well on media with minimum quantity of organic matter, equivalent by com- 

 position to that in natural water. 



Determination of the number of living bacteria should involve the use 

 of water from the investigated water-body. The water should be collected 

 in bottles, then decanted into 10 ml test tubes and sterilized in an auto- 

 clave. 



Since sterilization partially destroys the carbonates making the 

 water more alkaline, following autoclaving, the test tubes should be 

 placed into an atmosphere of carbon dioxide rendering the water neutral. 

 The test-tubes are subsequently placed in a stand in the order shown in 

 Figure 2. To the first test tube is added 1 ml of water by sterile 

 pippette. After thorough mixing, 1 ml of its contents is transferred 

 into the second test-tube. The process is repeated until the third test 

 tube is reached where subsequent transfers are performed in three repli- 

 cates for a greater statistical confidence. For majority of water bodies, 

 6-7 dilutions should be made. The seventh or the eighth test-tube serves 

 as control. Then the test tubes are placed for 7 days in a thermostati- 



2 3 4 5 6 7 8 

 SEQUENTIAL DILUTIONS 



Figure 2. Scheme of the order of test tubes in determination of living 

 bacteria by titer method. Numbers represent the sequence of dilutions. 



58 



