The Killer Effect of Paramecium and Its Causative Agents

Frequently, nonrelated organisms form endocytobioses resulting in organisms with different characters compared with those of the individual partners alone. Endocytic bacteria of the genus Caedibacter in host ciliates of the genus Paramecium enable their host to kill sensitive paramecia. These paramecia therefore are called “killers” and the phenomenon was named “killer trait” (Sonneborn in Proc. Am. Philos. Soc. 79:411–434, 1938). In their natural environment these endocytic bacteria enable their hosts to outcompete uninfected forms (Kusch et al. in Protist 153:47–58, 2002). Endocytobioses become more complex when the intracellular bacteria are infected with phages, such as in the three partner association Paramecium-Caedibacter-bacteriophage. Interaction of the different partners results in the formation of a proteinaceous intracellular structure called a “refractile body” (R-body) within the bacteria. Physiological tests demonstrated that R-bodies are a prerequisite for Caedibacter to confer the killer trait to their host, but they are not the toxin that kills sensitive paramecia (Preer et al. in Proc. Natl. Acad. Sci. USA 39:1228–1233, 1953). In this chapter we focus on the Caedibacter species, with additional brief remarks on other endocytic killer bacteria and on R-bodies produced by free-living bacteria.