132 BACTERIA, YEASTS, AND MOLDS 



in our houses, a fact of much importance in connection 

 with the problem of food preservation. 



Acidity. For still another reason molds and bacteria 

 do not commonly flourish upon the same material. The 

 former, as we have seen, grow best upon acid substances; 

 but most bacteria cannot endure acids, preferring a slightly 

 alkaline food. Hence fruits, which are acid, decay by 

 molding, while meats, which are not so acid or are alka- 

 line, decay by bacterial action. The presence of acid or 

 sourness in food will check its decay. Some food (cran- 

 berries) may be actually too sour for bacteria growth. 



Where Bacteria may be found 



We may almost say they are to be found everywhere 

 upon the surface of the earth. This is not strictly true, 

 since a few places seem to be free from them ; for exam- 

 ple, the middle of deserts and the bottom of the deep 

 oceans. But wherever on the surface of the earth ani- 

 mals or plants are found, there, in the earth, the air, and 

 all bodies of water, are also found bacteria. 



Air. Bacteria are so extremely minute that they are 

 capable of floating in the air for a long time and of being 

 blown by the winds almost indefinitely. Consequently it 

 is almost impossible, at least in inhabited localities, to 

 find any air that does not contain them. The number 

 that may be present in the air varies with the density of 

 human population. We find them more abundant in city 

 than in country air ; more abundant, as a rule, in houses 

 than out of doors ; more abundant in the air of rooms 

 well filled with people than in empty rooms, since they 



