24 ORGANISMS, SAPROPHYTES, PARASITES 



matter and split it up into simple chemical compounds, so that 

 these can be utilized again in building up the higher plants, which 

 in their turn are needed directly or indirectly to support the life of 

 the higher animals. Without higher plants, herbivorous animals 

 could not exist, and without the latter, the carnivora would perish. 

 Certain microorganisms are, therefore, essential in maintaining the 

 cycle of life on our planet. Microorganisms do not always live in 

 the general outside world. They may be in or on other higher living 

 beings, as parasites, and yet they may not do any harm to their host 

 but may even benefit it and be a necessary element in its metabolism 

 and existence. So it is incorrect to consider all microbes as enemies 

 of mankind and domestic animals. It is true that some micro- 

 organisms are our greatest enemies, but many others are our greatest 

 friends. 



Saprophytes and Parasites. Most microorganisms of both vege- 

 table and animal types exist in the outside world. They derive their 

 nutrition from dead organic material, which they split up in their 

 metabolism, and they are known as saprophytes. The lowest micro- 

 organisms of a vegetable type, the saprophytic bacteria, exist almost 

 everywhere on and near the surface of our earth. We find them in the 

 air, in the water, in the soil, on the surface of the bodies of animals 

 and plants, in decaying substances, etc.; they are, as it is also ex- 

 pressed, ubiquitous. 



Organisms which live on or in a living host, and which utilize 

 some of its material for their own nutrition, are known as parasites. 

 Some microorganisms can live only as parasites, and can never thrive 

 and multiply in the outside world, as, for instance, the microorganism 

 causing tuberculosis, the tubercle bacillus, in its various forms. 

 Such organisms are called strict or obligate parasites. 



There are other microorganisms which can exist both as sapro- 

 phytes and as parasites, such as those which cause anthrax in man and 

 animals. This anthrax bacillus can exist and multiply on meadows, 

 in manure, in the ground, and can also invade the blood of 

 animals, where it greatly increases in numbers and causes the disease 

 known as anthrax, or splenic fever. Microorganisms which can 

 exist both as saprophytes and as parasites are called facultative para- 

 sites or facultative saprophytes, while those which can live only in 

 the oustide world and never as parasites are called strict or obligate 

 saprophytes. 



When parasites live on the outside of their host, they are spoken of 

 as ectogenous parasites; when they live inside the body, as entogenous 

 parasites. 



Commensales. It must not be supposed that all parasites are 

 harmful; many are perfectly harmless. These are called commen- 

 sales. 



The bacterium known as the colon bacillus is of this type and 

 lives in the colon and other parts of the large intestines of man and 



