The macronuclei of certain isolates of Paramecium caudatum are inhabited by the bacterial endonucleobiont Caedibacter caryophila. These symbionts confer upon host paramecia a killer trait, which is clearly associated with the presence of R bodies in the endosymbionts.
R bodies are unique inclusion bodies of caedibacteria (which are obligate endosymbionts of Paramecium) and of certain free-living bacteria of the genus Pseudomonas. They have been grouped into four classes on the basis of morphology and behaviour. A fifth R body type, introduced in this study, is named the Cc R body to indicate its presence in bright particles of Caedibacter caryophila. Type Cc R bodies are approximately 0.8 μm in width and diameter. They unroll in a telescopic fashion from the inside and are distinguished by a tapered inner and a blunt outer terminus. They are further associated with icosahedral bacteriophages, which stick to the inner terminus. Cc R body proteins show a ladder-like pattern of stained bands on polyacrylamide gels and exhibit a certain degree of antigenic cross-reactivity with type 51 R bodies.