224 ESSENTIALS OF BACTERIOLOGY 



on one plate that there is on the other; thus the i c.c. serves as 

 control. 



Water that is very rich in germs requires dilution with ster- 

 ilized water fifty to one hundred times. Fewer colonies will 

 be found on agar than on gelatin, even at the same temperature. 



Special Media and Preparation. Sodium chlorid must not 

 be used in the preparation of media for water analysis. The 

 reaction of most culture-media should be +i per cent, to 

 phenolphthalein. 



Sugar broths should be neutral, and must be sterilized care- 

 fully in steam and not overheated, to prevent inversion of the 

 sugar. 



Examination for Bacillus Coli and Sewage Bacteria. 

 Instead of examining for typhoid bacilli, sewage contamination 

 is best indicated by the presence of the colon group of 

 organisms. 



The committee of the Public Health Association recom- 

 mends the following procedure : 



Two Methods. Method a. Preparation of an agar plate 

 with a known volume of water, using lactose litmus-agar and 

 incubating at 40 C. Bacillus coli will show its presence by red 

 colonies (acid fermentation of the sugar) ; further testing is then 

 needed to fully identify. 



Method b. Cultivation, at 40 C., of a measured quantity 

 of water in a fermentation tube containing a sugar broth. 

 If gas appears, a portion of the liquid is plated as in method a. 



Additional Details. If in twenty-four hours no red colonies 

 appear in the agar-lactose litmus Petri dishes, Bacillus coli is 

 considered absent, providing the sample was a polluted one, 

 so that the bacilli,, if present, would be in a concentrated form. 

 Only i or 2 c.c. of water can be used, because the ordinary 

 water-bacteria spread rapidly and contaminate the other bac- 

 teria. 



If acid-forming colonies are found, five or six are fished for 

 subcultures on slanted agar, in fermentation tubes, milk, 

 gelatin, peptone solution, and nitrate broth. 



If the water is not strongly contaminated, an underground 



