18 THE NATURE AND IDENTIFICATION OP BACTERIA 



separate " tribe " called the Thiohacilleae. The Entero- 

 bacteriaceae are divided into a number of tribes, some of which 

 are shown in Table I; the divisions in this case are based on 

 less well-defined characteristics and are still the subject of 

 debate amongst taxonomists. 



The next subdivision of the tribe is into genera. Again the 

 basis of subdivision may be morphological as in the separation 

 of Streptococci which are spherical organisms dividing about 

 one axis to form long chains, from Diplococci in which the 

 organisms occur in pairs rather than in chains. The separation 

 of genera within the Micrococcaceae is similar: when division 

 takes place evenly about three axes to give cubical packets the 

 organisms are called Sarcina ; when division occurs about two 

 axes to give plates the organisms are called Micrococci ; a third 

 genus. Staphylococcus, used to comprise organisms dividing 

 unevenly to give " bunches of grapes," but the latest edition of 

 Bergey includes the Staphylococci within the Micrococci as 

 " variants." In the Bacillaceae two genera are differentiated 

 by oxygen tolerance, thus the Bacilli are strict aerobes and the 

 Clostridia, strict anaerobes. In many cases the differentiation of 

 genera within a tribe is based upon biochemical characteristics; 

 thus the Lactobacilli ferment glucose to produce lactic acid 

 only, while the Propionibacteria have a more varied array 

 of fermentation products including propionic acid. The 

 differentiation between Escherichia and Aerobacter also rests 

 largely on differences in fermentation (see Chap. VII). 



SPECIES 

 An organism can be allocated to a family and genus along 

 the lines already indicated, but the differentiation of species 

 within many genera is mainly a matter of biochemistry, that 

 is, the species are separated by their reactions in a number of 

 simple biochemical tests. If we examine the fermentation of 

 a number of sugars such as sucrose, glucose, maltose, fructose, 

 etc., by different organisms, we shall find, without going into 

 any detailed examination of the products, that some organisms 

 can ferment some sugars but not others. Further, if we add 



