26 LABORATORY GUIDE IN BACTERIOLOGY 



the cotton plug to stick to the glass, and later not only 

 occasion much annoyance to the person using the tube 

 but expose the medium in it to danger of contamina- 

 tion. 



EXERCISE 3. PREPARATION OF DEXTROSE AGAR 



Dextrose is added to agar for the demonstration of 

 gas-forming organisms. Dextrose is decomposed by 

 these bacteria with gas formation, the gas appearing 

 as bubbles in the medium. 



Dextrose agar is prepared by adding a definite 

 amount of dextrose, usually i per cent, to filtered agar. 



Note. — Dextrose agar cannot be distinguished from plain 

 agar by appearance. It is therefore necessary either to label 

 the tubes or to separate dextrose agar tubes from plain agar 

 tubes in a basket by tying a piece of string or inserting a piece of 

 paper between. 



EXERCISE 4. PREPARATION OF PEPTON GELATIN 



1. Weigh the saucepan and measure 1,000 c.c. of 

 tap water into it. To this 200 to 300 c.c. of water 

 should be added to allow for evaporation. 



2. Dissolve 3 grams extract of meat and 10 grams 

 pepton. 



3. When boiUng dissolve 10 per cent gelatin in 

 cold weather and 12 per cent in hot weather. The 

 gelatin must be of the best quality (gold label) and 

 should be dissolved slowly, taking a few leaves at a 

 time, and with constant stirring. 



4. When completely dissolved, adjust the reaction 

 as directed in the preparation of agar. Gelatin con- 

 tains an appreciable amount of acid and it will require 

 more NaOH solution for neutraUzation than agar. 



