CHAPTER XXXVIII \- 

 BACTERIA 



Photomicrograph of spirilla. Photomicrograph of cocci. 



What are parasitic bacteria? How can bacteria be studied? What 

 conditions favor the growth of bacteria? What is the relation of bac- 

 teria to food preservation ? 



Bacteria possess no chlorophyll. They cannot make their own 

 food ; they are dependent upon other organisms for their nour- 

 ishment. They, with many other plants lacking chlorophyll, be- 

 long to a subdivision of Thallophyta. All bacteria are not harm- 

 ful. Life on earth would probably cease were it not for the 

 activities of certain bacteria. The disease-producing bacteria are 

 known as pathogenic types. 



Structure of bacteria. When unstained, bacteria are colorless, 

 transparent cells with comparatively thick cell walls. The nu- 

 cleus is not organized, but nuclear material is scattered through 

 the cytoplasm. Sometimes the cell wall absorbs water, becomes 

 swollen, and forms a mucilaginous mass. The " mother of vine- 

 gar " represents a mass of cells of this type. Another example is 

 the mass of mucilaginous material that sometimes clogs the drain- 

 pipe of the ice box. In other types, the wall becomes gelatinous 

 and thick, and forms a capsule around the individual bacterium. 

 In certain bacteria, small granules are found, which are probably 

 reserve supplies of food. These particles absorb dyes readily and 

 hold the stain very effectively. Some bacteria have one or more 

 whiplike projections of protoplasm called flagella. They are the 



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