Bacteria, Helpful and Otherwise 441 



resume their activities and give evidence of life. The resist- 

 ance of these spores to adverse conditions is so great that they 

 are able to withstand high temperature and the action of strong 

 chemicals. 



Bacteria are present practically everywhere, in the air, in the 

 water, and in the earth. They exist in the fluids and tissues of 

 plants, animals, and man. Some forms are harmless and appar- 

 ently of no special value to man; other forms are of the great- 

 est service to him; and still others are very harmful, causing 

 dangerous communicable diseases among animals and man and 

 much injury to vegetable life. 



Other forms of plant life which resemble bacteria in many 

 respects are molds. The molds are usually visible to the naked 

 eye and grow in the form of threads. They form on moist foods 

 and aid in the process of decay. Some molds are useful in devel- 

 oping flavor in certain kinds of cheese. 



Experiments. A single bacterium cannot be seen with the 

 naked eye. They can, however, be seen in masses or groups, 

 called colonies. For scientific study, cultures or growths of 

 bacteria are made on mediums, substances on which bacteria 

 thrive, such as gelatin or agar-agar. The medium used and 

 the test tubes or the flat saucers, called Petri dishes, which con- 

 tain it, must be sterilized, or made free from germs, usually 

 by heat. The containers must be kept plugged or covered until 

 needed. 



To prepare an agar-agar medium, add to 1000 grams of hot 

 distilled water 10 grams of agar-agar, 5 to 10 grams of beef 

 extract, 10 grams of peptone, and 5 grams of salt. Boil the 

 mixture. Filter the mixture through absorbent cotton into ster- 

 ilized test tubes and seal the tubes. Each tube should be about 

 one-quarter full. When required for use, melt the contents of 

 a tube in a double boiler and pour while hot into sterilized 

 Petri dishes or test tubes. A gelatin medium may be formed 

 by substituting 100 grams of jello or French gelatin for the 

 agar-agar. 



To Show the Growth of Bacteria and Molds in Air. 

 Use four test tubes containing gelatin or agar-agar. Set aside 



