Kappa organism

In biology, Kappa organism or Kappa particle refers to inheritable cytoplasmic symbionts, occurring in some strains of Paramecium. Paramecium strains possessing the particles ("Killers") liberate into the culture medium a substance lethal to Paramecium not containing kappa particles.

Kappa particles are Feulgen-positive and stain with Giemsa after acid hydrolysis. The length of the particles is 0.2–0.5μ.

While there was initial confusion over the status of kappa particles as viruses, bacteria, organelles, or mere nucleoprotein, the particles are intracellular bacterial symbionts Caedibacter taenospiralis. Caedibacter taenospiralis contains cytoplasmic protein inclusions called R bodies which act as a toxin delivery system.