When on the 3rd day of creation − or the 5th, the Old Testament isn't quite clear here − the Bacteria and Archaea were summoned to choose their shapes, there was at once a run on rods and cocci; spirillae were also immediately sold out. The Alphaproteobacteria frolicked and adorned themselves with all available prosthecae, cell appendages, that they cherish to this day. A few Archaea had snatched the quadrangular chips − one is still around today, Haloquadratum walsbyi − after the ever faster phages had quickly distributed the fancy icosahedra among themselves. Thus, for one loitering sulfur-oxidizing Gammaproteobacterium from the Chromatiales, only the cube remained. Well, mumbled "Candidatus Thiosymbion cuboideus", then I'll just challenge the millimeters-long Stilbonematinae worm Catanema sp. "Guadeloupe" from the day 5 lot of creatures to a game of dice... Spoiler: the two have become close friends and still roll the dice today (Figure 1a).
Silvia Bulgheresi's lab at the Archaea Biology and Ecogenomics Unit of Vienna University, Austria, is interested in the remarkable efforts − one could easily talk about "contortions" − that certain bacteria make to successfully colonize the cuticle of nematodes as ectosymbionts (also called epibionts) a special yet not rare form of symbiosis. Two years ago, Philipp Weber described in detail "Candidatus Thiosymbion oneisti" and its lengthwise cell division in this blog (Nanne Nanninga had first introduced it in these pages). Now, Philipp Weber and colleagues have published their recent studies of "Candidatus Thiosymbion cuboideus", a relative of "Oneisti" (and the authors won't mind when I call the bacterium not by its full name hereafter but simply refer to it as "Cuby").
Viewed from above by scanning electron microscopy (SEM), Cuby cells lie somewhat disorganized and jumbled but tightly packed on the cuticle of their nematode host; a few appear to be in the process of dividing (see frontpage). Carefully prepared single cells revealed their real shape: tiny cubes with average edge lengths for non-dividing cells of 1.5×1.7×1.3 µm (Figure 1c). In relative terms, the calculated cell volume of ~3.3 µm3 is roughly 3-times that of a "standard" E. coli cell (1.3 µm3). Like Oneisti cells, Cuby cells arrange as a monolayer in which every cell is attached to the host cuticle. The rod‑shaped Oneisti cells stand upright and cramped like in a palisade, with one cell pole attached to the host (see here.) With the Cubies, it looks more like a cobblestone pavement (think of the infamous Paris-Roubaix bike race course). And rather than having the customary hemispherical, rounded "pole" they have a flat "polar plane" (Figure 1b).
Weber et al. (2021) confirmed the attachment of Cuby cells to the host cuticle at their "polar plane" by staining live samples with a fimbriae-specific antibody (Figure 2, 2nd row). You should know that working with the bacteria-coated worms in the lab is tricky: they survive sampling, and also transport when frozen, but cannot (yet) be taken into permanent culture. The same applies to the worms' epibionts: they're alive, that is grow and divide, as long as their hosts stay alive. Nevertheless, the authors showed that Cubies grow "sideways," that is, perpendicular to the "polar plane," because they found incorporation of the Alexa488 fluorophore-labeled peptidoglycan precursor ethynyl-D-alanyl-D‑alanine (EDA‑DA) into the extending cell wall at the "division plane," and starting from the (nematode-attached) polar plane (Figure 2, 3rd row). Despite their different cell shapes, Oneisti and Cuby have this "sideways" growth in common. Weber et al. (2021) assume that this is an adaptation to the strict requirement to keep the progeny firmly attached to the host at all times.
Cuby didn't stop to surprise the researchers when it came to cell division. It has, like Oneisti, the full set of genes encoding the regular division proteins including the tubulin homolog FtsZ (see our recent Pictures Considered #52.) In the test tube, FtsZ polymerizes into long, straight rods but inside cylindrical cells like growing E. coli cells one can observe them briefly as shorter side-by-side filaments spiraling along the inner membrane. Since they are occluded from the cells poles and also from the nucleoid − or better: from the two nucleoids in growing cells − they accumulate at midcell where cell division will take place. While the divisome proteins direct the synthesis of the septal peptidoglycan they simultaneously tether FtsZ arcs to the membrane, and the overlapping arcs give the appearance of a successively constricting "FtsZ" ring (this is a rather sketchy outline of a highly complex process, mind you). In Cuby, Weber et al. (2021) found FtsZ located as short filaments at the sideways-oriented division plane, and there preferentially at the proximal (nematode-attached) side aligning with commencing invaginations (Figure 2). A closer inspection by 3D structured illumination microscopy (3D-SIM) revealed that the short FtsZ filaments arrange without much overlap along the square-shaped division plane, including filaments with apparent kinks, unheard of so far for FtsZ (Figure 3). They did not observe septal closure. Thus, it remains an open question how the very last stage of cell division is orchestrated, which proteins are involved, and how they are spatially organized. It would be intriguing to learn if the "FtsZ square" ever changes shape into a "constricting ring" at any point or whether a bidirectional zipper-like mechanism starting form the proximal (nematode-attached) side does the job, which would nicely maintain the cubic cell shape.
But maybe the difference between a annular 'division zone' as in cells with a cylindrical or spherical shape, rods and cocci, and a more-or-less quadrangular 'division plane' as in Cuby isn't such a big deal after all? The researchers should look over the shoulders of colleagues who are struggling with such problems in Archaea (they certainly do, as they're affiliated with the Archaea Biology and Ecogenomics Unit of their university.) Also other "archaeologists" investigate cell division: Kanika Khanna had recently reported here in STC about it and the particular role of the archaeal FtsZ homolog, FtsZ1,: The Cake-Cutting Problem in the Archaeal World.