Bacteria of the genus Holospora (H. elegans, arrows) colonize the micronucleus (MI) of the ciliate Paramecium caudatum (‘endonuclear symbionts’). The bacteria actively invade the ciliate and the nucleus and are also given to the daughter cell during division. Some bacteria are also released to the environment to infect new hosts. The macronucleus is also visible (MA). By courtesy of Michael Butkay.