228 GENERAL MICROBIOLOGY 



EXERCISE 2. BACTERIOLOGICAL ANALYSIS OF WATER 

 SUSPECTED OF SEWAGE OR OTHER POLLUTION 



Apparatus. Sterile 500 c.c. flask for collecting sample; 

 litmus lactose agar shake; six litmus lactose agar tubes for 

 making plates; ten litmus lactose (or ordinary) agar slants 

 for pure cultures isolated; six salt-free gelatin tubes for 

 plates; ten tubes of gelatin for pure cultures isolated; two 

 litmus lactose bile fermentation tubes (p. 122, Prescott and 

 Winslow); ten tubes of litmus milk; ten tubes of Dunham's 

 peptone solution; ten tubes sterile esculin bile for B. coli 

 test (p. 129, P. and W.); ten tubes of nitrate peptone 

 solution; ten each fermentation tubes of dextrose, lactose 

 and saccharose broth; 99 c.c. dilution flasks; twelve sterile 

 Petri dishes; sterile 100 c.c. volumetric pipette; sterile 

 1 c.c. and 5 c.c. pipettes; sample of water or sewage from 

 source indicated by instructor; record sheet for recording 

 data; record sheet for pure cultures isolated. 



Cultures. B. coli, Bad. aerogenes, B. typhosus. 



Method. Water for this experiment may be obtained 

 from a lake, river, etc., just below a sewer outlet, or 

 from a surface well, etc., the source will be designated by 

 instructor. 



1. Collect the sample in the sterile 500 c.c. flask, using 

 all precautions as with an unpolluted water sample. (See 

 Exercise 1.) 



2. This sample must be analyzed at once. . 



3. Using a wide range of dilutions, plate immediately 

 in litmus lactose agar and in salt-free gelatin, making six 

 dilution plates in each medium. 



4. If the water is not suspected of great pollution, 0.1 

 c.c. of the sample may be plated directly, using low dilu- 

 tions for the remaining five plates. 



5. If pure sewage is to be plated, use dilutions 1 : 100, 

 1 : 1000, 1 : 10,000, 1 : 100,000, 1 : 1,000,000 and 1 : 10,- 

 000,000. 



