186 LABORATORY GUIDE IN BACTERIOLOGY 



3. After growth has taken place transfer from these 

 agar slants to the following media: Meat agar slant, 

 dextrose agar, litmus milk, wort gelatin, liquid wort. 



4. After 24 hours' incubation make stains with 

 gentian violet and by Gram's method, examine the 

 unstained cells in water, and make sketches. 



EXERCISE 2. A STUDY OF GAS EVOLUTION BY YEASTS 



1. Prepare 2 per cent solutions of the following 

 carbohydrates in wort: Dextrose, saccharose, lactose, 

 mannit, levulose, maltose. 



2. Fill three fermentation tubes from each of these 

 solutions and sterilize for three consecutive days in the 

 Arnold. 



3. Inoculate each set with the three yeast cultures 

 studied in Exercise i. 



4. Measure the percentage of gas evolved after 

 24 and 48 hours. 



5. Determine approximately the composition of the 

 gas formed (see p. 82), using a 4 per cent NaOH 

 solution. 



6. Tabulate the results, stating the amount and 

 composition of the gas formed from each carbohydrate 

 by the three species of yeasts. 



EXERCISE 3. A STUDY OF FILM FORMATION 



1. Prepare cultures in liquid wort from three species 

 of yeasts. 



2. After growth has appeared inoculate three flasks 

 containing sterile wort with each one of the yeasts. 

 This should be done after pouring the supernatant 

 liquid off, inoculating with a loop from the sediment. 



3. Incubate these flasks as follows: One set at 3 7 C., 



