Summary. 



1. The present Work is generally concerned with the true lactic acid bacteria, and 

 more particularly with those which are of importance to the dain,- industry-. The strains 

 described number 330 in all, most of these having been under observation for many years 

 in order to ascertain the constancy of their qualities. 



2. The true lactic acid bacteria form a great natural group of immotile, sporeless, 

 Gram-positive cocci and rods, which in fermenting sugar form chiefly lactid acid. In a 

 freshly isolated state however, some few species approach the pseudo lactic acid bacteria, 

 the coli and aerogenes bacteria, by forming fairly considerable quantities of by-products. 

 These latter consist principally of acetic acid and carbonic acid, at times also succinic 

 acid; more rarely, mannite and hydrogen. That we nevertheless reckon such species among 

 the lactic acid bacteria is due to the fact that they resemble them in all other respects, 

 and gradually lose, partially or entirely, the power of forming by-products, thus ending 

 by becoming true lactic acid bacteria. 



3. As the sources of energy are utilised more completely where the quantity of by- 

 products formed increases, the formation of these is a sign of vitality. An exception, 

 however, is the formation of acetic acid, which is proportionately greatest under unfavour- 

 able conditions, as for instance where air and acid abound, and at too high temperatures. 



4. The lactic acid formed can be either dextro-rotarj- or lævo-rotar}-. Where equal 

 quantities of these two acids are formed, we obtain pure inactive lactic acid. Strains of 

 bacteria which in milk form pure dextro-, or pure lævo-lactic acid, will also in any nutritive 

 broth likewise form respectively dextro- and lævo-lactic acid, whether the source of energy 

 be alcohols, aldoses, ketoses, pentoses, hexoses or polysaccharides. Strains which in milk 

 form equal quantities of dextro- and lævo-lactic acid will as a rule also under other con- 

 ditions maintain the equilibrium between the two acids, whereas strains which in milk 

 form more of the one than of the other will under unfavourable conditions generally 

 form only the one which they more easily produce. Even in milk, indeed, such bacteria 

 may end by forming only the one acid, and it may happen without reducing their total pro- 

 duction of acid. 



5. As the modification of the lactic acid is entirely independent of the stereochemical 

 structure of the sugars, and determined solely by the species of bacteria, we must suppose 

 that dextro- and lævo-lactic acid are formed each by their own particular enzyme. 



