LACTIC ACID FERMENTATION 197 



The organism grows best under anaerobic conditions, better, 

 therefore, below the surface of the medium than at the top. 



Lactobacillus caucasicus and certain related species have 

 been isolated from cheese and are probably important in the 

 ripening process. Still other forms have been isolated from 

 fermenting silage, sauerkraut, and other fermented foods. 

 Many of them will grow at moderate temperatures or room 

 temperatures and' a few of them apparently possess the 

 power of producing gas, particularly carbon dioxide. 



Bacteria belonging to the genera Bacterium, and Staphy- 

 lococcus are also capable of producing some acid in milk. 

 Members of the former genus are important in development 

 of lactic acid in certain foodstuffs. 



The Genus Bacterium. The organisms belonging to this 

 group are all Gram-negative nonspore-forming rods, which 

 develop readily upon the ordinary culture media, and are 

 usually either aerobic or facultative. The most important 

 forms are Bacterium aerogenes from milk and several 

 species associated with the fermentation of sauerkraut and 

 of silage. The organisms of .this group, when present in 

 milk, are usually termed the abnormal and undesirable 

 lactic acid bacteria. Some of them, however, may be desir- 

 able in the fermentation of other food products. Most of 

 the bacteria belonging to this group, particularly those of 

 importance in milk and the dairy industry, have the power 

 of producing gas from sugars. They develop also certain of 

 the volatile acids in addition to the formation of lactic acid. 



Bacterium aerogenes grows well on ordinary culture 

 media. The colonies on agar or gelatin are much larger and 

 more inclined to be slimy than those of the organisms 

 belonging to the genera Streptococcus or Lactobacillus. 

 For the most part they grow best at blood heat. Most of 

 them also develop well at lower temperatures. When suit- 

 able carbohydrates which they can ferment are present, the 

 organisms will develop anaerobically. In the absence of 



