Soldo: Axenic Culture of Paramecium 387 



duction is dependent upon the presence of the particle in the cytoplasm; im- 

 plicit here is that the vitamin is produced by the particles themselves. 



Discussion 



The symbiotic association between X particles and the host Paramecium is 

 an example of what is doubtless a widespread phenomenon in nature. Para- 

 mecium bursaria harbors an alga of the genus Chlorella in its cytoplasm in what 

 has been described as a symbiotic system.-'' Colicins in bacteria,-^ extranu- 

 clear particles responsible for cytoplasmic inheritance in yeast," and particle- 

 Uke inclusions found in many insect tissues"-^ -^ may be further examples. A 

 well documented case of an endosymbiote has been found for the flagellated 

 protozoan, Strigomonas}^ This organism, apparently, exists in association 

 with cytoplasmic bipolar-like bodies. The presence of these particles, as with 

 Paramecium, alters the nutritional requirements of the host. 



A distinctive feature of the A system is the ability of particle bearing animals 

 to release a toxin which causes the death of certain particle free detector strains, 

 but is without effect upon the X bearers themselves. In this respect the X 

 system bears a striking resemblance to colicin producing systems."^ Colicins 

 are antibiotic substances produced by certain bacteria, notably members of the 

 family Enterobacteriaceae. The ability of these bacteria to produce these 

 substances is believed to be due to the presence of a transmissible pathogenic 

 agent which is regarded as a bacterial virus. The analogy serves to illustrate 

 the degree to which the X particles may have incorporated themselves into the 

 genetic structure of the protozoan. 



Yet X particles, unlike viruses, are highly complex structures which resemble 

 bacteria in size, morphology, staining characteristics, chemical composition, 

 and, possibly, manner of reproduction. Studies concerning the chemistry of 

 the particles reveal the presence of protein, carbohydrate, phospholipid, and 

 nucleic acid (W. J. van Wagtendonk and R. Tanguay — personal communica- 

 tion). Moreover, antibiotics are particularly effective in reducing or eliminat- 

 ing the particles from the cytoplasm of the protozoan. (These data, obtained 

 for the first time with axenically cultivated animals, provide the strongest 

 evidence to date on the action of antibiotics on particles in Paramecium.) 

 Finally, the finding that X particles produce amounts of folic acid sufficient to 

 support the growth and reproduction of the protozoan carries with it the im- 

 plication that the complex enzymatic machinery necessary for the synthesis of 

 this compound is present in the particles themselves. 



A cknowledgments 



I wish to express my sincere thanks to Andrea M. Pascale and William E. 

 Ronca for their excellent technical assistance. 



The work described in this article was supported in part by a Government 

 Contract, SA-43ph-1929, to The Cancer Chemotherapy National Service Cen- 

 ter, National Cancer Institute, National Institutes of Health, Bethesda, Md. 



References 



1. SoNNEBORN, T. M. 1959. Kappa and related particles in Paramecium. Adv. Virus 

 Res. 6: 229. 



