452 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



of them being the largest rods found in milk. Some are motile and 

 some non-motile. Some representatives of this group produce little 

 or no odor, but many of the species develop very strong putrefactive 

 odors. Barny or cowy odors or other off-flavors sometimes found in 

 milk and dairy products may be caused by the action of this type 

 of bacteria. They are associated with filth and their presence in 

 milk indicates insanitary conditions of production or handling. 



PATHOGENIC ORGANISMS. This group includes all those species 

 which may gain access to milk, which are capable of causing specific 

 diseases in human beings. They are of the greatest importance to the 

 consumer. They do not appreciably affect the physical or chemical 

 properties of the milk, or produce any changes in its appearance, 

 flavor, or keeping quality which would indicate their presence. 

 Some of them do not even develop in milk, as is the case with the Bad. 

 tuberculosis. Others, as the diphtheria bacteria and typhoid fever 

 bacilli, may grow in milk with great rapidity. This group also con- 

 tains certain species which produce diarrhceal disorders, especially 

 in infants and young children. Some of them are probably organisms 

 which are also included in the peptonizing group. The specific 

 pathogenic organisms, possibly with the exception of Bact. tubercu- 

 losis, get into milk, either directly or indirectly, from human patients 

 suffering with the particular disease. 



FACTORS INFLUENCING THE DEVELOPMENT OF MICROORGANISMS IN 



MILK 



The number of microorganisms found in fresh milk shows its bac- 

 terial condition at that time ; but it gives little idea of the organisms 

 which may be found in the same milk at later periods. There are 

 many factors to be considered if we wish to study the development 

 of the various types which get into ordinary milk. These factors 

 may be considered briefly under the following heads: 



INITIAL CONTAMINATION. Fresh milk varies widely in the number 

 of organisms which it contains as a result of the conditions under 

 which it has been produced. There are differences not only in the 

 numbers of organisms but also in the species which may be found in 

 different samples of fresh milk. Both of these factors are important 

 in the later changes which may take place. The effect of numer- 

 ical initial contamination may be seen in the following tables where 



