HOW PLANTS BENEFIT AND HARM MANKIND 177 



needle point. Then the needle, which cools very quickly, is dipped 

 in a colony containing the bacteria we wish to study. This mass of 

 bacteria is quickly transferred to another sterilized plate, and this 

 plate is immediately covered to prevent any other forms of bacteria 

 from entering. When we have succeeded in isolating a certain 

 kind of bacteria in a given dish, we are said to have a pure culture. 



Bacteria cause Decay. Bacteria in several ways, either directly 

 or indirectly, affect mankind. First of all, they cause decay. All 

 organic matter, in whatever form, is sooner or later decomposed by 

 the action of untold millions of bacteria which live in the air, water, 

 and soil. To a considerable degree, then, these bacteria are useful 

 in feeding upon the dead bodies of plants or animals, which other- 

 wise would soon cover the surface of the earth to the exclusion of 

 everything else. Bacteria may thus be scavengers. They oxidize 

 organic materials, changing them to compounds of nitrogen that 

 can be absorbed by plants and used in building protoplasm. 

 Without bacteria and fungi it would be impossible for life to exist 

 on the earth, for green plants would be unable to get the raw food 

 materials informs that could be used in making food and living mat- 

 ter. In this respect they are of the greatest service to mankind. 

 When bacteria grow in sufficient numbers upon foods, meat, 

 fish, or vegetables, they spoil them, and may form poisonous sub- 

 stances called ptomaines. Such substances are formed as waste 

 products by the bacteria, and are given off into the material in 

 which the bacteria are living. Thus we, upon eating the food con- 

 taining these poisons, may become violently ill as the result of 

 ptomaine poisoning. Fish and meats that have been kept for 

 some time in cold storage are very easily spoiled, and should be 

 avoided. Jars of canned goods that have " worked," that is in 

 which bacteria or yeasts have caused fermentation, are often unfit 

 for food. 



Relation to Fermentation. They may incidentally, as a result 

 of this process of decay, aid in the process of fermentation. In 

 making vinegar the yeasts first make alcohol (see p. 172), vvhich 

 the bacteria change to acetic acid. The lactic acid bacteria 

 which sour milk, changing the milk sugar to an acid, grow very 

 rapidly in a warm temperature; hence milk which is kept cool or 

 which is pasteurized (that is, kept at a temperature of about 176 



HUNT. ES. BIO. 12 



