THE DISTRIBUTION OF MICRO-ORGANISMS 183 



temperatures. If the temperature is too low, and the water 

 freezes, more or less of the bacteria may be killed; if, on the 

 contrary, the temperature is not low enough there will be a 

 multiplication of the bacteria in transit. Special containers 

 with provision for packing in cracked ice should be employed 

 for shipment, and even then any considerable delay should be 

 avoided. 



The number of bacteria may be determined by making plates of 

 a definite quantity of the water with gelatin or agar. 1 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 neces- 

 sary to dilute the sample ten times or more with sterilized water. 

 The amount should be measured with a sterilized, graduated 

 pipette. The water is mixed with melted 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; or better, measure the water into 

 the Petri dish, and pour the melted medium in, and mix. 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. The number of colonies that develop is supposed to repre- 

 sent 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 tempera- 

 ture than in the incubator. This observation illustrates the fact 



1 For standard methods of water analysis see the report of a Committee of the 

 American Public Health Association, Journ. Inf. Diseases, Supplement No. i, May, 

 1905; also Report of Committee, American Public Health Association, New York, 

 1912. 



