RELATION TO ENVIRONMENT I2Q 



able amount of food substance. On the other hand, the ex- 

 cess of food is not without its effect. It is impossible for 

 many of the bacteria to live in foods that are too concentrated. 

 Reference has already been made to the fact that when the 

 bacterial cell is surrounded by a substance that has greater 

 osmotic pressure than the protoplasm of the cell the cell is 

 plasmolyzed. Bacteria are, of course, unable to grow under 

 these conditions. If, then, their food substance is too con- 

 centrated, they are unable to make use of it, and this fact is 

 taken advantage of in the preservation of a number of food 

 substances. Condensed milk, for instance, keeps because 

 the bacteria in it are unable to make use of the food which is 

 present there in a too concentrated form. The same thing 

 is true of sirups. At times chemical substances are added 

 which increase the osmotic pressure, as is true in the case 

 of brine. 



Chemical Composition of Food. Food must contain cer- 

 tain chemical elements. Those which are necessary are carbon, 

 oxygen, hydrogen, nitrogen, phosphorus, sulphur, iron, and cal- 

 cium. No substance is valuable as a food which does not fur- 

 nish the necessary chemical elements. It is true that not all 

 substances furnishing the necessary elements are necessarily 

 good food substances. It is said that one man's food is an- 

 other man's poison. So among bacteria ; one substance may 

 serve as a medium for the development for one kind of bac- 

 teria and be absolutely prohibitive to the growth of another. 



Water. Bacteria require water for growth. They may in 



