PART V. 
BACTERIOLOGY OF WATER AND SEWAGE 
Exercise 31. The Total Bacterial Content of Water 
MATERIALS: 
13 tubes of sodium caseinate agar 
13 sterile Petri dishes 
Case sterile 1 cc. pipettes 
1 sterile 10 cc. pipette 
4 9 cc. sterile water blanks 
1 99 cc. sterile water blank 
3 dextrose broth fermentation tubes 
2 samples of surface water 
1 sample of ground water 
1. Follows the dilution diagram (Fig. 19) and prepare plate cultures as 
follows: Lake water, 1/10 and 1/100 cc.; shallow well, 1 cc. and 1/10 cc.; 
ground water 1 cc. Make one “control” plate. 
2. When the duplicate plates all contain their dilutions and the sterile 
“control” is made, melt the agar and cool it to 45° C. Pour the plates, mixing 
the water and agar well. 
^ 3. When the agar is solid, incubate the plates for one week at room temper- 
ture. 
FIG. 19—DIAGRAM SHOWING DILUTIONS FOR WATER SAMPLES 
4. Use one of the water samples designated by the instructor and inoculate 
the 3 fermentation tubes after labeling as follows: 10 cc., 1 cc. and 0.1 cc. 
The 0.1 cc. is obtained by taking 1 cc. of the 1 to 9 dilution. 
5. Be careful that there are no air bubbles in the closed arm of the fermenta¬ 
tion tube and incubate the cultures 48 hours at incubator temperature. 
