Ciliates are frequently colonized by other micro-organisms. The large size of ciliate cells offers habitats for hundreds to thousands of bacteria in different compartments, such as cytoplasm, nuclei and even perinuclear spaces. Size, phagocytic feeding habit and other features appear to be favorable pre-adaptations of ciliates for symbiosis with bacteria. Certain intracellular bacteria are permanent symbionts that are not infectious, whereas others are highly infectious. Both types show specific adaptations. With their wide spectrum of phylogenetic positions, intracellular bacteria in ciliates show relationships to different taxa of free-living bacteria and even archaea. Certain symbionts may be deleterious for their host ciliates, whereas others may provide a selective advantage under appropriate conditions or even be essential for the host cells. Depending on the nature of a symbiont, its prevalence in a host population may be low or high. Symbionts that express a killer toxin affecting non-infected ciliates achieve high infection rates in a host population, whereas certain infectious bacteria may only show a low prevalence.

中文翻译：

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纤毛中的细胞内细菌。

纤毛虫经常被其他微生物定殖。纤毛状细胞的大面积为不同隔室中的数百至数千种细菌提供了栖息地，例如细胞质，细胞核乃至核周空间。大小，吞噬性摄食习性和其他特征似乎是纤毛虫与细菌共生的有利预适应。某些细胞内细菌是永久性共生体，没有传染性，而其他细菌是高度传染性的。两种类型都显示出特定的适应性。纤毛虫的细胞内细菌具有广泛的系统发育位置，它们显示出与不同种类的自生细菌甚至古细菌的关系。某些共生体对其宿主纤毛可能有害，而其他共生体在适当条件下可能提供选择性优势，甚至对于宿主细胞必不可少。根据共生体的性质，其在宿主种群中的流行率可能很低也可能很高。表达影响未感染纤毛虫的杀手毒素的共生菌在宿主群体中获得较高的感染率，而某些传染性细菌的感染率可能较低。