Stable coexistence between an archaeal virus and the dominant methanogen of the human gut.

The human gut virome, which is mainly composed of bacteriophages, also includes viruses infecting archaea, yet their role remains poorly understood due to lack of isolates. Here, we characterize a temperate archaeal virus (MSTV1) infecting Methanobrevibacter smithii, the dominant methanogenic archaeon of the human gut. The MSTV1 genome is integrated in the host chromosome as a provirus which is sporadically induced, resulting in virion release. Using cryo-electron tomography, we capture several intracellular virion assembly intermediates and confirm that only a small fraction of the host population actively produces virions in vitro. Similar low frequency of induction is observed in a mouse colonization model, using mice harboring a stable consortium of 12 bacterial species (OMM12). Transcriptomic analysis suggests a regulatory lysogeny-lysis switch involving an interplay between viral proteins to maintain virus-host equilibrium, ensuring host survival and viral persistence. Thus, our study sheds light on archaeal virus-host interactions and highlights similarities with bacteriophages in establishing stable coexistence with their hosts in the gut.

Spotlight on FtsZ-based cell division in Archaea.

Compared with the extensive knowledge on cell division in model eukaryotes and bacteria, little is known about how archaea divide. Interestingly, both endosomal sorting complex required for transport (ESCRT)-based and FtsZ-based cell division systems are found in members of the Archaea. In the past couple of years, several studies have started to shed light on FtsZ-based cell division processes in members of the Euryarchaeota. In this review we highlight recent findings in this emerging field of research. We present current knowledge of the cell division machinery of halophiles which relies on two FtsZ proteins, and we compare it with that of methanobacteria, which relies on only one FtsZ. Finally, we discuss how these differences relate to the distinct cell envelopes of these two archaeal model systems.

An ancient divide in outer membrane tethering systems in bacteria suggests a mechanism for the diderm-to-monoderm transition.

Recent data support the hypothesis that Gram-positive bacteria (monoderms) arose from Gram-negative ones (diderms) through loss of the outer membrane (OM), but how this happened remains unknown. As tethering of the OM is essential for cell envelope stability in diderm bacteria, its destabilization may have been involved in this transition. In the present study, we present an in-depth analysis of the four known main OM-tethering systems across the Tree of Bacteria (ToB). We show that the presence of such systems follows the ToB with a bimodal distribution matching the deepest phylogenetic divergence between Terrabacteria and Gracilicutes. Whereas the lipoprotein peptidoglycan-associated lipoprotein (Pal) is restricted to the Gracilicutes, along with a more sporadic occurrence of OmpA, and Braun's lipoprotein is present only in a subclade of Gammaproteobacteria, diderm Terrabacteria display, as the main system, the OmpM protein. We propose an evolutionary scenario whereby OmpM represents a simple, ancestral OM-tethering system that was later replaced by one based on Pal after the emergence of the Lol machinery to deliver lipoproteins to the OM, with OmpA as a possible transition state. We speculate that the existence of only one main OM-tethering system in the Terrabacteria would have allowed the multiple OM losses specifically inferred in this clade through OmpM perturbation, and we provide experimental support for this hypothesis by inactivating all four ompM gene copies in the genetically tractable diderm Firmicute Veillonella parvula. High-resolution imaging and tomogram reconstructions reveal a non-lethal phenotype in which vast portions of the OM detach from the cells, forming huge vesicles with an inflated periplasm shared by multiple dividing cells. Together, our results highlight an ancient shift of OM-tethering systems in bacterial evolution and suggest a mechanism for OM loss and the multiple emergences of the monoderm phenotype from diderm ancestors.

FtsZ-mediated fission of a cuboid bacterial symbiont.

Less than a handful of cuboid and squared cells have been described in nature, which makes them a rarity. Here, we show how Candidatus Thiosymbion cuboideus, a cube-like gammaproteobacterium, reproduces on the surface of marine free-living nematodes. Immunostaining of symbiont cells with an anti-fimbriae antibody revealed that they are host-polarized, as these appendages exclusively localized at the host-proximal (animal-attached) pole. Moreover, by applying a fluorescently labeled metabolic probe to track new cell wall insertion in vivo, we observed that the host-attached pole started septation before the distal one. Similarly, Ca. T. cuboideus cells immunostained with an anti-FtsZ antibody revealed a proximal-to-distal localization pattern of this tubulin homolog. Although FtsZ has been shown to arrange into squares in synthetically remodeled cuboid cells, here we show that FtsZ may also mediate the division of naturally occurring ones. This implies that, even in natural settings, membrane roundness is not required for FtsZ function.

SepF is the FtsZ anchor in archaea, with features of an ancestral cell division system.

