72 OUTLINES OF BACTERIOLOGY 



5. PHYSIOLOGICAL CLASSIFICATION. 



We may divide bacteria according to their mode of life, for there is 

 a great deal of difference between the various kinds of bacteria in this 

 respect. Thus, some feed on living organic-, others on dead organic- 

 matter, whilst to a third kind, the presence of organic matter exercises 

 an injurious influence on its growth. Others, again, are characterised 

 by the possession of special properties, e.g. the power of exhibiting 

 phosphorescence, of secreting colouring matters, and so on. In a 

 classification of this nature, it does not follow that the inclusion of an 

 organism in one group means that it is excluded from any of the 

 others. For descriptive purposes such a classification is extremely 

 useful. On a physiological basis we may divide the bacteria into 

 twelve groups : 



I. Pathogenic Bacteria. This term is applied to bacteria that feed 

 on living organic matter and producing injurious effects ; thus the 

 bacilli causing diphtheria, consumption, lock-jaw, and other diseases, 

 are pathogenic. Also bacteria attack wounded plants, often causing 

 their death. 



II. Saprophytic Bacteria. Applied to such as feed on dead organic 

 matter. When an animal or a plant dies, it becomes food for a host 

 of bacteria and other organisms, which are already present on its 

 surface, or which drop down from the atmosphere. These effect changes 

 which render the body a suitable feeding material for other bacteria 

 with different physiological properties. 



III. Chromogenic Bacteria. The term is used to designate those 

 bacteria, the growths of which, on the various nutritive media, show 

 distinct coloration. Thus, Sarcina ventticula shows a yellow, and 

 Micrococcus prodigiosus a red growth. Almost all the colours are 

 represented ; they are extremely useful as guides in the identification 

 of species. 



IV. Zymogenic Bacteria. These are bacteria that induce fermenta- 

 tive changes. The use of the term zymogenic is somewhat ambiguous, 

 because, as will be explained later, there is no general agreement as to 

 what constitutes a fermentative change. Speaking generally, it is 

 customary to apply the term zymogenic to those bacteria, small 

 amounts of which produce a very large amount of chemical change 

 in food and other substances, into which they have been inoculated. 

 Further, the activities of these bacteria, in so far as these changes 

 are concerned, are inseparable from their life-processes. A typical 



