PATHOGENIC ORGANISMS 17 



changes in the medium on which they are grown. In many 

 cases albuminous bodies are peptonized and gelatine is 

 liquefied. Many bacteria have the faculty of resolving 

 organic bodies into their simplest elements ; others, again, 

 have the property of converting ammonia into nitric and 

 nitrous acid. Certain microbes have the power of becoming 

 phosphorescent in the dark. These phosphorescent bacteria 

 are often seen on decaying plants and wood ; sometimes 

 in tropical climates the sea becomes luminous owing to 

 the presence of countless numbers of these organisms. 

 Again, they are frequently seen on the surface of dead fish, 

 particularly mackerel, which often become so bright as to 

 strongly illuminate the cupboard in which they lie. 



The particular class of parasites which produce disease 

 in man and the higher animals are termed ' pathogenic ' 

 bacteria. These pathogenic organisms may exert their 

 pernicious power in several ways. They may be injurious 

 on account of their abstracting nourishment from the 

 blood or tissues, or for the purely mechanical reason of 

 their stopping up the minute capillaries and bloodvessels 

 by their excessive multiplication. But the poisonous action 

 of most of the pathogenic bacteria is due to the chemical 

 products secreted or excreted by the organisms, and it is to 

 the circulation and absorption within the body of these 

 poisons that must be traced the disturbances of the animal 

 system which characterise disease. 



The various products of the metabolism of the bacteria 

 are known as 'ptomaines,' ' toxalbumoses,' 'ferments,' or 

 ' enzymes.' Many of these bodies may be elaborated by 

 micro-organisms when growing on artificial media or 

 articles of food. When meat or albuminous bodies undergo 

 decomposition, i.e., when the organisms of putrefaction 

 alight and develop on. them, the result may be the pro- 

 duction of intensely poisonous bodies, which are the cause 



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