254 ESSENTIALS OF BACTERIOLOGY 



Neutral Forms. Many species of air and chromogenic 

 varieties found in milk have no pathogenic properties, 

 neither do they affect the composition of the milk. 



Injurious Organisms. Human diseases, like typhoid, 

 diphtheria, and scarlet fever, may be conveyed through 

 milk, the infection coming from some one concerned in 

 handling the particular supply. The milk acts as a favorable 

 medium for the pathogenic organisms that accidentally find 

 their way into it. Animals wading in infected water have 

 infected the milk. Utensils washed in polluted water have 

 been found to be the cause in some epidemics of typhoid. 

 Carriers, persons who harbor the diphtheria and typhoid 

 bacteria, but who are not affected with illness, may likewise 

 start epidemics of a kind, especially if working about dairies. 

 Bacteria may enter milk from the animal, as Bacillus tuber- 

 culosis from diseased udder. Infantile diarrheas from the 

 putrefactive Bacillus coli group, streptococcic sore throat 

 from udder disease, are other forms of disease originating in 

 milk. 



Butter and Cheese. Butter is milk-fat separated by 

 creaming and churning, and as such partakes somewhat of 

 the bacterial nature of the milk from which it is derived. 

 The flavor of butter is due to the character of the acid bac- 

 teria used in souring the milk. By eliminating the gas-form- 

 ing bacteria and by keeping his starting cultures pure the 

 butter-maker can control and develop flavors as easily as 

 the wine-maker. Pure cultures of lactic acid are supplied 

 to butter-makers and used in creameries to inoculate sweet 

 cream and milk. Bacteria coming from unclean utensils, 

 polluted water, or dirty milk undoubtedly affect the flavor 

 and often produce a poor quality of butter. Disease bacteria 

 are not often conveyed through butter, although it is claimed 

 that Bacillus tuberculosis has been found in salted butter. 



Cheese. The fat and casein salts and sugar-of-milk sepa- 

 rated by curdling from the bulk of soluble portion of milk 

 constitutes cheese. The curdling is accomplished by acid 

 bacteria normally in milk, so-called acid curd cheeses, or by 



