VITAL PHENOMENA OF BACTERIA. 81 



If after the sugar is consumed not enough acid has 

 been formed to kill the bacteria, a similar change in 

 reaction now takes place to that in ordinary culture 

 media in the absence of sugar viz., the acid is neu- 

 tralized gradually, and in the end the reaction becomes 

 alkaline. 



Among the acids produced the most important is 

 lactic acid; also traces of formic acid, acetic acid, pro- 

 prionic acid, and butyric acid, and not infrequently 

 some ethyl-alcohol and aldehyde or ac'etone are formed. 

 Occasionally no lactic acid is present, and only the other 

 acids are formed. 



Various bacteria, as yet incompletely 'studied, possess 

 the property of producing butyric acid and butyl-alcohol 

 from carbohydrates. 



Some bacteria also seem to have the power of decom- 

 posing cellulose, found in the stomach and intestinal 

 contents of herbivorous animals and in marshy soils, 

 with the production of marsh-gas. 



Formation of Gas from Carbohydrates and Other Ferment- 

 able Substances of the Fatty Series. The only gas pro- 

 duced in visible quantity in sugar-free culture media is 

 nitrogen. If sugar is vigorously decomposed by bac- 

 teria, as long as pure lactic acid or acetic acid is pro- 

 duced there may be no development of gas, as, for 

 instance, with the B. typhosus on grape-sugar; but 

 frequently there is much gas developed, especially in 

 the absence of air. About one-third of the acid-pro- 

 ducing species also develop gas abundantly, this con- 

 sisting chiefly of CO 2 , which, according to Smith, is 

 always mixed with H. Marsh-gas is seldom formed 

 by bacteria, with the exception of those decomposing 

 cellulose. 



6 



