242 CLEAN MILK 



the growth of more than 200 bacterial colonies of countable size, 

 without suppression of some of the weaker forms. Should there 

 be in the material (milk, water, etc.) mixed with the standard 10 

 c.c. of jelly a very large number of growable bacteria, say 2,000,000, 

 the colonies produced would not generally greatly exceed 10,000 

 to 20,000 and these would be exceedingly small colonies — barely 

 visible to the naked eye. In other words, placing 2,000,000 bac- 

 teria in 10 c. c. of medium is like planting a thousand grains of corn 

 in a flower-pot full of earth — only a very few of the strongest and 

 most favorably situated will survive and grow up. To grow from 

 2,000,000 bacteria successfully, 2,000,000 separate colonies, they 

 should be planted in 100,000 c.c. of jelly — a totally impracticable 

 amount. Hence the practice is to reduce the number of bacteria 

 plated, rather than to increase the food, until the due proportion 

 of 200 bacteria per 10 c.c. of medium is reached, at which point all 

 present capable of growing at all have freedom to grow, without 

 the weaker suffering from the competition of the stronger forms. 

 To secure this end, samples containing large numbers of bacteria 

 per c.c. must be plated in such a manner that only such very small 

 fractions of a whole c.c. are placed in each plate as will contain 

 but 200 bacteria per fraction. It is of course impossible to de- 

 termine accurately what number of bacteria are present in a given 

 sample before plating (except by Slack's preliminary miscroscopic 

 count) ; otherwise, why make a plate count? Hence it is impossible 

 to determine exactly what fraction of 1 c.c. to use in order to secure 

 200 bacteria per plate. The practice is therefore to plate several 

 different fractions (say 1/10 c.c, 1/100 c.c, 1/1000 c.c, etc.) of the 

 original sample, and to select that plate which gives 40 — 200 bacteria 

 as the one which represents the true number of bacteria. The num- 

 ber of colonies in this plate, multiplied by the dilution it represents, 

 gives the number per c.c. 



Thus — If 2,000,000 bacteria (growable) exist in each c.c. of 



