CLASSIFICATION ACCORDING TO NUTRITION 49 



which have already been decomposed by saprogenic species, and contain 

 a mixture of organic compounds which serve as nutriment. Such bacteria 

 may be termed saprophile. 



In the case of some metatrophic species the chemical change effected 

 by their growth varies according to the nature of the medium they grow in 

 (polytrophism), while the action of others is restricted to some one specific 

 process (monotrophism, see Chap. III). The ability to grow in the living 

 tissues is possessed in various degrees by different species, they are faculta- 

 tive parasites (cholera, anthrax, and perhaps typhoid bacilli), but many are 

 totally unable to exist under such conditions (obligatory saprophytes, obligatory 

 metatrophic bacteria). 



Finally, the paratrophic group includes all those bacteria that can 

 exist only within the living tissues of other organisms. Paratrophic bacteria 

 occur either not at all outside the animal body (gonococci), or else only as 

 dust-borne spores or resting cells (tubercle, diphtheria). Their culture is 

 possible only under conditions closely resembling those of the living tissues 

 (on serum at blood-heat, for instance), and where there is freedom from 

 competition with rapidly growing metatrophic species. Some paratrophic 

 bacteria seem to flourish well even outside the animal body, if their growth 

 on culture media is any criterion, but the conditions here are so very 

 special that no conclusions can be drawn therefrom as to their behaviour 

 in nature. 



Using then the modes of nutrition as a basis for classification, we 

 distinguish the following groups : 



I. PROTOTROPHIC BACTERIA. 



Nitrifying bacteria, bacteria of root-nodules, sulphur and iron bacteria ; occur only in 

 the open in nature, never parasitic, always monotrophic. 



II. METATROPHIC BACTERIA. 



Zymogenic, saprogenic and saprophile bacteria ; occur in the open and upon the 

 external and internal surfaces of the body, sometimes parasitic (facultative parasites), 

 monotrophic or polytrophic. 



III. PARATROPHIC BACTERIA. 



Occur only in the tissues and vessels of living organisms, true (obligatory) parasites. 



It is worthy of remark that not only the bacteria but all other organisms 

 can be arranged in these three biological divisions. All green plants from 

 the simplest unicellular alga up to the largest forest trees are prototrophic ; 

 all fungi and animals are metatrophic, except parasitic forms, which are 

 paratrophic. 



Seeing that the spores of metatrophic bacteria are ubiquitous, it is not 

 surprising that all liquids offering suitable conditions of nourishment should, 

 if left uncovered and exposed to the air, soon become turbid from the 



