23 



101 



tritive broth always form dextro- or lævo-Iactic acid, whether the source 

 of energy be alcohols, aldoses, ketoses, pentoses, hexoses or polysac- 

 charides^). Those strains which in milk form |)urely inactive lactic acid 

 — i. e. with like quantities of dextro- and lævo-acid — will as a rule 

 also under other conditions maintain the equilibrium between the two 

 acids, whereas strains which in milk form more of the one than of the 

 other will under less favourable conditions generally only form the acid 

 which they most easily produce. Indeed, we do not even need here to alter the nutri- 

 tive substrate, as even in milk, these bacteria can in the course of years end by being only 

 capable of producing the one acid. We have numerous examples, for instance, of cases 

 where strains of the species Slreplobacleriuin casei, which in a freshly isolated state forms, 

 besides dextro-lactic acid, also smaller or larger quantities of lævo-lactic acid, have after 

 a more or less considerable lapse of time been found capable only of forming dextro-lac- 

 tic acid, and that without any decrease in the total production of acid. 



These investigations, then, distinctly show that the modification of 

 lactic acid is altogether independent of the stereochemical structure of 

 the sugars, and depends entireh^ upon the species of bacteria. \Vc must 

 therefore presume that dextro- and lævo-lactic acid are formed each 

 by its own independent enzyme. Strains which are equally supplied with both (as 

 for instance Thermobaclerium helvelicum) will under all conditions form purely inactive 

 lactic acid, whereas strains which can more easily produce one of the enzymes than the 

 other, may often entirely lose the power of producing the latter, and thus the faculty of 

 forming the corresponding lactic acid. 



Nitrogen Sources, In contrast to the pseudo lactic acid bacteria, the true ones 

 do not thrive with ammonia salts or single amino-acids as source of nitrogen. We have 

 in this latter respect tested all our strains of bacteria with aspartic acid, but none of 

 them showed any signs of growth. The true lactic acid bacteria demand just as compli- 

 cated nitrogenous food as the animals, viz. genuine proteins or the entire complex of 

 amino acids therein contained. Even incomplete proteins, such as gelatine (without addi- 

 tion of other nitrogenous nourishment) generally proves, as in the case of animals, an 

 extremely bad nitrogenous food. As the lactic acid bacteria — save for the few species 

 capable of liquefying gelatin — do not in a living state give off any proteolytic enzymes, 

 the nitrogenous food must be given in a state of solution or in colloid form. Of all genuine 

 proteins, the most suitable seems to be casein in the form in which it is found in milk. 

 Even better, however, in many cases, is paracasein (or rather, perhaps, the peptones 

 which rennet gradually forms from casein): this -will be seen from Table VH, where we 

 have noted the quantities of acid formed by various species of our lactic acid bacteria 

 in the same milk with and without addition of rennet. The rennet employed was rendered 

 germ-free by filtration through a sterilised CHAMBERLAND-filter^). The effect of the rennet 



') Herzog and Hörth have, in their work 'Zur Stereochemie der Milchsäuregärung (Zeitschr. f. 

 physiol. Chemie 1909. Bd. 60. p. 131) arrived at the same result. 



-) As such filtration, which keeps back some nitrogenous matter, weakens the rennet very con- 

 siderably, it is necessary to start with a very concentrated solution. We dissolved 10 Hansen tablets 

 in 200 cm.' of water, and used 2 drops of the filtrate per 10 cm.- milk. The nitrogen content of the 



