110 BACTERIOLOGY OF WATER, AIR, MILK, ETC. 



should be sterile. Sterilization may be effected by heat or by rinsing 

 with a little sulphuric acid and subsequently washing out thoroughly 

 with the suspected water before collection. The utmost care must 

 be exercised that the ringers do not come in contact with the glass 

 stopper or the neck of the bottle while filling it. If the specimen is to be 

 sent some distance, it should be packed in ice to prevent bacterial 

 development. Frankland states that a count of 1000 became 6000 in 

 6 hours and 48,000 in 48 hours. In water packed in ice for a consider- 

 able time, however, the bacterial count may diminish. 



2. If collecting from city water supplies, secure the sample direct 

 from the mains and let the water run from the tap a few minutes be- 

 fore collection. If the water be taken from a pond, stream or cistern, 

 be sure that the specimen comes from at least 10 inches below the sur- 

 face. As sedimentation is the most important method in self-purifica- 

 tion of rivers and ponds, it will be understood that any stirring up of 

 the mud on the bottom will enormously increase a bacterial count. 



Quantitative Bacteriological Examination. 



i. Deliver definite quantities of the water to be examined into 

 tubes of liquefied gelatin or agar and plate out the same or into a 

 series of Petri dishes. The water should be deposited in the center 

 of the plate and the melted gelatin or agar poured directly on the 

 water and then, carefully tilting to and fro, mix the water and the 

 media. One set of plates should be of gelatin and incubated at 

 room temperature; a similar set should be of lactose litmus agar and 

 incubated at 38 C. If the water is highly contaminated, it is neces- 

 sary to dilute it; thus, with river water, which may contain from 2000 

 to 10,000 bacteria per c.c., a dilution of i to 100 would be desirable. 



Ordinarily it w r ill be sufficient to deliver from a sterile graduated 

 pipette .2, .3, and .5 c.c. of the water in each of 2 sets of plates: one set 

 for gelatin, the other for agar. 



When gelatin is not at hand or convenient to work with, the gelatin 

 plates may be replaced by others of lactose litmus agar for incubation 

 at room temperature. After 24 hours at 38 C. or 48 hours at 20 C., 

 the count should be made. 



Example: Forty colonies were counted on the gelatin plate con- 



