VITAL ACTIVITIES OF BACTERIA 153 



production of alkali. The formation of acid occurs sometimes 

 with and sometimes without the production of gas. Exces- 

 sive acid production may cause the death of the bacteria 

 from the increase in the acidity of the medium. If after the 

 sugar is consumed not enough acid has been formed to kill 

 the bacteria, or inhibit their multiplication, the acid may be 

 neutralized gradually and in the end become alkaline due to 

 the production of alkalies. 



Of the acids produced by the action of bacteria on sugar 

 the most important is lactic acid. There are usually if not 

 always traces of formic acid, acetic acid, proprionic acid and 

 butyric acid. Not infrequently some ethyl- alcohol and alde- 

 hyde or acetone are formed. Occasionally no lactic acid is 

 present, and only the other acids are formed. Various bac- 

 teria, as yet incompletely studied, seem to possess the prop- 

 erty of producing butyric acid and butyl alcohol from carbo- 

 hydrates. A few bacteria seem to have the power of decom- 

 posing cellulose. 



Formation of gas from carbohydrates. In the decompo- 

 sition of sugar by bacteria gas is not formed, so long as pure 

 lactic acid or acetic acid is produced. 



aCti n f 

 bacteria 



Dextrose Lactic acid. 



TT O 4- 9C IT O 



6 H 12 6 +<j bacteria AHA. 



A large number of the acid-producing species produce more 

 or less gas consisting chiefly of CO 2 and H. Traces of other 

 gases are reported. In sugar free media nitrogen is the only 

 gas produced in visible quantities. Marsh gas -is seldom 

 formed by bacteria, with the exception of those decomposing 

 cellulose. Grimbert 1 ascribed the greater production of CO., 

 to a greater formation of alcohol and the more abundant 

 production of H to a greater formation of acid in accordance 

 with the following formulae: 



1 Grimbert. Ann. de 1'Inst. Pasteur, Vol. VII (1893) p. 353. 



