42 PRACTICAL DAIRY BACTERIOLOGY 



Bacteria Producing Both an Acid and a Ferment. The 



peptonizing bacteria usually fail to produce acid and, indeed, 

 make the milk slightly alkaline. Among the acid forming bac- 

 teria are some which produce a peptonizing enzyme, like those 

 just mentioned, but they produce acid at the same time. This 

 type is thought by Gorini to have an important part to play in 

 the ripening of some cheeses 1 ; but there is no sufficient proof 

 of this at present. One variety of this class has been found 

 to be the cause of a bitterness in some types of cheese, bring- 

 ing great losses upon the dairymen. They make the milk sour 

 and are not, therefore, readily detected in normal milk. They 

 can easily be found by the ordinary litmus gelatin plate 

 methods. . 



NEUTRAL TYPES 



A large number of milk bacteria apparently play no part 

 in dairy problems, and, therefore, we need not consider them 

 except by a passing word. Upon studying the litmus gelatin 

 plates (page 289), we will find small colonies that are not 

 acid, that do not liquefy the gelatin, but appear simply as 

 whitish or greyish dots, some on the surface and others under 

 the surface of the gelatin. If we will remove and study these 

 by themselves, we will find most of them to be neutral organ- 

 isms. They do not sour or curdle the milk, nor do they pro- 

 duce any changes in it that are noticeable under any ordinary 

 conditions. Neither do they seem to have any pathogenic effect 

 upon persons drinking the milk, although they may be present 

 in considerable quantities. Up to the present time, therefore, 

 we have no reason to regard them of any special significance to 

 dairymen. We need consider them no farther, beyond stating 

 that they are frequently present in considerable numbers. 



1 Gorini. Cent. f. Bact. II., viii., p. 137, 1902. 



