SYSTEMATIC CLASSIFICATION 17 



biochemical studies have been carried out with members of 

 the order Euhacteriales in which the organisms exist as 

 separate individuals and do not show any form of mycelium 

 or filaments. The order is subdivided into families, partly 

 on a morphological basis and partly on a chemical basis. 

 Eleven of the families differentiated by Bergey are included in 

 Table I. The Nitrobacteriaceae comprise organisms which can 

 carry out an oxidation of inorganic material as source of energy. 

 The Azotohacteriaceae and the Rhizobiaceae are both capable 

 of utilising atmospheric nitrogen as nitrogen source, the latter 

 carrying out the fixation process only when living in symbiosis 

 with a host-plant (see Chap. X). The Micrococcaceae are 

 usually Gram-positive spherical organisms, while the Gram- 

 negative coccal organisms are placed in the Neisseriaceae ; the 

 spherical organisms which divide to form chains, the Strepto- 

 cocci, are not included in these groups but are classed in the 

 Lactobacteriaceae since they carry out a simple lactic acid 

 fermentation of glucose, and generally resemble the Lacto- 

 bacilli in their nutritional and chemical characteristics. The 

 rod-shaped organisms are divided into several families: the 

 Bacillaceae being those organisms which can form spores under 

 suitable conditions, the non-sporing rod-shaped organisms are 

 subdivided and most of the organisms of this group with which 

 we shall be dealing in this book are classed in the Entero- 

 bacteriaceae which comprises many of those bacteria normally 

 found in the intestinal flora. 



TRIBES AND GENERA 



In many cases families include organisms which fall clearly 

 into sub-groups. Thus the Lactobacteriaceae are subdivided 

 into spherical organisms, Streptococdeae, and rod-shaped 

 organisms, Lactobacilleae. Again, within the family Nitro- 

 bacteriaceae, we have organisms using inorganic nitrogenous 

 substances as oxidation substrate, the Nitrobacterieae, and 

 other organisms, obtaining energy from the oxidation of 

 sulphur compounds, which are consequently placed in a 



cHEM. A. B. 2 



