CHAPTER II. 

 PHYSIOLOGY. 



9. Conditions essential for growth. The growth 

 of bacteria, like all other living organisms, bears a direct 

 relation to their external surroundings. Certain condi- 

 tions are absolutely essential before life can develop. 

 Other conditions though often advantageous are not of 

 such vital importance. 



10. 1. Food-supply. Concerning the character of 

 the food- supply necessary for different species there is a 

 very great difference. Many of the disease- producing 

 forms are very particular in the selection of their food. 

 With those forms that are usually found in milk, such 

 delicacy of choice is not common. 



A food substance to be available for bacteria must be 

 in solution, as it must pass through the cell-wall by 

 absorption. Bacteria can live upon solid substances, but 

 they form chemical substances that enable them to render 

 soluble the necessary food material. 



The essential food elements that must be present are 

 nitrogen, carbon, and oxygen, together with minute quan- 

 tities of mineral elements. The nitrogen and carbon are 

 more available when in the form of organic compounds 

 than as simple inorganic salts. Albuminous or proteid 

 substances are best adapted for the nitrogen supply, while 

 sugars are available for the carbonaceous part of the 

 food. The nitrogenous element is, however, the most 

 indispensable. 



Concentration of medium. If the fluid is too dense, 

 bacteria cannot grow. Condensed milk or syrup is too 



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