98 



20 



acid, but as a rule, they lose this tendency after continued cultivation in the laboratory, 

 and will thus, in course of time, be found to differ in no essential degree from other lac- 

 tic acid ])actcria. In the system which we shall later set up for the lactic acid bacteria, We 

 have neverllieless separated off the betabacteria and betacocci as a distinct sub-group. 



At the close of Table III, we have shown, for purposes of comparison, the acid pro- 

 duction of a few other bacteria. It is highly surprising to find that the two coli strains 

 — despite abundant air formation — ■ apparently form over 100% acid from the fermented 

 sugar. The explanation, however, is that these bacteria only form a small quantity of lac- 

 tic acid, but ferment the greater part of the sugar to succinic acid and acetic acid, thereby 

 giving three equivalents of acid for every two obtained by lactic acid fermentation. 

 Ce//,2 0« = 2 C^H, OCOOH = C^H,{COOH\ + CH^COOH -j- H., 

 1 grape sugar = 2 lactic acid = 1 succinic acid -f- 1 acetic acid + 1 hydrogen. 



By calculating the acid formed as lactic acid, then, we have thus put the yield of acid 

 1 3 too high. Baclerium piodir/iosLiin forms acids similar to those formed by the coli bacteria, 

 but as it also transforms a quantity of the sugar completly into gas, there is a distinct wastage 

 here. Transformation into gas is, however, most marked in the caseof theaerogenes bacteria, 

 and especially No. 2, which has fermented all the sugar in the milk without coagulating it. 



A point of no slight interest is the question whether a given lactic acid bacterium will 

 under all circumstances form the same amount of by-products, especially volatile acids, 

 and whether it a'ways forms the same modification of lactic acid. 



Here, above all, the quantity of sugar plays a part, for if there is not any more 

 sugar than the occurring bacteria can easily ferment, part of the lactic acid formed 

 will be further transformed. This is plainly seen from Table II where the quantity of 

 acid formed is considerably less than the fermented quantity of sugar in the cases 

 where all the sugar has been fermented, i. e. where the figures are put in parenthesis. 



As regards the formation of acetic acid, Kayser^) has already shown that this increases 

 with the supply of air, and Barthel^) has pointed out that it is as a rule greater where 

 the conditions of life (f. in. the temperature) arc unfavourable. In accordance with 

 this We have found a relatively far greater quantity of acetic acid in cultures without 

 chalk than in those with. True, the quantity of acetic acid formed in milk cultures with 

 and without chalk is about the same, but as a far greater quantity of sugar is fermented 

 when chalk is added, it follows that from the sugar fermented more acetic acid is formed 

 when chalk is omitted than when it is added. Some examples of this are shown in Table 

 IV. We have reckoned one molecule hexose as giving three molecules of acetic acid. 



The quantity of volatile acid formed by the lactic acid bacteria dei)ends not only 

 upon the conditions of life, but varies also in other ways. Barthel found, for instance, 

 that .Sr. fæcium No. 19, in a freshly isolated state, formed from the sugar fermented 

 :}9% volatile acid, whereas we, nine months later, under the same experimental condi- 

 tions, found only 13% volatile acid. The bacterium in question exhibited no sign of 

 weakening, but formed, indeed, more total acid in milk than it did in a freshly isolated state. 



Sugars whose number of carbon atoms is divisible by six do not as a rule alTect the 



') Contribution a l'étude de la fermentation lactique. Pari.s 1>S!»4. 

 -) Ceiitialblatt f. Bakteriologie. II. Abt. 1900. Bd. IV, p. 420. 



