86 ACTIVITIES OF BACTERIA. 



bouillon, 0. 1 % of sugar sufficed to kill the cholera vibrio 

 in a few days, 0.2% the Bact. typhi, and 0.3% most bac- 

 teria. In solutions rich in peptone the sugar produced 

 less harm. 



Since by many varieties the acid-production with the 

 reduction of sugar is very rapid and intense, one designates 

 this metabolism, brought about at the expense of the 

 carbohydrates, as fermentation (compare p. 64). Be- 

 cause not rarely gas is produced in abundance, this desig- 

 nation also seems proper to the laity. 



If, after the sugar is exhausted, the quantity of acids 

 produced is not such as to kill the bacteria, then in the 

 nutrient medium, now free of sugar, other metabolic pro- 

 cesses occur and the acids are neutralized and the reaction 

 becomes even alkaline. 



Among the acids produced (besides the carbonic acid, 

 to be spoken of under "gas-formation") the most impor- 

 tant, so far as we know, is lactic acid ; almost always 

 there are at least traces of formic acid, acetic acid, pro- 

 pionic acid, butyric acid, and also not rarely some ethyl 

 alcohol, aldehyd, or acetone. More rarely lactic acid is 

 absent and only the other acids are produced. 



For obtaining and separating the acids one proceeds 

 somewhat as follows: One-half liter of peptone bouillon 

 containing from 2% to 5% of grape- or milk-sugar is placed 

 in liter flasks, and about 10 gm. of carbonate of calcium 

 added to each. The acids produced unite with the calcium 

 carbonate as soluble salts, and carbonic acid escapes. The 

 reaction of the fluid remains neutral, and that is the prin- 

 cipal thing ; a strong acid reaction would prematurely 

 hinder further growth of the bacteria. 



When growth ceases (after eight to fourteen days), the 

 insoluble carbonate is filtered off, and from the fluid, with 

 neutral reaction, the alcohol, aldehyd, and acetone, etc., 

 present are removed by distillation, thus reducing very 

 much the amount of fluid. The three mentioned sub- 

 stances are tested for together by Lieben's iodoform re- 

 action. To the slightly warm fluid in a test-tube are added 

 five to six drops of a pure 10% aqueous solution of caustic 

 potash ; then drop by drop a weak solution of iodid of 

 potassium is added until a brown color appears, and the 



