64 
6. Count the colonies in the plate cultures and tabulate the results as shown 
in Table XII on the regular note paper. 
TABLE XII.—Bacterial Content of Waters 
Kind ofj 
sample ! 
Dilu¬ 
tion 
Colonies 
per plate 
Bacteria 
per cc. 
Kinds of 
colonies 
Gas from 
bacteria in 
10 cc. 
1 cc. 
0.1 cc 
• 
7. Make a careful examination of the fermentation tubes for growth and 
gas formation. 
Note: Each fermentation tube containing gas indicates the presence of a 
member of the common intestinal bacteria. The most commonly occurring 
forms are: Escherichia coli and Aerobacter aerogencs. On the assumption that 
at least one of the organisms is present in the tube, the approximate number 
in one cubic centimeter of water may be calculated. 
(a) What is the nature of the organisms developing from ground 
water as compared to those from surface water? 
( b ) What quality of water is indicated by the results? 
(c) What chemical tests should go with these tests? 
(d) What is meant by the presumptive test for Escherichia coli? 
(e) What is the sanitary significance of Escherichia coli in water? 
