27 
Exercise 14. The Influence of the Composition of the Sub¬ 
stance on the Types of Fermentation that will Occur in It. 
MATERIALS: 
4 100 cc. portions of sterile broth 
8 sterile test tubes 
1 300 cc. flask of sterile sand 
4 sterile Petri dishes 
N/10 sulphuric acid 
Sterile glucose solution 
Pure cultures of Psuedomonas fluoresce ns, Saccharomyces 
cerevisiae and Aspergillus species. 
1. Treat four 100 cc. portions of sterile broth, pH 7.0 as follows: 
No. 1. Leave as it is. 
No. 2. Add 5 cc. of 20 per cent glucose solution. 
No. 3. Add 1 cc of N/10 sulphuric acid, giving a reaction of ap¬ 
proximately pH 4.5. 
No. 4. Add glucose and acid. 
2. Mix well with gentle shaking; and carefully pour two sterile test tubes, 
one-third full from each flask. Be sure the tubes are labeled properly. 
3. Pour each of four sterile Petri dishes one-third full of sterile sand. 
4. Carefully label the dishes and moisten the sterile sand to saturation 
with one of the broths prepared above. 
5. Inoculate the tubes and dishes as follows: 
Broth No. 
Tul 
bes 
Dishes 
1 
1 
2 
Mold 
Bacteria 
Yeast 
2 
Bacteria 
Yeast 
Mold 
3 
Bacteria 
Yeast 
Mold 
4 
Bacteria 
Yeast 
Mold 
Use a culture of Psuedomonas fluorescens for the bacteria, Saccharomyces 
cerevisiae for the yeast and Aspergillus for the mold. 
6. Incubate the cultures in the desk for two weeks. Examine them at 
the end of the first and second weeks for signs of growth. Construct a table 
showing the type of fermentation that has taken place in each case. Record 
the presence of gas, odor and the reaction. 
7. (a) What do the results of this exercise indicate in regard to the con¬ 
ditions favoring and opposing the growth of bacteria, yeasts, 
and molds? 
(b) Construct a diagram of the pH range of hydrogen ion concentra¬ 
tion showing the optima for bacteria, yeasts, and molds? 
(See Fig. 15). 
