94 



10 



exactlj' answered to the previous ones, by testing them both with bacteria which we 

 knew fermented the substance in ^question and also with bacteria which we knew were 

 unable to do so. Many of these sugars, however, are very expensive, and it was therefore 

 ordered that as little as possible of them should be used. Owing to the strong fermentation 

 of many of our strains, it is, as already mentioned, necessary to use at least 2% of the su- 

 gars. The question is, however, whether even this amount is sufficient in all cases — for 

 when desiring to ascertain with certainty what sugars can be fermented, it is necessary 

 that the test should be made under optimal conditions in every respect; also as regards the 

 concentration of the sugar. Table II shows the acid production of several of our strains 

 with various quantities of grape sugar. As we generally reckon the quantity of acid formed 

 in 7oo> the quantity of sugar employed is here also expressed in °/oo- In order to see at all 

 how the lower sugar concentrations behave, we have as a rule worked with an inferior source 

 of nitrogen (W with 0.3% JV); only where the bacteria did not thrive sufficiently on this 

 we have used the better sources (C and I'with 0.5% N) but by this means, all the sugar 

 is generally fermented in the lower concentrations i), so that the results here will be mis- 

 leading. 



It will be seen from Table lia, that the Streptococci are very little affected by the 

 concentration of sugar, not until 15% (loO^/oo) is reached — • more rarely 10% — does the 

 effect become pronounced. Most frequently, the optimum is found to lie at or 2% 



sugar; only in the case of Slieplococcus cremoris is it first reached at 5 or 10%. Table lib, 

 shows that the betacocci exhibit an even greater sugar concentration than the last-men- 

 tioned species, while the tetracocci (micrococci and sarcinæ) resemble the maj ority of strepto- 

 cocci in this respect. The sugar optimum of the thermobacteria lies at 2, 5 or 10% sugar, 

 and according to Table lie, most of the other rod-shaped lactic acid bacteria are much 

 the same. An exception is formed by the microbacteria, which prefer the lowest concen- 

 trations^). The main result of the investigation, however, is that We can without 

 hesitation employ 2% sugar, whether we wish to study one or another 

 species of lactic acid bacteria, as in no case will the quantity of acid 

 formed therewith differ essentially from the quantity formed at optimal 

 sugar concentration. 



The investigations mentioned apply only to grape sugar. In the case of the polysac- 

 charides, the optimum of sugar concentration lies as a rule somewhat higher, though not 

 so much as to be of any practical importance. The more or less complete exclusion of 

 air may also affect the conditions here in question. Table Ild. shows, by way of example, 

 how Thermobaclerium cercale (Bacillus Delbiiicki) appeared under difterent experimental 

 conditions. The nitrogen content in all tubes was 0.5%, and the quantity of acid formed 

 is as usual expressed in °/oo- 



Even more important than knowledge of the sources of energy themselves is the 

 knowledge of the manner in which they are utilised; it is this factor which determines 



') Where this is the case, the quantity of acid formed is placed in parenthesis in the tables. 

 We have also investigated coli and aerogenes bacteria in their relation lo the quantity of 

 sugar, and found that they throve almost equally well with small or larger quantities. The aerogenes 

 bacteria, however, form hardly any acid with 'l. or 1 "lo sugar, as they are able to convert this slight 

 amount of sugar almost entirely into gas. 



