J4O MANUAL OF BACTERIOLOGY. 



small samples. These samples should be examined as 

 quickly as possible, for the water bacteria increase rapidly 

 in number after the samples have been collected. When 

 transportation to some distance is unavoidable the samples 

 should be packed in ice. 



The number of bacteria may be determined by making' 

 plates of a definite quantity of the water with gelatin or 

 agar. The amount examined ordinarily is I c.c. When 

 the number of bacteria is very large, a smaller quantity 

 must be taken, and it may be necessary to dilute the sample 

 ten times or more with sterilized water. The amount should 

 be measured with a sterilized, graduated pipette. The water 

 is to be mixed with liquefied gelatin or agar in a tube which 

 has been allowed to cool after melting. After thorough 

 mixing, remove the plug, burn the edge of the tube in the 

 flame, hold in a nearly horizontal position until cool, and 

 pour into a sterilized Petri dish. The number of colonies 

 may be counted on the third or fourth day; the later the 

 better, as some forms develop slowly and may not present 

 visible colonies for several days; but the plates are often 

 spoiled after three or four days by the profuse surface 

 growths of certain forms or by the rapid liquefaction of 

 gelatin, if that be used, by other forms. The number of 

 colonies that develop is supposed to represent the number 

 of individual bacteria contained in the quantity measured. 

 That will probably not always be the case, however, as 

 colonies may develop from a clump of bacteria which have 

 not been separated from one another by the mixing process. 

 Abbott has shown that the number of colonies is usually 

 larger on gelatin plates than upon agar plates, and at the 

 room temperature than in the incubator. This observation 

 illustrates the fact that there are doubtless many kinds of 

 bacteria that do not find favorable conditions for develop- 

 ment on ordinary culture-media. The reaction of the 



