BACTERIAL MILK ANALYSIS 239 



to each other. It therefore follows that in a crowded 

 plate, i.e., over two hundred colonies, many will not 

 develop. This is easily proven by making a higher 

 dilution. 



g. Spreaders and molds, by their rapid surface 

 growth, merge with other surface colonies and obscure 

 deeper ones. 



h. Samples kept in the collecting case at 34 F. for 

 varying periods have shown a tendency to decrease in 

 the number of bacteria which will develop into colonies. 

 Samples kept in dilution water for several hours have 

 shown a marked decrease in the number of bacteria 

 which will develop into colonies. 



On account of these reasons strict adherence to 

 standard procedure is of especial importance, since 

 there are so many points where disagreement may 

 result if uniform technique is not followed. 



Since at best only approximate results can be 

 reached in the numerical determination of bacteria in 

 milk, and since from the varying methods in use at 

 present counts from different workers are usually in- 

 comparable, those methods which have given best re- 

 sults as a whole should be united upon and adopted by 

 all, that a bacterial count on a sample of milk may mean 

 the same if made in any standard laboratory. 



Collection of samples: Quantity of milk required for 

 analysis. The minimum quantity of milk necessary for 

 making an ordinary bacteriological examination is ten 

 cubic centimeters. When making examinations for 

 certified milk, if possible a pint or quart bottle should 

 be taken and brought to the laboratory unopened. 



Collecting apparatus. In collecting milk samples 

 for bacteriological examination it is essential that the 

 sample be taken and kept in such a manner as to prevent 

 either any addition of bacteria from without or multi- 

 plication of the bacteria originally present. Bottles, 

 tubes, pipettes, etc., used in the collection of samples, 

 besides being washed, shall be sterilized with dry heat 

 for an hour at or about 160 C., or to the charring point 

 of cotton. 



In the selection of " certified milk" samples it is 



