A Sugar Bacterium. 



75 



solution, or with yeast-water. The bacteria lay dormant only at the 

 bottom, as experiments showed. 



It is evident from the foregoing that of all the sugars tried, saccha- 

 rose is the one which favours the growth of the bacterium, but even in 

 saccharon the growth is distinctly favoured by the addition of yeast 

 extract. Some experiments were consequently started to test the 

 effect of the yeast extract. 



Raw Yeast-Water. 



Fresh yeast, squeezed and drained, and then ground up with kiesel- 

 guhr and extracted with water, was allowed to stand all night and 

 "filtered through porcelain. Employed alone it was of no use as a 

 medium for the growth of the bacterium ; nor would the latter develop 

 in this raw yeast-water, to which 5 per cent, of cane-sugar was added. 

 The latter fact was thought to be possibly due to the raw yeast-water 

 liaving inverted the saccharose, and we have seen that glucose and 

 levulose are not suitable media ; and we explained similarly the failure 

 of saccharon-Mayer's solution, to which this raw yeast-water was 

 •added, as well as failures with brown sugar, and with beet extract 

 similarly made up. That failure should follow with dextrin, with 

 maltose, with levulose, milk-sugar, and with beer wort similarly made 

 Tup, was only to be expected from experience with these media con- 

 cocted with boiled yeast-water. 



Further experiments, however, led to the conviction that matters 

 were more complicated than would be implied by this explanation. 



At one stage in the investigation, being impressed by the stimulus 

 to growth afforded by the addition of (boiled) yeast- water to the saccha- 

 rose solutions, we tried the effect of adding sugar to the raw yeast 

 extract and sterilising by filtration through porcelain only, and were 

 surprised to find that no growth whatever occurred at any temperature, 

 e.g., 18°, 23°, and 30° C. This was afterwards explained as above — 

 the yeast extract inverts 'the sugar before it has time to filter. 



We then tried a series of experiments as follows : — Cultures at 

 17 — 20°, 25°, and 31° C, were made in 10 per cent, saccharon + 

 10 per cent, yeast-water mixed raw and sterilised by filtration only; 

 no traces of growth occurred in 10 days. The same failure was 

 realised with 10 per cent, saccharon alone sterilised by filtration only ; 

 with yeast extract only sterilised by filtration only ; and with yeast 

 extract sterilised by filtration and then boiled before adding to the 

 filtered sterile 10 per cent, saccharon solution. In no case did any sign 

 of growth occur. 



It is therefore clear that the filter either holds back some body 

 necessary for the nutrition of the bacterium, or destroys it in its passage 

 through the pores. Also that raw yeast extract in some way spoils the 



