Paramecium strains possessing the particles are known as "killer paramecia".
They liberate a substance also known as paramecin[1][2] into the culture medium that is lethal to Paramecium that do not contain kappa particles.
Kappa particles are found in genotypes of Paramecium aurelia syngen 2 that carry the dominant gene K.[3][4] Kappa particles are Feulgen-positive and stain with Giemsa after acid hydrolysis.
[5] While there was initial confusion over the status of kappa particles as viruses, bacteria, organelles,[6] or mere nucleoprotein,[7] the particles are intracellular bacterial symbionts called Caedibacter taeniospiralis.
[8] Caedibacter taeniospiralis contains cytoplasmic protein inclusions called R bodies which act as a toxin delivery system.