Most archaea divide by binary fission using an FtsZ-based system similar to that of bacteria, but they lack many of the divisome components described in model bacterial organisms. Notably, among the multiple factors that tether FtsZ to the membrane during bacterial cell constriction, archaea only possess SepF-like homologs. Here, we combine structural, cellular, and evolutionary analyses to demonstrate that SepF is the FtsZ anchor in the human-associated archaeon Methanobrevibacter smithii. 3D super-resolution microscopy and quantitative analysis of immunolabeled cells show that SepF transiently co-localizes with FtsZ at the septum and possibly primes the future division plane. M. smithii SepF binds to membranes and to FtsZ, inducing filament bundling. High-resolution crystal structures of archaeal SepF alone and in complex with the FtsZ C-terminal domain (FtsZCTD) reveal that SepF forms a dimer with a homodimerization interface driving a binding mode that is different from that previously reported in bacteria. Phylogenetic analyses of SepF and FtsZ from bacteria and archaea indicate that the two proteins may date back to the Last Universal Common Ancestor (LUCA), and we speculate that the archaeal mode of SepF/FtsZ interaction might reflect an ancestral feature. Our results provide insights into the mechanisms of archaeal cell division and pave the way for a better understanding of the processes underlying the divide between the two prokaryotic domains.

High-resolution ultramicroscopy of the developing and adult nervous system in optically cleared Drosophila melanogaster.

The fruit fly, Drosophila melanogaster, is an important experimental model to address central questions in neuroscience at an organismic level. However, imaging of neural circuits in intact fruit flies is limited due to structural properties of the cuticle. Here we present a novel approach combining tissue clearing, ultramicroscopy, and data analysis that enables the visualisation of neuronal networks with single-cell resolution from the larval stage up to the adult Drosophila. FlyClear, the signal preserving clearing technique we developed, stabilises tissue integrity and fluorescence signal intensity for over a month and efficiently removes the overall pigmentation. An aspheric ultramicroscope set-up utilising an improved light-sheet generator allows us to visualise long-range connections of peripheral sensory and central neurons in the visual and olfactory system. High-resolution 3D reconstructions with isotropic resolution from entire GFP-expressing flies are obtained by applying image fusion from orthogonal directions. This methodological integration of novel chemical, optical, and computational techniques allows a major advance in the analysis of global neural circuit organisation.

Host-Polarized Cell Growth in Animal Symbionts.

To determine the fundamentals of cell growth, we must extend cell biological studies to non-model organisms. Here, we investigated the growth modes of the only two rods known to widen instead of elongating, Candidatus Thiosymbion oneisti and Thiosymbion hypermnestrae. These bacteria are attached by one pole to the surface of their respective nematode hosts. By incubating live Ca. T. oneisti and T. hypermnestrae with a peptidoglycan metabolic probe, we observed that the insertion of new cell wall starts at the poles and proceeds inward, concomitantly with FtsZ-based membrane constriction. Remarkably, in Ca. T. hypermnestrae, the proximal, animal-attached pole grows before the distal, free pole, indicating that the peptidoglycan synthesis machinery is host oriented. Immunostaining of the symbionts with an antibody against the actin homolog MreB revealed that it was arranged medially-that is, parallel to the cell long axis-throughout the symbiont life cycle. Given that depolymerization of MreB abolished newly synthesized peptidoglycan insertion and impaired divisome assembly, we conclude that MreB function is required for symbiont widening and division. In conclusion, our data invoke a reassessment of the localization and function of the bacterial actin homolog.

Asynchronous division by non-ring FtsZ in the gammaproteobacterial symbiont of Robbea hypermnestra.

The reproduction mode of uncultivable microorganisms deserves investigation as it can largely diverge from conventional transverse binary fission. Here, we show that the rod-shaped gammaproteobacterium thriving on the surface of the Robbea hypermnestra nematode divides by FtsZ-based, non-synchronous invagination of its poles-that is, the host-attached and fimbriae-rich pole invaginates earlier than the distal one. We conclude that, in a naturally occurring animal symbiont, binary fission is host-oriented and does not require native FtsZ to polymerize into a ring at any septation stage.

Size-independent symmetric division in extraordinarily long cells.

Two long-standing paradigms in biology are that cells belonging to the same population exhibit little deviation from their average size and that symmetric cell division is size limited. Here, ultrastructural, morphometric and immunocytochemical analyses reveal that two Gammaproteobacteria attached to the cuticle of the marine nematodes Eubostrichus fertilis and E. dianeae reproduce by constricting a single FtsZ ring at midcell despite being 45 µm and 120 µm long, respectively. In the crescent-shaped bacteria coating E. fertilis, symmetric FtsZ-based fission occurs in cells with lengths spanning one order of magnitude. In the E. dianeae symbiont, formation of a single functional FtsZ ring makes this the longest unicellular organism in which symmetric division has ever been observed. In conclusion, the reproduction modes of two extraordinarily long bacterial cells indicate that size is not the primary trigger of division and that yet unknown mechanisms time the localization of both DNA and the septum.

Growth in width and FtsZ ring longitudinal positioning in a gammaproteobacterial symbiont.

Rod-shaped bacteria usually grow in length and place their FtsZ ring and division site at midcell, perpendicular to their long axis [1,2]. Here, we provide morphometric and immunocytochemical evidence that a nematode-associated gammaproteobacterium [3,4] grows in width, sets a constricting FtsZ ring parallel to its long axis, and divides longitudinally by default. Remarkably, the newly described FtsZ ring appears to be not only 90° shifted with respect to model rods, but also elliptical and discontinuous. This reveals an unexpected versatility of the gammaproteobacterial cytokinetic machinery.