6o Elements .of Water Bacteriology. 



on the other hand is almost entirely wanting among the 

 commoner saprophytic bacteria, and therefore lactose is 

 most commonly used in making sugar agar, i per cent 

 being added to the medium just before the final filtration 

 (between steps 15 and 16 in the standard process of media 

 making given on p. 209). In pouring the plate a cubic 

 centimeter of sterile litmus solution should be added. 

 After incubation the colonies of the acid-forming organ- 

 isms will be clearly picked out by the reddening of the 

 adjacent agar. Only those colonies which are sharply 

 colored should be considered as significant, since certain 

 bacteria of the hay-bacillus group produce weak acid 

 and faint coloring of the litmus. 



When polluted waters are examined in this manner the 

 number of organisms developing on the lactose-agar plate 

 will be very high, almost equalling in some cases the total 

 count obtained on gelatin. Chick (Chick, 1901), using 

 a lactose-agar medium with the addition of one-thousandth 

 part of phenol, found, of colon bacilli alone, 6100 per c.c. 

 in the Manchester ship canal, 55 to 190 in the polluted 

 River Severn, and numbers up to 65,000 per gram in 

 roadside mud. In an examination of water from the 

 Charles River above Bost6n, total 37-degree counts 

 ranging from 9800 to 16,900 have been found. The 

 average result of 56 examinations of Boston sewage from 

 July to December, 1903, showed 5,430,000 bacteria per 

 c.c. at 20 degrees, and 3,760,000 per c.c. at 37 degrees, 

 of which 1,670,000 were acid formers. The average of 



