98 Elements of Water Bacteriology. 



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 found generally 

 successful: After the dextrose tubes have been incubated 

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

 which 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 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, 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- 



