Structure of the Bacteria. 5 



and suitable food supply and with moisture. The tem- 

 perature must also range between certain limits, and finally, 

 the oxygen requirements of the. organism must be con- 

 sidered. 



Food supply. Most bacteria are capable of living on 

 dead, inert, organic. matter, such as meats, milk and vege- 

 table material, in which case, they are known as sap- 

 rophytes. In contradistinction to this class is a smaller 

 group known as parasites, which derive their nourishment 

 from the living tissues of animals or plants. The first 

 group comprise by far the larger number of known organ- 

 isms which are concerned for the most part in the decom- 

 position of organic matter. The parasitic group includes 

 those which are the cause of various communicable diseases. 

 Between these two groups there is no sharp line of division, 

 and in some cases, certain species possess the faculty of 

 growing either as parasites or saprophytes, in which case 

 they are known as facultative parasites or saprophytes. 



The great majority of bacteria of interest in dairying 

 belong to the saprophytic class ; only those species 

 capable of infecting milk through the development of dis- 

 ease in the animal are parasites in the strict sense of the 

 term. Most disease-producing species, as diphtheria or 

 typhoid fever, while parasitic in man lead a saprophytic 

 method of life so far as their relation to milk is concerned. 



Bacteria require for their growth, nitrogen, hydrogen, 

 carbon, oxygen, together with a limited amount of 

 mineral matter. The nitrogen and carbon are most avail- 

 able in the form of organic compounds, such as albuminous 

 material. Carbon in the form of carbohydrates, as sugar 

 or starch, is most readily attacked by bacteria. 



Inasmuch as the bacteria are plant-cells, they must im- 



