CHAPTER XXX 



MICROBES AND THEIR CONTROL 



Questions, 1. Of what use are microbes ? 2. Can we make them in- 

 crease in numbers if we wish to? How? 3. How can we make them 

 diminish in numbers ? 4. How do microbes cause disease ? 5. How are 

 diseases carried ? 6. How can we keep microbes from causing disease ? 

 7. How can we keep diseases from spreading ? 8. How can we find out 

 under what conditions bacteria thrive and under what conditions they 

 die out ? 9. How can we tell whether a method of treating microbes 

 produces the results that we wish ? 



236. Bacteria. One-celled plants and one-celled animals are 

 sometimes spoken of collectively as microbes, which means 

 sinall living things. Occasionally one hears the name germs ap- 

 plied to the microbes that are known to cause disease. Germ 

 suggests something out of which life can develop. We have the 

 same expression applied to the embryo of a seed (see page 67), 

 and to the reproductive cells of plants and animals (p. 512! 

 Chap. XLIII). Germs of disease are structures, or bodies, that 

 have been found by strict experimentation to give rise to more 

 like themselves, and to bring about a diseased condition in the 

 organism in which they live. 



Bacteria are among the simplest plants. Each individual 

 {bacterium) consists of a single cell, with fairly distinct cell 

 wall but lacking chlorophyl. There are three main groups of 

 bacteria, classified according to the general form of the cell. 

 The bacilli, or rod-shaped bacteria, include those that cause 

 tuberculosis, typhoid, and diphtheria. The cocci (kok'si), or 

 ball-shaped bacteria, include those that cause pneumonia and 

 pus. The spirilla, or spiral-shaped bacteria, include those that 

 cause relapsing fever (see Fig. 146). 



The bacteria have no special organs for receiving food, and 

 the cell wall permits no solid particles to come through, as is 



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