CHAPTER 8 



MICROSCOPIC ANIMAL FORMS AS ANTAGONISTS 



The microscopic animal world inhabiting the soil and water basins com- 

 prises protozoa, insects and insect larvae, nematodes and other worms. 

 Their relationships to the microbiological flora of soils and waters are 

 varied. Many, if not most, of these animals feed upon the bacteria and 

 fungi, as well as upon the smaller animal forms. Some carry a bacterial 

 population in their digestive tract and appear to depend upon these 

 bacteria for some of the digestion processes. Many of the animal forms 

 are parasitized by bacteria and fungi. Some of these forms are subject 

 to the action of specific substances produced by microbial antagonists. 

 No detailed discussion will be presented of these varied relationships, 

 but attention will be directed to a few specific phenomena which have a 

 bearing on the subject under consideration. The ability of higher ani- 

 mals to produce antibacterial substances has been amply established. 

 Some of these substances are well characterized, as in the case of ly- 

 sozyme found in mammalian tissues and secretions (262, 264) and in- 

 hibins found in fresh human urine (180). 



INTERRELATIONSHIPS BETWEEN PROTOZOA 

 AND BACTERIA 



The lower animal forms inhabiting the soil, manure piles, and water 

 basins often utilize bacteria in the synthesis of their foodstuffs. Al- 

 though many of the smallest organisms, namely the protozoa, are able 

 to obtain their nutrients from simple organic compounds and mineral 

 salts, they frequently depend upon the bacteria to concentrate the nu- 

 trients present in dilute forms in the natural substrate. It has been 

 shown (102), for example, that when carbohydrates are present in 

 water in very low concentration, the protozoa may not be able to use 

 them in that dilute formj however, the bacteria can assimilate these 

 carbohydrates and can build up extensive cell substance, and the pro- 



