WATER TESTING 



The samples of water to be tested are from wells, springs, res- 

 ervoir, and swimming pool. Two kinds of media are used agar for 

 plate cultures to give the bacterial count, and a bile salts medium 

 in fermentation tubes to show gas production. The bile salts in- 

 hibit the growth of nearly all bacteria other than the intestinal 

 forms. More than 10 '< of gas in the tubes after incubating two 

 days at 37 C. is regarded as a positive presumptive test for B. coli 

 and the water is considered unsafe for. drinking purposes. When 

 confirmation i? desired further tests are necessary. 



No absolute standard can be given to determine the potable 

 quality of water from the number of bacteria in it. Water con- 

 taining thousands of germs to the cubic centimeter may be far 

 less dangerous than one containing but two germs if one of these 

 two be a typhoid bacillus. It is not the number that proves dan- 

 gerous, it is the kind. The agar count at 37 C. eliminates the 

 water flora but not the soil bacteria. Harrison states that for deep 

 waters the agar count should generally -not exceed 10 per cc. and 

 for surface water not over 100 per cc. 



Four agar plates are made from each sample using 1 cc., .5cc., 

 .2 cc., and .1 cc. respectively from the sample of water which has 

 been shaken vigorously twenty-five times. Five bile salts fermen- 

 tation tubes are inoculated with 1 cc. each. 



The work should be done in the following order. 



Make records in note book giving each sample an experiment 

 number and one page. 



Mark Petri dishes and tubes with locker number, sample 

 number, and amount of water added. 



With pipettes graduated to tenths of a cubic centimeter add 

 the required amounts of water to the plates and fermentation 

 tubes. These pipettes have eleven marks or graduations. They 

 are marked beginning at the top o, .1, .2, .3, .4, .5, .6, .7, .8, .9, and 

 1. Use the forefinger, never the thumb, on the top of the pipette 

 to control the flow of water from the pipette. When 1 cc. is de- 

 sired draw water up to the top mark and let out into the plate or 

 tube until the lowest graduation marked 1 cc. is reached. This 

 gives 1 cc. of water. When fractional parts of a cubic centimeter 

 are desired draw water to the top mark and let out till it reaches 

 the graduation marked with the number of tenths required. Each 

 of the divisions represents .1 cc. One pipette is required for each 

 sample. In handling pipettes never let the lower part touch any- 

 thing but the sample to be tested. Keep the fingers off from it. 

 Let the case of pipettes lie on the table never stand it on end. 



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