66 ELEMENTS OF WATER BACTERIOLOGY. 



well separated, but a second plate, inoculated from the 

 dilution water with a straight needle instead of a loop, 

 furnishes a desirable safeguard. 



The litmus-lactose-agar plates made in this manner 

 should be incubated for from twelve to twenty-four hours at 

 the body temperature (37), at the end of which time if B. 

 coli is present red colonies upon a blue field will be visible. 

 If a pure culture of B. coli is obtained, the litmus-lactose- 

 agar plate may become blue again after forty-eight hours 

 owing to the formation of amines and ammonia by the 

 action of the bacteria upon the nitrogenous matter present. 

 If the dilution is too low, the resulting colonies will be 

 small and imperfectly developed, making it difficult to be 

 sure of pure cultures for the subsequent tests. A great 

 number of colonies will also prevent the change of reac- 

 tion from acid back to alkaline. Since many bacteria 

 ferment lactose with the formation of acid, it is erroneous 

 to regard all colonies as those of B. coli; in all cases 

 several colonies from each plate should be isolated upon 

 agar streaks and further studied in subculture. 



In the selection of those red colonies which are to be 

 fished from the litmus-lactose-agar plate the appearance 

 of the growths must be closely noted. A colony of irregu- 

 lar contour, surrounded by a very faint area of reddening, 

 will probably belong to some member of the B.. mycoides 

 group (Winslow and Nibecker, 1903); small, compact, 

 bright-red colonies are characteristic of the streptococci, 

 and Gage and Phelps (Gage and Phelps, 1903) have 

 pointed out that of these there are two types, one of a 



