132 ELEMENTS OF WATER BACTERIOLOGY 



should be made from this enrichment medium after 

 6-12 hours at 37°. 



Isolation of Pure Cultures from the Enrichment Tube. 

 In case one does not rely upon a " presumptive " test 

 alone but desires to study the organisms present in 

 detail the isolation upon a solid medium, usually 

 litmus-lactose-agar in this country, must follow the 

 enrichment process. Since the enrichment tube was 

 inoculated with a known amount of water all further 

 work is purely qualitative, and it is only necessary to 

 obtain such a number of colonies upon the lactose plate 

 that the isolation of a pure culture shall be easy. In 

 practice the following procedure has been found gen- 

 erally successful. After the enrichment tubes have 

 been incubated for 12 to 24 hours at 37°, from those 

 which show gas, one loopful is carried over to a tube 

 containing 10 c.c. of sterile water, and of this water one 

 loopful is taken for the inoculation of the plate. 

 Ordinarily this will give colonies which are sufficiently 

 well separated, but a second plate, inoculated from the 

 dilution water with a straight needle instead of a loop, 

 furnishes a desirable safeguard. With practice it is 

 possible to effect a proper seeding more rapidly by 

 barely touching the tip of a straight needle to the broth 

 in the fermentation tube and transferring this directly 

 to the agar. The touch must be a very light one, how- 

 ever, or the colonies on the plate will be too thick for 

 proper isolation. 



The litmus-lactose-agar plates made in this manner 

 should be incubated for from 1 2 to 24 hours at the body 

 temperature (37°), at the end of which time, if B. coli is 



