72 EXPERIMENTAL DAIRY BACTERIOLOGY 



Formation of indol. Many bacteria produce a number of 

 aromatic compounds, the most important of which is indol, 

 one of the series associated with nitrogenous decomposition, 

 as in fecal matter. Indol is produced only in the absence of 

 sugar. Sugar-free broth must thus be used. The incubation 

 of the meat infusion must be sufficiently prolonged to re- 

 move the sugar, but not so extended as to permit of indol 

 formation. The use of peptone solution is preferable for this 

 purpose; to make this solution, 10 grams of peptone are dis- 

 solved in 1000 cc. of water, tubed, and sterilized. 



The broth is inoculated and incubated for four days at 

 37 C. To test for indol add 1 cc. of 0.01 per cent solution 

 of sodium nitrite, mix, add a few drops of concentrated sul- 

 phuric acid, allowing it to flow down the side of the tube, 

 forming a layer at the bottom. If indol is present, a pink or 

 red ring forms at the junction of the two liquids, due to the 

 formation of nitroso-indol. The tubes should be allowed to 

 stand for half an hour before final judgment is recorded. 



Exercise. Inoculate tubes of sugar-free broth from the cultures 

 furnished. Test for indol as described. 



Relation to oxygen. The relation of the bacteria to oxygen 

 may be determined by noting the character of the growth in 

 the fermentation tube. If the tubes are heated in the steril- 

 izer shortly before inoculation, the oxygen dissolved in the 

 medium will be driven off, and in the closed arm anaerobic 

 conditions will prevail. If the developing growth is limited 

 to the open arm of the tube, the organism is an obligate 

 aerobe ; if confined to the closed arm, it is an obligate an- 

 aerobe ; if, however, the growth is present in both open and 

 closed portions of the tube, it possesses the faculty of growing 

 under either condition, and is called a facultative organism. 



