192 



114 



neutral liquid. Only Sc. licjiiefaciens and some few tetracocc.i give off proteolytic enzymes in a 

 living state. These liquefy gelatin and can act in a slightly acid liquid, but are most 

 powerful, however, with neutral reaction. 



14. The cocci which split up casein decompose it gradually through peptones to 

 amino-acids; the casein-splitting rod forms, however, peel off the mono-amino-acids from 

 the casein molecule without previous formation of peptone. From the peptojies, the lactic 

 acid bacteria appear to form a quantity of polypeptides, which are not precipitated 

 by phosphotungstic acid. 



15. The true lactic acid bacteria are incapable of breaking down amino-acids, and they, 

 therefore, in splitting up proteins, do not form more ammonia than is present in the 

 protein molecule as such. 



16. In accordance with this, the true lactic acid bacteria are, in contrast to the 

 pseudo lactic acid bacteria, unable to thrive with single amino-acids or ammonia salts 

 as source of nitrogen, but demand a nitrogenous nourishment as complicated as do the an- 

 imals, viz. genuine proteins or the entire complex of amino-acids therein contained. 



17. As the lactic acid bacteria are as a rule not provided with ectoenzymes, the 

 proteins must be given in a state of solution or in colloid form. Casein is particularly 

 suitable in the finely divided form in which it occurs in milk. The digestion is, however, 

 rendered easier as a rule by the addition of rennet. Among other genuine proteins, gluten 

 and legumin (dissolved in sodium phosphate) can be used. Gelatin, on the other hand, 

 is a very bad source of nitrogen. Many lactic acid bacteria will comparatively rapidly 

 lose the power of utilising casein, just as on the other hand they can gradually accustom 

 themselves to other sources of nitrogen. 



18. Inactivised blood serum is, even with the addition of potassium phosphate, a 

 poor source of nitrogen even for pathogenic streptococci. Liebig's extract of meat, and 

 casein pepton, on the other hand, are excellent sources of nitrogen for all lactic acid bac- 

 teria. The microbacteria, however, do not thrive so well with the meat extract. Yeast 

 extract had a very specific action, being an extremely bad source of nitrogen for patho- 

 genic streptococci, but by far the best nitrogen source for the thermobacteria and strepto- 

 bacteria. Nevertheless, casein peptone is generally to be preferred, on account of its light 

 colour. Witte peptone is a far poorer source of nitrogen than casein peptone, and as it 

 gives abundant deposits with acid, it is ill-suited to solid substrates intended for culti- 

 vation of strong acid formers. In agar stab cultures, for instance, the stab is rendered al- 

 together invisible. 



19. Many lactic acid bacteria do not grow at all with only Va % \Vf7/e peptone, an- 

 swering to 0.07 .% N. All thrive the better, the greater the quantity of assimilable nitrogen 

 at their disposal. The betacocci and betabacteria, however, are impeded by concentrations 

 of 2 % N. In the case of casein peptone or yeast extract, the effect is not as a rule increased 

 to any essential degree by using more than corresponds to 0.5 % N. For our nutritive 

 substrates therefore, we always use the last mentioned sources of nitrogen at these con- 

 centrations. 



20. That the lactic acid bacteria thrive better with increasing concentration of 

 nitrogen, is essentially due to the fact that the organic nitrogenous nourishment acts as 

 a inilTer, and the fermentation of sugar is therefore increased with increasing quantity of 



