THE BACTERIA AND VIRUSES 349 



3. Chlamydobacteriales. — Cells arranged in chains with an investing 



sheath. 



Chlamydobacteriaceae. The only family, the so-called " Iron 

 Bacteria," e.g., Cladothrix. 



4. Thiobacteriales.—The " Sulphur Bacteria." Fornn sulphur 



granules in the cells. 



(i) Rhodobacteriaceae. Purple Bacteria, e.g., Chromatium. 



(2) Beggiatoaceae. Filamentous, no pigment, e.g., Beggiatoa. 



(3) Achromatiaceae. Unicellular, no pigment, e.g., Thiophysa. 



5. Myxobacteriales. — Cells grouping themselves into variously shaped, 



macroscopic fruiting bodies, in which the rods become encysted 

 in groups. 



Myxobacteriaceae. The only family, e.g., Myxococciis, Poly- 



anguim. 



6. Spirochaetales.-~Thin flexuous filaments with terminal flagella or 

 an undulating membrane attached along the body. Parasites. 



Spirochaetaceae. The only famih', e.g., Treponema pallidum, the 

 causative organism of Syphilis. 



Metabolism 



It has long been realized that the microscopical appearance of these 

 very simple organisms is insufficient to distinguish species from one another, 

 so that the identity of a bacterium rests on its biochemical characteristics 

 rather than on its morphology. These properties are not always constant, 

 however, which has led to many difficulties, and the whole question of the 

 constancy of bacterial species is still siibjiidice. Few things are more surpris- 

 ing than the extraordinary range of chemical reactions which can be displayed 

 by the minutest and seemingly least difl'erentiated of organisms. A single 

 species, Pseudomonas putida, has been shown to assimilate as many as seventy- 

 seven different substances as sources of Carbon. It is this chemical activity, 

 in conjunction with their almost universal distribution in nature, which 

 gives Bacteria their peculiar importance. 



Among the most characteristic activities is fermentation, and it is upon 

 tests of this power as applied to various sugars, polysaccharides, glucosides 

 and alcohols that the discrimination of species is usually based. The 

 organisms are grown in a culture fluid to which the fermentable substance 

 and an indicator are added. A small inverted tube is immersed in the fluid 

 to act as a gas-trap, and if fermentation occurs it is shown by the production 

 of acid, with or without gas as well. 



Bacteria var\- widely in their power of synthesizing their food materials, 

 and they may be divided into three classes in this respect. 



I. Bacteria which can utilize ammonia as a source of Nitrogen and 

 simple carboxylic acids as sources of Carbon. Some of the 

 commonest soil organisms are in this group, e.g., Escherichia coli 



