Paramecium
These are paramecia which secrete, into the medium around them, particles which kill other paramecia. This killer trait is caused by kappa particles, which are symbiotic bacteria. The kappa particles are only found in paramecia with a dominant K gene. The killing is done by smaller particles which are defective DNA phages. The kappa bacterium is just one of many found in natural populations of Paramecium aurelia.p243/4