454 VETEEINAEY HYGIENE 



Light is as a rule unfavourable to the growth of bacteria, 

 and direct sunlight frequently kills them. Destruction may 

 also be produced by certain chemical agents, and this is the 

 basis of disinfection. Finally, the growth of bacteria may 

 be influenced by the products of their own metabolism, or 

 by the presence of other species of micro-organisms. 



Prochicts of Bacterial Growth. — According to the biological 

 conditions under which organisms work they may be 

 classified under three heads. 



1. Prototropliic bacteria are characterized by their syn- 

 thetical tendency, whereby they build up chemical com- 

 pounds out of simple elements. The best examples of this 

 are the nitrogen bacteria on the roots of leguminous plants 

 (p. 389), and the nitrifying bacteria of soils (p. 391). 



2. Metatropliic bacteria are characterized by their work 

 being chiefly of an analytical character, by which they 

 break up organic compounds, both nitrogenous or non- 

 nitrogenous. An example of the former is the process of 

 putrefaction (saprogenic bacteria), and of the latter the 

 process of fermentation (zymogenic bacteria). 



A third group of bacteria (the Saprophile) live princi- 

 pally on the material decomposed by the saprogenic or 

 zymogenic organisms. 



Saprogenic bacteria break down the complex proteid 

 molecule by means of enzymes (proteolytic enzymes, 

 trypsines), and in this way resemble the process of digestion. 

 The final products are nitrogen, ammonia, carbonic acid, 

 sulphuretted hydrogen, etc. Finally, these simple sub- 

 stances under the influence of the paratrophic bacteria are 

 rendered available as plant food, and thus their cycle 

 begins afresh. 



In a similar way organic matter which contains no 

 nitrogen is also broken down ; the carbo-hydrate molecule 

 is attacked by enzymes, the final product being chiefly 

 carbonic acid. The breaking down of insoluble molecules 

 is preceded by their being changed into soluble molecules 

 by means of enzymes (invertin, maltose, etc.). 



The biological group of pigment bacteria produce pig- 



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