114 



36 



acids which are not precipitated by phosphotungstic acid, it is thus likely that the rod 

 forms in question simply peel off the monoamino acids from the casein 

 molecules. That this really is so, we have shown in the case of a single thermobacterium 

 {Tbm. helveticum), as the nitrogenous components not precipitated with phosphotungstic 

 acid could not be further split up by boiling with hydrochloric acid. As they showed the 

 same formol titration figure before and after boiling, they must consist of free amino 

 acids. It is somewhat different with the nitrogenous decomposition pro- 

 ducts, formed by lactic acid bacteria from Witte peptone; they must 

 evidently contain a quantity of cornplex compounds (polypeptides) not 

 precipitated by phosphotungstic acid, as otherwise, DX could not in the great 

 majority of cases be greater than FA'. 



Disregarding the few species capable of liquefying gelatin, the proteolysis produ- 

 ced by lactic acid bacteria is most active in old cultures with many dead cells. There is 

 no doubt that the hydrolysis of proteins — like that of the sugars — is due 

 to endoenzymes, and does not therefore spread in the nutritive substrate 

 until the cells have become weakened, or even autolysed, It is the pro- 

 teolytic enzymes of the lactic acid bacteria which occasion the ripening of cheese, and this 

 process only reaches distinct development long after the sugar in the cheese has been 

 fermented, when the lactic acid bacteria have ceased their activity, being either dead, 

 or in a latent state. The fact that lactic acid bacteria only decompose pro- 

 teins to amino acids, but do not decompose the latter any further, makes 

 the maturing of cheese equal to the process of digestion, and not to that 

 of putrefaction, which is particularly characterised by a breaking down of 

 amino acids. 



We need not here go into the proteolytic qualities of the different species. They are 

 least developed in the betacocci and betabacteria, but even among the betacocci, we may 

 find a few strains, isolated from milk and dairy produce, able to split up casein to a 

 perceptible degree. On the other hand, casein-splitting cocci may, when they have not 

 for some time been cultivated in milk, lose their power of utilising casein as nitrogenous 

 nourishment, and thus often the power to grow in milk at all, even where they are other- 

 wise able to ferment milk sugar. The bacteria have, as mentioned, their own power 

 of gradual adaptation to a new nitrogenous food, and great caution 

 should therefore be observed in taking their proteolytic qualities as 

 species character. 



Relation to Common Salt. In order to be perfectly sure as to the most favourable 

 conditions of nourishment for a bacterium, it is necessary not only to know how they 

 behave with the various carbon and nitrogen sources, but also with different nutritive 

 salts. We have, however, already referred to the attitude of lactic acid bacteria towards 

 the most important one of these, viz. potassium phosphate, and will here restrict our- 

 selves to a brief mention of sodium chloride, not because this salt is necessary to their 

 development, but because lactic acid bacteria are often found in substrates such as cheese, 

 sour cabbage, not to speak of anchovy pickle, in which common salt is abundant. 



Table XI a and b shows the effect produced by greater or smaller quantities of sodium 

 chloride upon the formation of acid. The casein peptone broth employed, which according 



