Bacteria of the genus Caedibacter are endocytobionts of Paramecium. Mostly, the endocytobionts colonize the cytoplasm and sometimes the macronucleus of a host cell (Fig. 1). Infected paramecia may be recognized by the fact that they release toxins produced by Caedibacter that kill sensitive paramecia. Paramecia bearing Caedibacter are therefore called "killers". Host cells of Caedibacter are resistant against the toxins. This killer trait was first described by Sonneborn in 1938, who observed that sensitive paramecia exhibit distinct morphological symptoms upon ingestion of toxic particles (at that time designated kappa-particles) released by the killer strain and ultimately die (Sonneborn 1938). Evidences for a bacterial nature of the particles were found by Preer (1950), Hamilton and Gettner (1958) and Dippel (1958). After the initial discovery of killer bacteria, others were found that acted differently on sensitive cells. In the case of Caedibacter, the toxic effect is associated with refractile inclusion bodies (R bodies) found inside the bacteria (Schmidt et al. 1988). R bodies consist of highly insoluble protein ribbons (Fig. 2). They unwind under certain conditions and are associated with the toxicity of the bacteria. Besides Caedibacter, several free-living bacteria were found, too, that have the ability to produce R bodies.

J. Kusch

Department of Ecology, Faculty of Biology, TU Kaiserslautern, Erwin-Schroedinger-Str. 13/14, 67663 Kaiserslautern, Germany H.-D. Görtz (e-mail: [email protected])

Department of Zoology, Biological Institute, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany

Progress in Molecular and Subcellular Biology Jörg Overmann (Ed.) Molecular Basis of Symbiosis © Springer-Verlag Berlin Heidelberg 2005

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