RELATIONS OF PROTOZOA TO BACTERIA 157 



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 (381). 



The ability of protozoa to destroy bacteria was said (426) 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 ckri, as well as by species of Fusarium and 

 Penicill'ium,. 



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

 the development or bringing about the destruction of the latter (133, 

 584). Certain plant pathogenic bacteria inedible by amebae were found 

 to produce a toxin that was harmful to these amebae. In some cases, the 

 protozoa were capable of developing a certain resistance to specific 

 bacterial products (721). The toxic action of some bacteria against 

 Paramecium could be overcome by the presence of a flagellated proto- 

 zoan Oikomonas (382). 



On the basis of the ability of protozoa to utilize bacteria as food, 

 Singh classified (855) the latter into 3 groups: (a) edible forms, (b) 

 inedible but harmless to protozoa, (c) forms toxic to protozoa. Pig- 

 ment-producing bacteria are inedible and some are toxic j these comprise 

 the Ps. aeruginosa and the S. m^arcescens groups. 



Since some amebae, like Hartmanella castellanii, function as phago- 

 cytes, they are believed (545) to offer excellent material for the study 

 of the effect of antibiotic substances upon pathogenic bacteria in the 

 presence of these amebae, the latter not being affected, as by penicillin, 

 for example. 



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

 tozoa (9C0) J it remains to be determined to what extent these factors 

 can be classified with antibiotic substances. 



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. 



