25 
Exercise 12. The Fermentation of Carbohydrates by Bacteria 
MATERIALS: 
2 fermentation tubes of dextrose broth containing brom cresol 
purple 
2 Durham tubes of lactose broth containing brom cresol purple 
1 culture each of Streptococus lactis and Aerobacter aerogenes 
1. Inoculate one Durham tube of lactose and a fermentation tube of 
dextrose broth with each of the organisms furnished. 
2. Place the cultures in the incubator and examine them in 48 hours. 
3. Observe the reaction of the broth in the tubes. Note whether gas has 
been formed in both lactose and dextrose broth. If gas has been formed in the 
fermentation tubes, measure it and determine its composition as shown by 
the instructor. 
4. Compare the color of the indicator in the Durham tubes with a set of 
standard solutions containing brom cresol purple. Record the change in reac¬ 
tion as shown on the pH scale of hydrogen ion concentrations. (See Fig. 15.) 
(а) What is the range in pH of brom cresol purple? 
(б) What was the change in the reaction of the sugar broths 
where the bacteria grew? (See the diagram Fig. 15). 
(c) What acids do bacteria most commonly form from dextrose 
and lactose? 
( d ) What was the gas formed in the fermentation tube? 
(e) What gaseous substances are usually formed in the decom¬ 
position of carbohydrates? 
( f ) What are the steps in the conversion of an insoluble carbo¬ 
hydrate like starch into acid by bacteria? 
(g) Why is the bacterial decomposition of carbohydrates of more 
industrial importance that that caused by molds? 
(/?) What is the advantage to bacteria of being able to ferment 
carbohydrates? 
(0 How does the action of yeast on sugars differ from the action 
of bacteria? 
