DETERMINATION OF ORGANISMS 65 



obtained with little extra trouble by adding a sugar 

 and some sterile litmus to trie agar medium and 

 observing the fermenting powers of the organisms 

 present, as first suggested by Wurtz (Wurtz, 1892) 

 for the separation of B. coli from B. typhi. It hap- 

 pens that the most abundant intestinal organisms, 

 belonging to the groups of the colon bacilli and the 

 streptococci, decompose dextrose and lactose with the 

 formation of a large excess of acid. The decomposi- 

 tion of the latter sugar, 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 1 6 in the standard process of media-making 

 given on p. 102). In pouring the plate a cubic centi- 

 meter of sterile litmus solution should be added. 

 After incubation the colonies of the acid-forming 

 organisms will be clearly picked out by the redden- 

 ing of the adjacent agar. Only those colonies which 

 are sharply colored should be considered as significant, 

 since certain bacteria of the hay-bacillus group pro- 

 duce 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 



