EXAMINATION OF WATER AND SEWAGE 147 



EXERCISE 3. QUALITATIVE AND QUANTITATIVE ' DE- 

 TERMINATION OF B. COLI AND STREPTOCOCCI 



1. Take one sample and inoculate 10 fermentation 

 tubes, containing dextrose or lactose broth, each with 

 I c.c. of the sample. Similarly inoculate 10 fermenta- 

 tion tubes with i c.c. of each of the dilutions. 



Note. — This experiment requires 40 fermentation tubes and 

 may be divided among several students. This same experiment 

 should be repeated with lactose bile in place of dextrose or lactose 

 broth and the results compared. 



2. Incubate the fermentation tubes at 37° C. 



3. Measure the amount of gas in the closed arm after 

 24 hours and replace in the thermostat. 



4. Measure the gas again after 48 hours and deter- 

 mine the composition. See p. 81 for directions. 



5. Make smears of each tube and apply Gram's 

 stain to determine the presence of streptococci. 



6. Calculate the number of B. coli and streptococci 

 present according to the formula given on p. 86. Sub- 

 stitute the number of tubes containing streptococci 

 for the letter N in the formula to determine the num- 

 ber of streptococci. 



Note. — ^Each fermentation tube containing gas in the closed 

 arm shows the presence of B. coli. On the assumption that at 

 least one of the organisms is present in the tube the approximate 

 number in i c.c. of water is calculated. 



EXERCISE 4. EXAMINATION OF WELL WATERS 



Secure three samples of well water, make dilutions 

 of 1:10 and 1 : 100 and examine in the same manner as 

 directed in Exercises 2 and 3. Compare the results 

 with those obtained in Exercise 3. 



