106 



28 



the ordinary saprophytic streptococci^). If several nitrogen sources be mixed together, 

 giving a greater certainty of obtaining all the requisite building material for bacterium 

 protein, then the specific qualities of the various sources in themselves will as a rule be 

 eflaced. 



The favourable efïect of phosphoric acid upon the lacticacid fermentation is seen most 

 distinctly by comparing the Wq and ^Y; less markedly in the experiments with casein 

 ])eptone and different quantities of phosphoric acid, as the casein pepton in itself contains 

 some phosphoric acid. As a rule, it does not seem that anything essential is gained by 

 adding more than 2*'/oo K^HPO^, and a single strain (Sbm. planlarum No. 1) even appears 

 to be quickly satiated with phosphoric acid^). We have also tried some outside species 

 of bacteria, with dilTerent sources of nitrogen. Table VIII shows that the propionic acid 

 bacteria exibit a great partiality for yeast extract, whereas the omnivorous coli bacteria 

 of course exhibit a far slighter sensibility in regard to the species of the nitrogen source 

 (as also, by the way in respect of temperature and many other factors) than the true lac- 

 tic acid bacteria. As regarfls the aerogenes bacteria, the acid formation can here no longer 

 be taken as any measure of the vital activity, as these bacteria turn more of the sugar 

 into gas the better they thrive, and under favourable conditions, the inoculated tubes 

 become even less acid than the control tubes. 



Having investigated the ([uestion of which nitrogen sources the diflerent species of 

 bacteria prefer, it is then necessary to determine at what degree of concentration they 

 should preferably be employed. As with sugars, the colour tone and price of the nutritive 

 substrates are increased with increasing concentration, and it is therefore better to keep 

 a little below the optimal limit than to exceed it. It is obvious, of course, that the amount 

 of acid formed cannot be any thoroughly valid measure of the optimal concentration of 

 nitrogen, as all organic nitrogenous nourishment acts as a bufïer, and therefore a greater 

 (juantity of acid may thus possibly be formed — without exceeding the hydrogen ion con- 

 centration detrimental to the various species of bacteria — the more nitrogenous nourish- 

 ment is given. Nevertheless, we have also in the present instance kept to this measure, 

 not only because it gives a numerical expression of the vital activity of the lactic acid 

 bacteria, but also because further study has shown that lactic acid bacteria really grow 

 most rapidly, and are able to ferment most sugars (i. e. become most abundantly sup- 

 plied with enzymes) in those nutritive substrates in which they form most acid. 



For the experiments shown in Table IX, we used a 2% solution of grape sugar, with 

 the necessary salts, and Witte peptone or casein peptone as source of nitrogen. Of Witte 

 j)eptone, We used 1/25 2,5, 10 and 15%, answering to the quantities of nitrogen shown in 

 the table. These are, in the case of the 10 and 15% Witte peptone broth, somewhat lower 

 than they should be, as some ingredients of the Witte peptone are no longer fully soluble 

 at these concentrations. As regards the casein peptone, we used partly the solution (with 



') In the case of these streptococci, yeast extract ai)pcars to be as good a source of nitrogen as 

 casein peptone, judging merelj' from the (juantity of acid formed; this is, however, due to the fact that 

 tiie yeast extract lias fully as great a buffer action as the casein peptone, a feature which we shall 

 refer to more fully later on. 



As the e.\i)erinient w;is repeated several times, llic ligures given are not due to accidental cir- 

 cumstances. 



