146 MICROSCOPIC ANIMAL FORMS AS ANTAGONISTS 



by the presence of protozoa. Thus, an interrelationship among micro- 

 organisms which was at first thought to be antagonistic actually has 

 proved to be associative (943). The protozoan Oikomonas termo was 

 found capable of living at the expense of a large number of bacteria, 

 namely 83 per cent of those tested. The fact that Oikomonas causes 

 many species of bacteria to flocculate was suggested as explanation for 

 the ability of the protozoa to digest these bacteria (378). 



The ability of protozoa to destroy bacteria was said (414) to be re- 

 sponsible for the protection of certain plants against attack by plant 

 pathogenic bacteria and fungi. This was said to hold true of attack of 

 potatoes by Bacterium aroideae and of other plants by Pseudomonas 

 hyacinthi and Pseudomonas citri, as well as by species of Fusarium and 

 Penicillium. 



Various bacteria may exert a toxic action upon protozoa, thus limiting 

 the development or bringing about the destruction of the latter (122, 

 545, 687). Certain plant pathogenic bacteria inedible by amebae were 

 found to produce a toxin that was harmful to these amebae. The toxin, 

 however, appeared to be without effect on the flagellate Cercomonas, 

 which could eat all these bacteria partly or completely (826), In some 

 cases, the protozoa were capable of developing a certain resistance to 

 specific bacterial products (687). 



Certain factors in the medium seem to affect the encystment of pro- 

 tozoa (874) 5 it remains to be determined to what extent these factors 

 can be classified with antibiotic substances. 



RELATIONS OF PROTOZOA TO FUNGI 



The presence of Colfoda and other infusoria in an active form was 

 found to repress the growth of Verticillium dahliae in culture media 

 and to prevent infection of tomato plants by this pathogeny Colfoda 

 was also active in soils and reduced the incidence of wilting (87). 



Myxamoebae of the slime mold Dictyostileum discoideum also live 

 upon bacteria. They are able to utilize the gram-negative somewhat 

 better than the gram-positive types, with certain few exceptions. Bac- 

 terial spores are also ingested by these organisms, but they are not di- 

 gested. The ability of various fungi to destroy protozoa and nema- 

 todes has been studied in detail by Drechsler (183). 



