Ckapter 24 



BACTERIA AND DISEASE 



Bacteria are probably the most universally distributed of all 

 plants. They are especially abundant in the air where they are 

 carried on dust particles, in water, in the alimentary tract of the 

 human body, and in the upper layers of soils, where certain 

 forms play an important part in soil fertility (see Chapter 5). 

 Since bacteria are colorless plants, they must depend upon 

 organic foods for their energy. Many species utilize dead plant 

 or animal materials for their nourishment, but there are likewise 

 many which attack living plants or animals, causing charac- 

 teristic diseases of those organisms. These latter are known as 

 plant, or animal, pathogens, or are referred to merely as patho- 

 genic bacteria — meaning disease producing individuals. 



If body size were a criterion of potency, the bacteria could 

 be entirely neglected in this discussion, since they are the smallest 

 members of the plant kingdom, the largest bacteria being about 

 one-twenty five hundredth of an inch long and one-twenty five 

 thousandth of an inch wide. A billion of the smaller bacteria 

 could be contained in a single drop of water; and about one 

 hundred and twenty five thousand, placed end to end would 

 form a line one inch long. Bacteria are single-celled plants 

 (see pp. 45-46) which reproduce by simple fission. They are 

 capable, under favorable conditions, of dividing as often as once 

 every twenty minutes. Thus an enormous bacterial population 

 may result from a single parent cell at the end of twenty four 

 hours. Fortunately, the bacteria poison themselves by their own 

 waste products without reaching such numbers, but it is easy to 

 see how food may spoil quickly, or disease strike suddenly, with 

 the great potential reproductive capacities of these organisms. 



Bacteria may be classified into three groups on the basis of 

 shape; the rod-shaped bacteria or bacilli, the spherical bacteria 



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