RESUMO
Eukaryotes have been threatened by microbial infections throughout their phylogenetic history. Today, infections are found in all taxa of Eukaryotes. The diversity of infectious microorganisms most of them intracellular and their biological strategies is remarkable and by far exceeds what we know about human pathogens. The search for infections in the wide spectrum of protozoa has led to the discovery of more and more intracellular bacteria and other microorganisms. This paper provides an introduction into the diversity of intracellular microorganisms in protozoa and gives a few examples of such symbioses that have been studied in detail. Special emphasis is put on infection of the ciliate Paramecium by Holospora bacteria, which invade the host nuclei with high efficacy.
