244 



Bacteriology of Water 



II. The Determination of the Purity of the Water from the 

 Standpoint of Sewage Pollution. — The chief interest in ordinary 

 bacteriological examinations centers about sewage contamination as 

 indicated by the presence of numbers of the Bacillus coli group. It 

 is therefore recommended that the B. coli group be considered as 

 including all non-spore-bearing bacilli which ferment lactose with gas 

 production and grow anaerobically on standard solid media. The 

 formation of lo per cent, or more of gas in a standard lactose broth 

 fermentation tube within twenty-four hours at 37°C. is presumptive 

 evidence of the presence of members of the B. coli group, since the 

 majority of the bacteria which give such a reaction belong to this 



Fig. 80. — Frost's plate counter, for counting colonies of bacteria on Petri dish 

 or plate cultures. The cross-lines divide the figure into square centimeters. The 

 numbers at the top of the figure iijdicate the area in centimeters of the various 

 discs. The area of each sector (a and b) is one-tenth of the whole area. 



group. To determine this one carries out the so-called presumptive 

 test. 



A. Presumptive Test.- — For this purpose one employes lactose 

 bouiUon. It is prepared by the addition of 3 grams of beef 

 extract and 5 grams of peptone to the 1000 cc. of distilled water, 

 heating slowly on a steam bath to at least 6s°C., until the ingredients 

 are dissolved. The lost weight is then made good and the reaction 

 adjusted by titration to -f i, when it is cooled to 25°C., and filtered 

 through filter-paper until clear. It then receives an addition of i 

 per cent, of chemically pure lactose, after which it is distributed in 



