Holospora and related bacteria are a group of obligate Paramecium symbionts. Characteristic features are their infectivity, the presence of two distinct morphotypes, and usually a strict specialization for a single Paramecium species as host and for a nuclear compartment (either somatic or generative nucleus) for reproduction. Holospora caryophila steps out of line, naturally occurring in Paramecium biaurelia and Paramecium caudatum. This study addresses the phylogenetic relationship among H. caryophila and other Holospora species based on 16S rRNA gene sequence comparison analyzing the type strain and seven new macronuclear symbionts. Key aspects of Holospora physiology such as infectivity, symbiosis establishment and host range were determined by comprehensive infection assays. Detailed morphological investigations and sequence-based phylogeny confirmed a high similarity between the type strain of H. caryophila and the novel strains. Surprisingly, they are only distantly related to other Holospora species suggesting that they belong to a new genus within the family Holosporaceae, here described as Preeria caryophila comb. nov. Adding to this phylogenetic distance, we also observed a much broader host range, comprising at least eleven Paramecium species. As these potential host species exhibit substantial differences in frequency of sexual processes, P. caryophila demonstrates which adaptations are crucial for macronuclear symbionts facing regular destruction of their habitat.

Hypersaline environments are the source of many viruses infecting different species of halophilic euryarchaea. Information on infection mechanisms of archaeal viruses is scarce, due to the lack of genetically accessible virus–host models. Recently, a new archaeal siphovirus, Haloferax tailed virus 1 (HFTV1), was isolated together with its host belonging to the genus Haloferax, but it is not infectious on the widely used model euryarcheon Haloferax volcanii. To gain more insight into the biology of HFTV1 host strain LR2-5, we studied characteristics that might play a role in its virus susceptibility: growth-dependent motility, surface layer, filamentous surface structures, and cell shape. Its genome sequence showed that LR2-5 is a new strain of Haloferax gibbonsii. LR2-5 lacks obvious viral defense systems, such as CRISPR-Cas, and the composition of its cell surface is different from Hfx. volcanii, which might explain the different viral host range. This work provides first deep insights into the relationship between the host of halovirus HFTV1 and other members of the genus Haloferax. Given the close relationship to the genetically accessible Hfx. volcanii, LR2-5 has high potential as a new model for virus–host studies in euryarchaea.

The bacterial and archaeal cell surface is decorated with filamentous surface structures that are used for different functions, such as motility, DNA exchange and biofilm formation. Viruses hijack these structures and use them to ride to the cell surface for successful entry. In this review, we describe currently known mechanisms for viral attachment, translocation, and entry via filamentous surface structures. We describe the different mechanisms used to exploit various surface structures bacterial and archaeal viruses. This overview highlights the importance of filamentous structures at the cell surface for entry of prokaryotic viruses.

Archaeal viruses are the most enigmatic members of the virosphere. These viruses infect ubiquitous archaea and display an unusually high structural and genetic diversity.