52 THE BACTERIOLOGICAL EXAMINATION OF MILK 



minute sub-surface colonies and small air bubbles contained in 

 the medium. If the colonies on the plates are comparatively few, 

 it is more satisfactory to count the whole of them, and with the 

 assistance of the divisions on the black disc this can be done 

 without difficulty. If numerous, however, it will suffice to count 

 say half the number of sectors, to estimate from them the average 

 number per sector, and multiply by the total number of sectors 

 on the counting card. This will give approximately the total 

 number of organisms on the plate, and the subsequent calculations 

 will be as follows : — 



.:rx>'X^ = number of organisms per c.c. of the original milk, ;ir 

 being number of organisms on the plate, y the multiple of fraction 

 of the final dilution taken, and .s-the amount of such dilution. 



Example. — Given -05 of a dilution of i in 500, and 98 as the 

 total number of organisms on the plate. Then 



98 X 20 X 500 = 980,000 — total number of organisms per c.c. 



or. Given -2 of a i in 5000 dilution, and 476 as the total number of 

 organisms. Then 



476 X 5 X 5000= 11,900,000 — total number of organisms per c.c. 



At least two and if possible three plates should be counted for 

 each sample of milk, and the average of these taken as the approxi- 

 mate number of organisms per c.c. of the milk. 



Petri dishes. — Those of 9 centimetres internal diameter will be found the 

 most convenient. They should be carefully selected, and any showing 

 unevenness on the interior bottom surface should be rejected. A regular 

 distribution of organisms over the plate is impossible when the medium is of 

 varying depth. 



A supply of these dishes should always be kept sterile ready to hand. The 

 best way of ensuring this is to wrap up each dish in the form of a small paper 

 packet before sterilisation,^ and only remove the paper covering when /the 

 dishes are required for use. In making the packet the dish should be placed 

 lid downwards, so that when finished the folds come at the bottom side 

 when the plate is in proper position. After sterilisation for one hour at 

 i5o°C. they are taken out of the hot air steriliser, and stored in a drawer 

 or other convenient place free from dust. If stored with the folded side of the 

 paper downwards and the paper unbroken, there is practically no chance of 

 contamination under ordinary circumstances. To avoid carbonisation of the 

 paper care should be taken that the dishes during sterilisation do not come in 



1 A very suitable paper for this purpose is that known to the trade as " 22^ 

 white Demy." 



