220 . CIVIC BIOLOGY 



Reproduction in bacteria. Bacteria multiply by division, 

 which is even more simple than the buckling of yeast. The 

 cell, when mature, divides transversely into equal halves. 

 Under favorable conditions a bacterium may divide every 

 twenty minutes to half an hour. Can you calculate the 

 progeny of a single bacillus for twenty-four hours? 



Bacteria do not grow and reproduce without food, and their 

 astonishing power of multiplication helps us to understand the 

 altered condition of milk and meat if kept in a warm place for 

 even a few hours. 



Some species develop spores within the cell and these are 

 much more difficult to kill than the bacteria themselves. 



Conditions favorable for the multiplication of bacteria. Like 

 other plants, bacteria demand food, moisture, oxygen, and 

 warmth for growth. Remove any one of these conditions and 

 they will either cease to multiply or die. 



Moisture. Bacteria grow only in liquids or moist sub- 

 stances. Dry foods and those containing less than 20 or 30 

 per cent of water they cannot attack. Drying weakens and 

 kills many bacteria. Spores, however, are much more resis- 

 tant to continued drying than the vegetative or growing cell. 



Why should houses not be allowed to become damp? Why is meat 

 salted and dried? Why is canned fruit sealed? What influence has 

 sugar in preserving fruit? Why are such foods as molasses, condensed 

 milk, flour, seeds, and grain bacteria-proof? 



Temperature. Temperature affects growth of bacteria. As 

 in higher plants, there is a temperature known as the optimum 

 at which each species thrives best. A tubercle bacillus grows 

 within a range of 5 degrees, while a few other species can 

 grow anywhere within a range of 50 degrees. 



Bacteria do not multiply during the time they are exposed 

 to low temperature, but their vitality is not affected ; the tu- 

 bercle bacillus has been exposed to a temperature of liquid air 



