The toxic symbiont Caedibacter caryophila in the cytoplasm of Paramecium novaurelia

Endosymbiotic bacteria were observed to inhabit the cytoplasm of the the fr eshwater ciliate Paramecium navaurelia. Transmission electron microscopy an d toxicity tests with sensitive paramecia showed that the endosymbionts bel ong to the genus Caedibacter. The bacteria conferred a killer trait to thei r host paramecia. The production of a proteinaceous inclusion body ("R-body ") in the bacterial cell makes them toxic to other paramecia after they bec ome enclosed in food vacuoles. R-bodies of Caedibacter sp were associated w ith phages, which are known in most other Caedibacter species to code for t he R-body proteins. The killer-effect of P, novaurelia on sensitive P, caud atum strains was of the "paralysis" type, which is a characteristic of the symbiont species Caedibacter caryophila. Until now C. caryophila was known to inhabit the macronucleus of Paramecium caudatum only. Sequencing of the 16S rRNA-gene proved that Caedibacter sp from the cytoplasm of P, novaureli a belongs to the species C. caryophila as well. The rDNA-sequence of 1695 b p length differed in a total of only 1 bp from the corresponding gene in C, caryophila from the macronucleus of P. caudatum. The results indicate that the infection of specific host cell compartments may depend on host genes, but not on different traits of the infecting symbiont species. The occurre nce of killer and sensitive paramecia strains together in one pond is discu ssed with respect to the competitive advantage of the killer trait.