1 8 PRACTICAL DAIRY BACTERIOLOGY 



Relation to Moisture. Bacteria will grow only in the pres- 

 ence of considerable quantities of moisture; indeed, they de- 

 mand more moisture than most organisms. They will hardly 

 grow at all unless there is 30 per cent, of moisture in the ma- 

 terial in which they are living, and even then the growth is 

 slow. On the other hand, they flourish most luxuriantly in 

 localities where the water is from 90 to 100 per cent. Hence, 

 as materials dry, bacteria will cease to grow in them, and 

 any substance that can be dried can be thoroughly protected 

 from their action. This explains why dried fish and dried 

 meat, fruits, dried milk, etc., will keep indefinitely. The drying, 

 however, does not actually kill the bacteria, for although they 

 do not grow when the water is extracted from them, they may 

 remain alive for weeks, months, or even years. In other 

 words, it is impossible to depend upon drying as a means of 

 destroying bacteria, for while many individuals will fail to live, 

 many others do not seem to be injured at all by the drying, 

 and are capable of resuming life again as soon as they find 

 moisture. 



Relation to Food. Bacteria are just as dependent upon food 

 as are larger organisms, and they feed upon a greater variety 

 of materials, perhaps, than any other organisms. There are 

 some that appear to be capable of feeding upon what is called 

 inorganic food; that is, upon mineral materials. The vast ma- 

 jority of bacteria feed upon organic material, by which is 

 meant substances that ha\e been produced by other animals or 

 plants. Their relation to food is very much like that of the 

 animal kingdom, and in general they feed upon the same kinds 

 of food that animals do : meats, gluten, milk, eggs, etc. It is of 

 importance to note, however, that whereas they feed with great 

 readiness upon proteid foods, like the lean part of meat, white of 

 eggs, gluten of wheat, legumen of beans, peas, etc., they do 

 not feed, as a rule, upon sugar or starches. Yeasts, on the 

 other hand, are adapted to feed upon sugars. We find, there- 



