WHY DO FOODS SPOIL? 



337 



Colonies of bacteria growing on culture 

 media in a Petri dish. 



decay. This can be easily done by a simple experiment. 



It has been found by scientists that bacteria grow well in 



a medium made by cooking 



beef broth with either gelatin 



or agar, a substance obtained 



from a Japanese seaweed. 



It is poured, while still boil- 

 ing hot, into small Petri 



dishes. These are glass 



dishes (see picture, page 336), 



which have loosely fitting, 



overlapping covers. Then 



the dishes and their contents 



are heated in order to kill all 



life that might exist in them. 



After one of these dishes has cooled, it is exposed to 



the air for a very short time and then covered and put 



in a warm place. Another dish containing media which 



was not exposed is left as a control. If these dishes are 



left in a warm place for two or three days, spots appear 

 on the surface of the culture medium 

 of the dish that was exposed. 

 These spots may be gray, orange, 

 brown, or even red in color. If a 

 tiny speck from one of these spots is 

 removed and examined under a high- 

 powered microscope, it will be found 

 to be made up of great numbers of 

 spirilla tiny rod, spiral, or ball-shaped bodies. 

 These bodies have been proven by 

 scientists to be bacteria. If some of 

 these bacteria are placed on food, 

 they will develop rapidly into more 



Three ^ forms of Bacteria. colonieg of bacteria of the Same kind 



and the food will spoil. Thus we 



bacilli 



Are all exactly alike within 

 a given group ? 



H. & w. sci. I 23 



