98 Elements of Water Bacteiiology. 



ment culture. In the first case a measured quantity of 

 water is added and the number of colonies of B. coli 

 corresponds to the number of bacteria in the portion 

 plated. ' In the second case, 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 foimd generally 

 successful: After the dextrose tubes have been incubated 

 for twelve to twenty-four hours at 37 degrees, from those 

 whic^ show gas, one loopful is carried over to a tube con- 

 taining 10 c.c. of sterile water, and of this water one loop- 

 ful 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 de- 

 sirable safeguard. With practice it is possible to eflEect 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, however, 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 twelve to twenty-four hours 

 at the body temperature (37 degrees), at the end of which 

 time, if B. coli is present, red colonies upon a blue field 

 will be visible. The litmus-lactose-agar plate may be- 



