A Sugar Bacterium. 



81 



One or two features of the fermentations call for comment. The 

 progress of the fermentation in an acid liquid was so great as to sug- 

 gest that an enzyme was excreted by the organism, but careful 

 search proved that this was not the case. Both the conjoint organ- 

 ism and its two constituents are normally aerobic, but in all cases the 

 fermentations will proceed, though with slightly less vigour, in an 

 atmosphere of C0 2 . The decomposition effected by the bacterium is 

 accompanied by only a very slow evolution of this gas, not greater 

 indeed than would be due to its respiration. In one experiment which 

 was carried on for two months, there was an output of 0-005 gram of 

 C0 2 per day, the gas being absorbed by caustic potash as it was given 

 off, and the fermentation being consequently aerobic. 



The intimate association of the two organisms in the lumps of jelly 

 suggested a symbiotic relationship. Experiments made to ascertain 

 how the two affected each other failed however to bear out this view. 

 The influence of the bacterium we have seen to be largely shown in 

 the formation of relatively considerable quantities of both acetic and 

 succinic acids. Cultivations of the yeast were consequently made in 

 the presence of different proportions of acid, with the expectation of 

 finding that an acid medium would be advantageous to it. 



Three flasks were prepared, each containing 90 c.c. of 10 per cent, 

 solution of cane-sugar to which 10 c.c. of yeast- water were added. 

 These were then carefully sterilised. To flask A, acetic acid was added 

 till O'l per cent, was present ; B contained 0*25 per cent., and C 0*5 per 

 cent, of the same acid. Each was then infected with 1 c.c. of a pure 

 culture of the yeast, and they were allowed to ferment at the labora- 

 tory temperature (about 10° C). A control was prepared in a fourth 

 flask, no acid being added. The fermentation which resulted in A 

 was about equal to that in the control ; that in B was less vigorous, 

 and that in C was very feeble. After two days, while the fermentation 

 was still active, they were all filtered on to tared filters, and the 

 quantity of yeast weighed. The control was then found to contain 

 0'09 gram of yeast, while the other flasks contained respectively — A, 

 0-106, B, 0-079, and C, 0-027 gram. So far as the growth of the yeast 

 was concerned, 0-1 per cent, of acetic acid was favourable ; but 0*25 

 per cent, was slightly inhibitory, while 0*5 per cent, was markedly so. 



The alcohol was then distilled off' and estimated. A contained 2 per 

 cent., B 1*33 per cent., and C 1 per cent., while the control contained 

 1*66 per cent, of the spirit. These figures agree with the conclusion 

 based upon the weight of the yeast. The same results were obtained 

 when lactic acid was substituted for acetic. 



As in the original experiments made with cane-sugar, the bacterium 

 produced more acetic acid than the highest proportion used in these 

 fermentations, it is clear that the bacterium does not conduce in this 

 way to any increase of growth or activity of the yeast. 



