THE RELATION OF MICROORGANISMS TO MILK 451 



Representatives of this group are found constantly in milk and other 

 dairy products. Their presence in milk can be demonstrated by 

 placing a sample of milk in a corked bottle, and incubating at 37. The 

 acidity of the milk increases rapidly at first, due to the growth of the 

 members of the two previous groups. These ordinary acid-forming 

 organisms are soon inhibited by the appearance of free acid, but the 

 acidity of the milk nevertheless continues to increase slowly, and 

 with this continued increase a change in flora is noted, the short, 

 plump bacilli ceasing to predominate and long slender rods constantly 

 increasing in numbers. The source of this group is undoubtedly 

 the alimentary tract of the animal. 



Characteristics of the Coccus Group*- -This group is well represented 

 by the bacteria which form the characteristic flora of the udder. They 

 vary greatly in size and in other properties. They retain Gram's 

 stain; many are chromogenic, the color ranging from a white to a 

 deep orange. They grow slowly on all ordinary culture media, but 

 the growth is not necessarily meager. Generally they are aerobic, 

 although many grow under anaerobic conditions. Gelatin may be 

 liquefied or not. Milk may or may not be curdled, the curd often 

 resembling that formed by rennet-like enzymes. They produce no 

 lactic acid, but only acetic, propionic, butyric and caproic acids, 

 and hence cannot be classed as lactic bacteria. 



BACTERIA HAVING No APPRECIABLE EFFECT ON MILK. This 

 group is made up of many different forms. They produce no changes, 

 during the normal life of market milk, which can be detected either 

 by the eye or the taste. They do not develop very rapidly in milk, 

 and some species gradually disappear while others increase in numbers. 

 Many of the organisms in this group are chromogenic, orange and 

 lemon yellows being among the more common forms. They are 

 mostly cocci and do not liquefy gelatin. From the standpoint of the 

 commercial milkman these organisms are of little significance and this 

 is probably also true from the standpoint of the consumer. 



THE CASEIN-DIGESTING OR PEPTONIZING BACTERIA. These organ- 

 isms digest the casein either with or without coagulation. Many of 

 them cause the milk to curdle. The reaction is alkaline. The curdling 

 agent is a rennet-like enzyme. They liquefy gelatin. Most of the 

 organisms of this group are rods of various shapes and sizes, some 



* Prepared by E. G. Hastings 



