An Acanthamoeba sp. containing two phylogenetically different bacterial endosymbionts.

Acanthamoebae are ubiquitous free-living amoebae and important predators of microbial communities. They frequently contain obligate intracellular bacterial symbionts, which show a worldwide distribution. All Acanthamoeba spp. described so far harboured no or only a single specific endosymbiont phylotype, and in some cases evidence for coevolution between the symbiotic bacteria and the amoeba host has been reported. In this study we have isolated and characterized an Acanthamoeba sp. (strain OEW1) showing a stable symbiotic relationship with two morphologically different endosymbionts. 16S rRNA sequence analysis assigned these symbionts to the candidate genus Procabacter (Betaproteobacteria) and the genus Parachlamydia (Chlamydiae) respectively. Fluorescence in situ hybridization and transmission electron microscopy confirmed the affiliation of the endosymbionts and showed their co-occurrence in the amoeba host cells and their intracellular location within separate compartments enclosed by host-derived membranes. Further analysis of this stable relationship should provide novel insights into the complex interactions of intracellular multiple-partner associations.