GROWTH OF BACTERIA 399 



Bacteria are probably found on all cooked foods which have, 

 been exposed to the air, as well as on the surfaces of all fruits 

 and vegetables. 



The relation of various conditions to the growth of bacteria. 

 Most bacteria thrive best in darkness. Direct sunlight kills them 

 after a few hours of exposure. Even spore forms may be killed by 

 direct sunlight. The food of bacteria must be organic and must 

 be slightly moist. The best temperature for bacterial growth on 

 culture media is between 20 and 40 C., although bacterial life 

 is possible at absolute temperature ( 273 C.) and at 160 C. 

 The temperature varies with the particular type of organism. For 

 example, the human tuberculosis bacillus grows best at blood 

 heat, 37.5 C. ; the bird tuberculosis bacillus grows best at about 

 42 C. Sudden changes in temperature are more or less detrimen- 

 tal to bacteria. If changes are slow, the spore formers will have a 

 chance to form their spores. Cold checks the growth of bacteria 

 and continued freezing or alternate freezing and thawing may 

 sometimes kill them. Different bacteria are killed at different 

 temperatures. Active typhoid bacteria are killed when subjected 

 to a temperature of 60 C. for thirty minutes. Ordinarily, all 

 bacteria are killed at 100 C. Moist heat is found more effective 

 for killing bacteria than dry heat. 



Methods of food preservation. Substances may be freed from 

 bacteria, or the action of the bacteria on the substance may be 

 materially decreased by depriving the organisms of some condi- 

 tion that is necessary for their life or growth. Some of the 

 methods of preserving foods against the action of bacteria are 

 dehydration, refrigeration, canning, smoking, and addition of 

 preservatives. 



If a food is thoroughly dried, particularly by the sun, the bac- 

 teria cannot thrive. The sun, in all probability, will kill them. 

 Dryness is also an unfavorable condition for the growth of bac- 

 teria. Refrigeration preserves foods because cold checks the 



