14 MICROORGANISMS IN SOILS AND WATER BASINS 



fection by different bacteria, fungi, and protozoa. The more advanced 

 the animal body is in the stage of evolution, the more numerous are its 

 ills, most of which are caused directly or indirectly by microorganisms. 



The microbial agents causing thousands of diseases of plant and ani- 

 mal life have now been recognized and even isolated and described. In 

 many cases these disease-producing agents are closely related morpho- 

 logically to others that lead a harmless existence in soils or water 

 basins i many of the saprophytes, for instance, are found to be of great 

 benefit to man and to his domesticated plants and animals. This sug- 

 gests the probability that pathogenic microorganisms represent certain 

 strains of soil and water-inhabiting types that have become adjusted to 

 a parasitic existence. During their life in the host, they multiply at a 

 rapid rate and produce substances toxic to the body of the host. The re- 

 sult is that the host is incapacitated for a certain period of time, until it 

 succeeds in building up resistance against the invading organisms. It 

 may thus overcome the injurious effect of the pathogen or it may be 

 killed if such resistance cannot be effected. In the first instance, a tem- 

 porary or permanent immunity against the specific disease-producing 

 microbe or its close relatives may result. The host is often able to sur- 

 vive the attack without being able to destroy the invading microbes j if 

 it again attains a normal form of life, it is designated as a carrier of the 

 disease-producing agent. 



Pathogenic organisms pass their existence in the living body of the 

 plant or animal. They spread from one host to another by contact or 

 through a neutral medium, such as water, milk, or dust where they may 

 remain alive and active for varying lengths of time, or they reach the 

 soil or water basins in the excreta of the host. If the host is killed by 

 the infecting microbes, they may survive for some time upon the rem- 

 nants of what was once a living animal or plant and thus find their way 

 into the soil and water basins. 



Considering the millions of years that animals and plants have ex- 

 isted on this planet, one can only surmise the great numbers of microbes 

 causing the numerous diseases of all forms of life that must have found 

 their way into the soil or into streams and rivers. What has become of 

 all these pathogenic bacteria? This question was first raised by medical 

 bacteriologists in the eighties of the last century. The soil was searched 



