CHAPTER IV 

 BACTERIA 



By EDWIN O. JORDAN 

 Professor of Bacteriology in the University of Chicago 



Bacteria are unicellular organisms, for the most part very 

 small. Considerable differences in size, however, are observed. 

 A certain large, rod-shaped species studied by Schaudinn measures 

 from 50/i to 6oai in length and from 4yLi to 5^1 in width. On the 

 other hand the bacillus of influenza averages about 0.5^ in 

 length and 0.2 ix in width. The average rod-shaped bacterium, 

 such as is found in water and soil, measures about 2/i in length 

 and about 0.5/^ in diameter. Some microorganisms are known 

 to exist which are so small that they will pass through the pores of 

 the finest Berkefeld filter and remain invisible under the most 

 powerful lenses, but it is not surely established that all these so- 

 called ultramicroscopic organisms belong to the group of bacteria. 



For the methods of studying bacteria, special laboratory man- 

 uals or guides should be consulted. A number of such guides are 

 in existence, among which may be mentioned Heinemann (191 1) 

 and Frost (1905). In any case a proper familiarity with laboratory 

 methods can be gained only with the assistance of a skilled labora- 

 tory instructor possessed of individuality and resource. 



Bacteria are generally classed as plants rather than animals, 

 but, as is well known, the dividing line between animals and plants 

 is an entirely arbitrary one, and there is no general agreement 

 among naturalists respecting what shall constitute a determina- 

 tive plant or animal characteristic. It is largely considerations of 

 convention and convenience that place them among the plants. 

 From their lack of chlorophyl and the fact that they multiply by 

 division or fission the bacteria are classed as Schizomycetes or fission 

 fungi. 



Within the group of bacteria themselves classification is, for 

 practical purposes, especially important, but because they are so 



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