QUANTITATIVE BACTERIOLOGICAL ANALYSIS. 9 



of the colonies are able to develop in the plates. Gelatine plates, 

 however, can rarely be kept under observation beyond the fourth 

 or fifth day, owing to liquefying organisms destroying the plates. 

 Up to the present time the quantitative analyses of most sources 

 of water supply have been made by means of faintly alkaline 

 gelatine-peptone plates, consequently, if it is desired to compare 

 the results with previous analyses, this medium must be used. 



THE PREPARATION OF " WATER PLATES." 



The water to be examined should be gently shaken for a few 

 minutes ; in this way all clumps of bacteria will be broken up 

 and each organism will produce only one colony when it 

 develops in the Petri dish. Forcible shaking of the sample 

 must be avoided as it may cause the death of some of the micro- 

 organisms. The next step is to determine the amount of water 

 which should be used for each plate, as it is necessary that each 

 organism shall have room to develop without touching its 

 neighbours. The simplest method of arriving at an idea of the 

 number of bacteria present in the water is to remove ^V c<c * f 

 the water from the bottle and deposit it on a cover-glass ; 

 then by staining and counting the microbes on the cover-glass 

 preparation it is possible to determine very roughly the number 

 of micro-organisms present in a c.c. of the water. When work- 

 ing with Petri dishes having a diameter of four inches, it is best 

 to use such an amount of water as will give not more than 300 

 colonies in each plate. If the water contains a very large 

 number of bacteria it must be diluted with sterile tap-water. 

 Usually three plates are made from each sample, containing 

 respectively T V c.c., J c.c. and J c.c. of the water diluted or not 

 as required. In order to measure the calculated amount of 

 water required for each plate, accurately graduated pipettes 

 must be used. One c.c. pipettes graduated in y^ths are most 

 useful. Each pipette must be thoroughly cleaned, the upper 

 end plugged with cotton-wool, and the lower end placed in a 

 test-tube, the mouth of which is firmly plugged with cotton- 

 wool. The test-tubes and pipettes are then sterilised at 150 C. 

 for three hours and kept in metal boxes until required for use. 

 The Petri dishes, wrapped in thin paper, are also sterilised at 

 the same temperature. 



