BACTERIA IN WATER 1025 



"A comparison of the positive results obtained in the various dilutions 

 of the water or sewage planted into the lactose bile gives a good idea of 

 the relative amount of contamination in the various samples examined. 



"Quantities of Water Tested. For ordinary waters, 0.1, 1.0 and 10.0 c.c. 

 shall be used for the colon test. For sewage and highly polluted surface 

 waters, smaller quantities shall be used; and for ground waters, filtered 

 waters, etc., the quantities shall be larger, if necessary to obtain positive 

 results. The quantities shall vary preferably in the tenfold manner in- 

 dicated. Single tests with quantities which give ordinarily a positive result 

 or ordinarily a negative result are in themselves of but little account for 

 quantitative determinations. The range in quantities studied shall be sufficient 

 to allow the quantities needed for both a positive and a negative result to 

 be recorded for each sample. When this is done, the results of several tests 

 allow an approximate estimate of the number of B. coli per c.c." 



The identification of colon bacilli so obtained should then be 

 undertaken. The following table, again taken from the report of 

 the A. P. H. A., will be of help : 



