GROWTH OF BACTERIA IN NORMAL MILK. 2 1 



dealer, a dilution of one hundred to six hundred is commonly 

 most satisfactory, but in warm weather this is too slight a dilu- 

 tion; and in the case of milk that has been obtained under 

 uncleanly conditions a larger dilution is necessary. Milk 

 which has been kept for one to two days in the laboratory 

 requires, of course, higher dilutions; the oldest milk samples 

 which we have studied required a dilution of 1-250,000 or even 

 more. These high dilutions make it impossible to detect the 

 presence of bacteria which are present in old milk in very small 

 numbers only. 



After dilution, a series of plates is made from each sample of 

 milk and allowed to develop at ordinary room temperature for 

 three or four days before an examination is made. The plates 

 are then carefully studied for a differentiation of colonies. A 

 partial differentiation is extremely simple and quite accurate. 

 An examination of the plates after three or four days' growth 

 enables one at a glance to differentiate the liquefying bacteria 

 from the non-liquefying, and those which produce an acid re- 

 action from those which produce an alkaline reaction or no 

 reaction at all. It is also possible to differentiate the liquefiers 

 readily into those which liquefy rapidl}' and tho.se which liquefy 

 slowly. A differentiation thus into four types, viz., into those 

 producing an acid or an alkaline reaction and into rapid and 

 slow liquefiers is extremely easy, very accurate, and can be 

 made at a glance. 



A further differentiation requires more careful study. We 

 have, however, learned to separate about thirty species from 

 each other by a study of their colonies in this litmus gelatin. 

 For example, we have found that the most common organisms 

 in milk which is a few hours old are three types of acid bacteria 

 easily separated upon our litmus gelatin plates as follows: 



1. B. acidi ladici (Esten) is, in this locality at least, the 

 most common organism in milk. It produces a small, dense, 

 intensely acid colony, growing only under the surface of the 

 litmus gelatin, and showing after growing for about four days 

 slight irregularities or spines on the edges. It is very easily 

 distinguished. 



2. B. acidi ladici II. (Conn) is also very common but not 

 so abundant as the first. It produces an extremely minute 

 transparent colony, hardly visible even with the lens, and quite 

 invisible with the naked eye. 



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