CHAPTER XLII 

 THE BIOLOGY OF BACTERIA 



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Of all the groups of non-green plants, the ever-present bacteria and 

 their effects are most often discussed among educated people every- 

 where. Although the great majoritv of the human race has never seen 

 bacteria, every individual has had both direct and indirect contacts with 

 them. The forms of some of the common species of bacteria are shown 

 in Fig. 229. They are the causes of many diseases and of much of the 

 decay of organic substances. Certain pleasant and unpleasant flavors of 

 foods such as cheese, milk, butter, and eggs result from bacterial action, 

 and bacteria influence our lives in many other ways. As a matter of 

 fact, household sanitation, sewage disposal, refrigeration, quarantine, 

 vaccination, aseptic surgery, personal hygiene, and all of our splendidly 

 organized and vitally necessary methods of guarding and purifying water 

 have developed as we have learned how and where bacteria live and 

 how they affect plants and animals, including ourselves. 



Some of the factors affecting growth of bacteria are water, tempera- 

 ture, certain gases, mineral salts, light, and food. Such factors also 

 influence the growth of green plants, but not necessarfly in the same way 

 or to the same degree. A particular species of bacteria does not grow 

 in all environments, but the diversity of species is so great that there 

 are few habitats in which some kinds of bacteria do not live. Certain 

 species of bacteria are widely distributed over the earth. Others live 

 only in very specific habitats. For example, some grow only in certain 

 species of plants and animals, and sometimes only within certain tissues 

 of the host. 



Some bacteria manufacture their own food; others secure it from liv- 

 ing or non-living sources. A few species are unique in that they can 

 synthesize sugar and other foods from water, carbon dioxide, and in- 

 organic salts. Most species, however, depend upon an external source of 

 sugar, and sometimes of protein also. At one end of the scale are the 

 bacteria that derive energy from light, or from compounds containing 

 reduced nitrogen, iron or sulfur, and synthesize sugars and other foods. 



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