Infection and maintenance of Holospora obtusa, A macronucleus-specific bacterium of the ciliate Paramecium caudatum

The gram-negative bacterium Holospora obtusa is a macronucleus-specific symbiont of the ciliate Paramecium caudatum, which invades the host cell via a food vacuole, infects its macronucleus and grows exclusively in the nucleus. From infection experiments, we showed that a property of the macronucleus that is necessary for it to be recognized and infected by H. obtusa is commonly provided by P. caudatum, P. multimicronucleatum and 14 species of the P. aurelia complex, but not by P. jenningsi, P. bursaria, P. trichium, P. duboscqui, Didinium nasutum, Blepharisma japonicum, Pseudourostyla levis, seven species of Euplotes or Tetrahymena thermophila. Furthermore, it was also shown that the bacteria that infect the macronuclei of P. multimicronucleatum and the P. aurelia species complex always disappear from the nuclei within 5 days and the infected bacteria are maintained stably in the host nuclei in only 13 out of 22 strains of P. caudatum. The results indicate that the species specificity of the habitat of H. obtusa is not simply a matter of its ability to penetrate the host nuclear membrane but depends on unknown factors that exist only in certain strains of P. caudatum.