CHAPTER X 



PHYSICAL EFFECTS PRODUCED BY MICROORG-ANISMS 



Practically all of the chemical changes brought about 

 by microorganisms result in physical changes becoming 

 apparent in the medium in which the organisms have 

 been growing. There are four of these changes, however, 

 distinctly physical in nature that are worthy of notice. 

 These are : the production of light, the development of 

 heat, changes in viscosity of medium, and changes in 

 osmotic pressure. 



Light Production by Microoeganisms 



The property of producing light, that is, of being pho- 

 togenic or phosphorescent, is one which is common to plants 

 and animals of a great many different types. A number 

 of species of bacteria have been described as photogenic. 

 Practically all of them have been found in sea water or 

 upon food materials, such as salt fish, taken from the sea. 

 These bacteria may be grown readily upon suitable culture 

 media, particularly media containing proper amounts of 

 salts. When grown upon an agar slant, for example, dis- 

 tinct light may be observed when the culture is examined in 

 a dark room. Sufficient light may be given off so that the 

 organisms may be photographed by their own light. The 

 bacteria which are found to be phosphorescent or photo- 

 genie are all aerobes, growing well only in the presence of 

 oxygen and not producing light in its absence. The phe- 

 nomenon of photogenesis in bacteria has not been adequately 

 studied, but there is no reason to suppose that it differs 

 materially from the same phenomenon observed in higher 



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