MICROORGANISMS AND HUMAN WELFARE 23 



IV. BACTERIA AS THE FOES OF MAN 



21. Injurious effects of bacteria. Most of the common 

 bacteria are either harmless or distinctly beneficial to man- 

 kind (1&-20). The experiments we tried with milk (17 ,B), 

 however, show that this kind of food soon sours unless it is 

 kept in a very cold place. Every housekeeper knows also 

 that meat and many other kinds of food quickly spoil if 

 they are not cooked or otherwise preserved. In a following 

 section we shall consider some of the methods that are used 

 to prevent this decaying action of bacteria. 



Unfortunately, too, there are certain germs 1 that find 

 favorable conditions for growth in living animal tissue, and 

 by their growth cause certain diseases, some of which are 

 tuberculosis, diphtheria, and typhoid fever. In later sections 

 we shall learn that these disease-producing bacteria are all 

 too common in dust, water, and foods ; but we shall likewise 

 see that scientists are fast learning effective methods of 

 preventing the ravages of these disease-producing bacteria, 

 which are called by Dr. Prudden " Man's Invisible Foes." 2 



22. Methods of food preservation. We saw in (17, A 

 and C) that bacteria thrive whenever they can get plenty of 

 food and moisture, and whenever the temperature is favor- 

 able for their growth. We also learned that, whenever any 

 one of these necessary conditions is wanting, bacteria 

 cease to carry on their functions. If, then, we wish to 



1 Disease-producing bacteria are commonly spoken of as germs or 

 microbes. 



2 In general it is unwise and unnecessary that boys and girls 

 should be taught much regarding the symptoms and effects of dis- 

 ease ; but since so much may be done to prevent these diseases that 

 we have mentioned and others that afflict mankind, it is essential 

 that the young should learn something of the deadly work of some 

 of the germs which are all too common. 



