468 MICROBIOLOGY OF MILK AND MILK PRODUCTS 



inhibiting action on certain undesirable organisms the Bact. lactis 

 acidi type of bacteria must be regarded as beneficial organisms, and 

 from the standpoint of the health of the consumer their presence in the 

 milk is to be welcomed rather than discouraged. As the value of 

 sour milk drinks becomes better known the importance of this group 

 of milk bacteria will be more fully recognized. 



Neutral or Inert Forms. In ordinary milk there is a large class of 

 bacteria which, so far as known, have no appreciable effect either 

 upon the composition of the milk or the health of the persons consuming 

 it. This group includes a number of species, many of them being 

 coccus forms, some of them appearing in plate cultures as chromogenic 

 colonies. They grow more or less freely in milk, depending upon the 

 conditions, but they are usually held in check by the acid-forming bac- 

 teria and do not constitute a very important part of the flora of normal 

 milk. They are, therefore, of little significance from the practical 

 standpoint except as they indicate the conditions under which the milk 

 has been produced and handled. 



Injurious Organisms. The diseases which may be carried by milk 

 are of two classes. 



Epidemic Diseases. The human diseases most commonly carried 

 by milk are typhoid fever, diphtheria and scarlet fever and occasionally 

 other diseases such as septic sore-throat, cholera and foot-and-mouth 

 disease. The first three are by far the most important of this group. 

 The outbreaks of typhoid fever which are traceable to milk occur 

 most frequently. There is a large accumulation of data showing the 

 occurrence of epidemics caused by infected milk. An epidemic caused 

 by the milk supply has certain characteristics which distinguish it 

 from epidemics resulting from other causes. A considerable number 

 of cases of the particular disease will appear almost simultaneously 

 and will be distributed along some particular milk route. Usually 

 the epidemic stops as suddenly as it began except for a few secondary 

 cases contracted from those first taken. The source of the disease 

 organisms is a human patient suffering from the disease. The infection 

 of the milk may be direct, as when a sick person handles a milk, 

 or it may be indirect as when a person caring for a patient also works 

 about the milk. In other cases it may be caused by contamination 

 of the water used in washing the utensils or by cows wading in water 

 of infected streams and getting the organisms on their body whence 



