for further growth. The fresh extract or primary culture 

 (Table II) showed a relative increase of 136,700 after 24 hours: the 

 secondary culture gave only 381 cells, whilst the tertiary culture showed 

 practical extinction of the organisms after upwards of 40 hours. 



The addition of minute (juautities of peptone to these supposedly 

 toxic extracts sufficed to bring the relative content to upwards of 65 

 million, and thus fully confirmed the results obtained from the study 

 of the extracts of untreated soils. 



Some indication of the presence of inhibiting substances is however 

 afforded by experiments in which portions of the extract used for the 

 tertiary culture were diluted to varj-ing degrees. 



hrs. 



Extract : Water, 2:1. . . . 1000 



: „ 1:1. . . 1000 



: ., 1:2. . . . 1000 



Undiluted Extract (boiled for 2 hours) . 1000 



* After 30 hours. 



