THE METABOLISM OF BACTERIA 143 



to 37 C. is the most favorable for the large majority 

 of micro-organisms. 



The maximum and minimum temperatures at which 

 growth takes place, as well as the optimum, are fairly 

 constant for each bacterium. 



Bacteria have been classified, according to their 

 optimum temperature, into 



MIN. OPT. MAX. 



1. Psychrophilic bacteria (chiefly 



water organisms o C. isC. 30 C. 



2. Mesophilic bacteria (includes patho- 



genic bacteria) 15 C. 37 C. 45 C. 



3. Thermophilic bacteria 45 C. 55 C. 70 C. 



The thermal death-point of an organism is another 

 biological constant; and is that temperature which 

 causes the death of the vegetative forms when the 

 exposure is continued for a period of ten minutes (see 

 pages 298-301). 



3. Light. Many organisms are indifferent to the 

 presence of light. On the other hand, light frequently 

 impedes growth, and alters to a greater or lesser extent 

 the biochemical characters of the organisms e. g., 

 chromogenicity or power of liquefaction. Pathogenic 

 bacteria undergo a progressive loss of virulence when 

 cultivated in the presence of light. 



4. Movements. Movements, if slight and simply of 

 a flowing character, do not appear to injuriously affect 

 the growth of bacteria; but violent agitation, such as 

 shaking, absolutely kills them. 



A condition of perfect rest would seem to be that 

 most conducive to bacterial growth. 



The Metabolic Products of Bacteria. Pigment Pro- 

 duction. Many micro-organisms produce one or more 

 vivid pigments yellow, orange, red, violet, fluorescent, 

 etc. during the course of their life and growth. The 

 colouring matter usually exists as an intercellular 

 excrementitious substance. Occasionally, however, it 



