102 ELEMENTS OF WATER BACTERIOLOGY. 



only i sample, while Baker and one of us (Prescott and 

 Baker, 1904) found the organism present in each of 

 50 samples of variously polluted waters when inoculation 

 was first made into dextrose broth from which litmus- 

 lactose-agar plates were made after various intervals. 



The isolation of these organisms either from plates or 

 liquid cultures is easy. On the lactose-agar plate, made 

 directly from a polluted water the colonies of the strep- 

 tococci may generally be distinguished from those of other 

 acid-formers by their small size, compact structure, and 

 deep-red color, which is permanent, never changing to blue 

 at a later period of incubation as the colonies of B. coli 

 may do. Developing somewhat slowly, however, they 

 may be overlooked if present only in small numbers. In 

 the dextrose-broth tube, streptococci will appear in abun- 

 dance after a suitable period of incubation. Prescott 

 and Baker in the work above mentioned found that with 

 mixtures of B. coli and streptococci in which the initial 

 ratios of the latter to the former varied from 1 : 94 to 208 : 1, 

 the colon bacilli developed rapidly during the early part 

 of the experiment, reaching a maximum after about 

 fourteen hours and then diminishing rapidly, while the 

 streptococci first became apparent after ten to fifteen 

 hours and reached their maximum after twenty to sixty 

 hours, according to the number originally present. 



Applying the same method to polluted waters, similar 

 periodic changes were observed; pure cultures of B. coli 

 were first obtained, then the gradual displacement of one 

 form by the other took place, and at length streptococci 



