LACTIC ACID FERMENTATION 193 



fermentative process apparently depends very largely upon 

 the kind of organisms bringing about the change, the carbo- 

 hydrate fermented and the temperature. Very frequently 

 the acid formed is of the inactive type, that is, it consists of 

 an equal mixture of the two acids, levo and dextro. 



Lactic acid is the principal fermentative product of many 

 kinds of bacteria belonging to the genera Streptococcus and 

 Lactobacillus. Bacteria belonging to other groups, however, 

 may sometimes produce small amounts of lactic acid asso- 

 ciated with larger amounts of succinic, acetic, butyric or 

 propionic acids. 



ORGANISMS PRODUCING LACTIC ACID 



The organisms producing lactic acid primarily belong 

 for the most part to the genera Streptococcus and Lacto- 

 bacillus. Organisms producing smaller amounts of lactic 

 acid and larger amounts of volatile acids, proteolytic fer- 

 ments, etc., in general belong to the genera Bacterium and 

 Staphylococcus. 



The Genus Streptococcus. There is much discussion in 

 the literature relating to the souring of milk as to whether 

 lactic acid production in milk is usually due to the presence 

 of streptococci or of short rods occurring in chains. This 

 has given rise to considerable confusion in names. It seems, 

 however, that the organisms frequently alluded to in liter- 

 ature as Bacillus lactis acidi or Bacterium lactis acidi are 

 for the most part, at least, streptococci and may be included 

 under the species name, Streptococcus lacticus. 



Another source of confusion in discussions of the strepto- 

 cocci as lactic acid producers has been the fact that the first 

 species of Streptococcus studied, S. pyogenes, as well as many 

 other species since described, are found associated with dis- 

 ease in man and animals. This led at one time to recommenda- 

 tion for the condemnation of milk which was found upon 

 microscopic examination to show chains of cocci. It seems to 



