DETERMINATION OF ORGANISMS. 45 



tion of the latter sugar is almost entirely wanting among 

 the commoner saprophytic bacteria, and therefore lac- 

 tose is most commonly used in making sugar agar, 2 per 

 cent being added to the medium just before the final nitra- 

 tion (between steps 15 and 16 in the standard process 

 of media making given on p. 120). In pouring the plate 

 a cubic centimeter of sterile litmus solution should be 

 added; and in counting, the colonies of the acid-forming 

 organisms will be clearly picked out by the reddening of 

 the adjacent agar. Only those which show this clearly 

 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, 55-190 in the polluted 

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

 side mud. In an examination of water from the Charles 

 River above Boston, total 37 counts ranging from 9800 to 

 16,900 have been found. Two twenty-four-hour exam- 

 inations of Boston sewage made at the Sanitary Research 

 Laboratory of the Institute of Technology during the 

 summer of 1903 gave an average of 3,660,000 bacteria 

 per c.c. on gelatin at 20 and 2,310,000 per c.c. on lactose 

 agar at 37 with 550,000 acid-formers. 



