PRODUCTS OF BACTERIAL ACTIVITY. 33 



is filled with sterilized glucose-bouillon and inoculated. Any 

 gases which may form will accumulate at the top of the arm 

 of the tube. The usual precautions are taken to prevent 

 escape of the gas, so that an accurate reading may be made. 

 To determine the presence of CO 2 , add sodium hydroxide 

 and shake ; the two will combine and the difference between 

 the original amount of gas and the amount now present is 

 the amount of CO 2 . Hydrogen gas is detected as follows : 

 Allow the gas in the long arm to diffuse into the bulb, mix 

 with air, remove the plug, and apply a match. An explosion 

 indicates hydrogen or marsh gas. In peptone-bouillon con- 

 siderable H 2 S is produced, which is detected easily by the 

 odor. The presence of gases may also be determined by 

 adding a little glucose or dextrose to nutrient agar. If gas 

 is formed, the agar will be split into chunks. 



Odors : In addition to the odors caused by the gases men- 

 tioned above, a number of very characteristic odors are per- 

 ceptible in cultures. Many of them are unpleasant. 



Aromatics : The most important aromatic produced is indol, 

 as it is one of the means of differentiating the typhoid from 

 the colon bacillus. Other aromatics are skatol, phenol, and 

 tyrosin. 



Liquefaction of gelatin : Many pathogenic and non-patho- 

 genic bacteria liquefy gelatin by the production of digestant 

 substances, such as trypsin. Anything that interferes with 

 the development of the germ prevents liquefaction. For in- 

 stance, the addition of carbolic acid or glycerin in excess. 

 When the bacteria are removed from the culture by filtration 

 the filtrate continues to liquefy. The liquefaction of gelatin 

 is a very valuable aid in the differentiation of bacteria, as 

 many of them liquefy gelatin in a constant and characteristic 

 manner. 



Fermentation : Bacteria produce certain metabolic products 

 called ferments, and the process by which these ferments are 

 formed is known as fermentation. Many of our food-products 

 are the result of fermentation caused by bacteria. The manu- 

 facture of wine, cheese, butter, and indigo is largely depen- 

 dent on such fermentation engendered by the bacteria which 

 these substances harbor. Some bacteria evolve proteolytic 



3 Bact, 



