90 



12 



elles thus require precisely the same enzymes to produce any effect, and 

 when we find that many pathogenic streptococci are capable of fermenting starch, and 

 this very strongly, it is doubtless because they have became accustomed, in the host orga- 

 nism, to glycogenei). 



Some experts may perhaps consider it quite superfluous to test the lactic acid bac- 

 teria with four hexoses which they are probably all able to ferment. We did not, however, 

 restrict ourselves to ascertaining merel3' whether any fermentation took place at all, but 

 determined in each particular case the exact quantity °/oo of acid formed, in order to find 

 out what carbon sources were preferred. Thus employed, the four hexoses are often of 

 as great importance for species determination as the carbon sources which are but rarely 

 fermented. It is highly characteristic of several species, for instance, that they prefer 

 lævulose to dextrose, or in the case of others, that they find it very difficult to attack man- 

 nose. As a rule, galactose is that of the four hexoses which is least fem- 

 mented. 



As with the milk, we also allowed the inoculated sugar broth tubes to stand for 14 

 days at the optimal temperatures of the respective bacteria before proceeding to titration. 

 In order to calculate the quantity of acid formed, the original acid grade of the sugar 

 broth tube was subtracted. Each tube was given exactly lOcm.^ sugar broth. Many 

 sugars are highly coloured by sterilisation in the nutritive liquids employed, which renders 

 titration (with phenolphtalein as indicator) extremely difficult. The ones which colour 

 most strongly are xylose and arabinose ; then come galactose, lævulose, rhamnose, dextrose, 

 mannose, maltose, lactose and dextrin, while the other (non-reducible) sugars are scarcely 

 affected. The more strongly the sugars are browned on sterilisation, the higher will be the 

 initial acidity of the nutritive substrate^). Even though this, owing to the buffers present, 

 may not influence the hydrogen ion concentration, the supply of buffers is itself reduced 

 thereby, and thus doubtless also the quantity of acid formed. As sterilisation is indispens- 

 able, it is consequentlyimpossible to get an absolutelyjust comparison between thcdiflerent 

 sugars, even when they are all dissolved in precisely the same nutritive liquid — which 

 of course we always did. It should be added, that the composition of the nutritive liquid 

 greatly influences the transformation of the sugar on sterilisation. As our sugar-substrates 

 contain no surplus of hydroxyl ions, it cannot be these which destroy the sugar; the trans- 

 formation is, on the contrary, the more marked according as the substrates are richer in 

 buffers. The reduction power of sugars is hardly impaired at all by sterilisation in pure 

 water, whereas it is diminished abt. 20% in yeast extract, which is the richest of our 

 substrates in respect of buffers. 



It need hardly be said that we have employed only the purest sugars, from Merck 

 and Kahlbauin, for our investigations, and we always made sure that new consignments 



') It is generally supposed that the primary division of raffinosc (melitriose) into lævulose and 

 melibiose is due to invcrtase. and it was also found that only saccharose-fermenting lacticacid bacteria 

 were able to ferment raffinose, but it is by no means all saccharose-fermenting lactic acid bacteria 

 which do ferment raffinose. 



'-') By way of example, the degree of acidity I cm ' NaOH to 10Ü cm.' nutritive liquid) for xylose, 

 grape sugar and cane sugar, with casein pepton as nitrogen source, was respectively 23, 13 and 10. 

 and with yeast extract as nitrogen source respectively 30, 20 and 15. 



