WATER 425 



(a) In the final counting of each plate, place the 

 plate over the counting disc, and centre it, if possible, 

 making its periphery coincide with one or other of the 

 concentric circles. 



(b) Remove the cover of the plate, and by means of 

 a hand lens count the colonies appearing in each of the 

 sectors in turn. Make a note of the number present in 

 each. 



(c) If the colonies present are fewer than 500, the 

 entire plate should be counted. If, however, they 

 exceed this number, enumerate one-half, or one- 

 quarter of the plate, or count a sector here and there, 

 and from. these figures estimate the number of colonies 

 present on the entire plate. In practice it will be found 

 that Pakes* disc is more suitable for the former class of 

 plate ; Jeffer's disc for the latter. It should be recol- 

 lected however that unless the plates have been care- 

 fully levelled and the medium is of equal thickness all 

 over it is useless to try and average from small areas 

 since where the medium is thick all the bacteria will 

 develop, where the layer is a thin one, only a few bacteria 

 will find sufficient pabulum for the production of visible 

 colonies. 



It will be noted that the quantities of water selected 

 for addition to each set of tubes of nutrient media 

 have been carefully chosen in order to yield workable 

 results even when dealing with widely differing samples. 

 Plates prepared in agar with o.i c.c. and in gelatin 

 with 0.02 c.c. can be counted even when large numbers 

 of bacteria are present in the sample ; whereas if micro- 

 organisms are relatively few, agar plate 4 and gelatine 

 plate i will give the most reliable counts. Again the 

 counts of the plates in a measure control each other ; for 

 example, the second and third plates of each gelatine 

 series should together contain as many colonies as the 

 first, and the second should contain about half as 

 many more than the third and so on. 



