PHYSIOLOGY OF MICRO-ORGANISMS 171 



so unstable that their presence in a decomposing substance is 

 influenced by access of air, temperature and moisture, and they 

 may quickly disappear or decompose. 



Micro-organisms also form fat-splitting or steatolytic enzymes, 

 and enzymes capable of transforming urea into ammonium 

 carbonate. 



NH 2 -CO-NH 2 +2H 2 O (fermentation) = (NH 4 ) 2 CO 3 . 



Various inorganic substances undergo chemical change under the 

 influence of microbic activity and some of these changes appear to 

 be due to enzymes. Specific examples will be considered in the 

 section on the soil bacteria. 



The toxins of bacteria are primary products built up by the cell. 

 The true bacterial toxins are of unknown chemical composition, 

 are labile like enzymes and stimulate the production of antitoxins 

 when they are injected into animals. They are the most poisonous 

 substances at present known. Analogous substances have been 

 found in some plants, ricin in the castor bean and abrin in the 

 jequirity bean, and the poisonous property of some kinds of snake 

 venom is due to the presence of substances similar in nature to 

 the bacterial toxins. These substances will be considered more 

 fully in a later chapter devoted to the relation of parasitic microbes 

 to their hosts. 



MUTUAL RELATIONS OF A MICROBE AND ITS ENVIRONMENT. 



Morphological Characters. It is evident that the phenomena 

 of growth taking place in a microbic pure culture depend not only 

 upon the particular kind of microbe present but also in a very 

 important way upon the chemical and physical structure of the 

 medium, the access of air and the temperature. Variations in 

 these latter may even bring about considerable alteration in the 

 form and structure of the individual cells. A common effect of 

 high temperature is the shortening of individual bacilli and spirilla 

 because of more rapid division and complete separation of the 



