CHAPTEE III. 



BACTERIA GENERAL CONSIDERATIONS MORPHOLOGY. 



Definition. Bacteria (singular bacterium) are very minute, uni- 

 cellular, vegetable microorganisms, of round, cylindrical, or spiral 

 shape, motile or immotile, which perform their nutritive function 

 without the aid of chlorophyl, and which multiply very rapidly by 

 binary division or fission. 



Position among Organisms. Bacteria have been placed as an inter- 

 mediary phylum between animals and plants by some biologists and 

 by others they have been classified as the lowest forms of animal 

 life, because many are motile. If we consider their mode of nutri- 

 tion without the aid of the chlorophyl of the higher plants and their 

 whole metabolism we find that they are nearest related to the higher 

 fungi. Their most logical classification is, therefore, among the plants. 

 On account of their mode of multiplication by division they are also 

 called fission fungi, or schizomycetes. Bacteria do not, like the higher 

 plants, show any differentiation into root, stem, or leaves, but consist 

 generally of a very simple single cell. 



Types. When bacteria were first studied by botanists and biolo- 

 gists in general (as, for instance, Naegeli and Zopf) it was believed 

 that they were very variable in form, and that one and the same 

 species might present itself alternately in ball-, rod-, or screw-form. 

 The botanist F. Cohn was the first to claim that this was an erroneous 

 impression, and that bacteria, like higher plants, were constant in 

 shape. This was proved beyond doubt by Robert Koch and his 

 followers. For the purpose of obtaining so-called pure cultures of a 

 single species of bacteria, Koch for the first time devised and used 

 solid culture media, which has enabled us to show beyond doubt that 

 bacteria are constant in form, and that each type only reproduces 

 itself. Three main types of bacteria are distinguished: 



1. The coccus (plural cocci). 



2. The bacillus (plural bacilli). 



3. The spirillum (plural spirilla). 



The coccus is a ball or spherical-shaped bacterium. The bacillus is a 

 cylindrical rod-shaped bacterium very much like a short, round lead 

 pencil. The spirillum is spiral or corkscrew-like in shape. It may 

 consist of a portion of a screw winding, or it may show several twists, 

 giving it the appearance of a complete corkscrew. In the former 

 case we speak of a vibrio, while in the latter we designate the complete 

 spiral as a spirillum (plural spirilla), or, better still, as a spirochete 

 (plural spirochetci). 



