fmicb-12-706426 August 23, 2021 Time: 14:50 # 1 REVIEW published: 27 August 2021 doi: 10.3389/fmicb.2021.706426 Prospects for the Mechanism of Spiroplasma Swimming Yuya Sasajima1 and Makoto Miyata1,2* 1 Department of Biology, Graduate School of Science, Osaka City University, Osaka, Japan, 2 The OCU Advanced Research Institute for Natural Science and Technology (OCARINA), Osaka City University, Osaka, Japan Spiroplasma are helical bacteria that lack a peptidoglycan layer. They are widespread globally as parasites of arthropods and plants. Their infectious processes and survival are most likely supported by their unique swimming system, which is unrelated to well-known bacterial motility systems such as flagella and pili. Spiroplasma swims by switching the left- and right-handed helical cell body alternately from the cell front. The kinks generated by the helicity shift travel down along the cell axis and rotate the cell body posterior to the kink position like a screw, pushing the water backward and propelling the cell body forward. An internal structure called the “ribbon” has been focused to elucidate the mechanisms for the cell helicity formation and swimming. The ribbon is composed of Spiroplasma-specific fibril protein and a bacterial actin, MreB. Here, we propose a model for helicity-switching swimming focusing on the ribbon, in which MreBs generate a force like a bimetallic strip based on ATP energy and Edited by: Zhaomin Yang, switch the handedness of helical fibril filaments. Cooperative changes of these filaments Virginia Tech, United States cause helicity to shift down the cell axis. Interestingly, unlike other motility systems, the Reviewed by: fibril protein and Spiroplasma MreBs can be traced back to their ancestors. The fibril Pananghat Gayathri, protein has evolved from methylthioadenosine/S-adenosylhomocysteine (MTA/SAH) Indian Institute of Science Education and Research, Pune, India nucleosidase, which is essential for growth, and MreBs, which function as a scaffold Chih-Horng Kuo, for peptidoglycan synthesis in walled bacteria. Institute of Plant and Microbial Biology, Academia Sinica, Taiwan Keywords: helix, MreB, fibril protein, SAH nucleosidase, electron microscopy, cooperativity, kink, evolution *Correspondence: Makoto Miyata [email protected] INTRODUCTION Specialty section: Spiroplasma is a parasitic bacterium that infects arthropods and plants globally (Regassa and This article was submitted to Gasparich, 2006; Harne et al., 2020b). Their interactions with hosts are mostly commensal but Microbial Physiology and Metabolism, sometimes pathogenic, causing economical damage to different industries (Regassa and Gasparich, a section of the journal 2006). Interestingly, Spiroplasma poulsonii is known to disrupt the sex ratio of Drosophila species Frontiers in Microbiology by killing males (Regassa and Gasparich, 2006; Harumoto and Lemaitre, 2018). Their successful Received: 07 May 2021 survival may be supported by a unique swimming mechanism, which may be advantageous for Accepted: 12 July 2021 translocation in the tissues of their hosts, because they do not stack due to high load to their Published: 27 August 2021 appendages as do flagella and pili, which are widespread in bacterial motility (Miyata et al., 2020; Citation: Nakamura, 2020). Spiroplasma possesses helical cell morphology and swims in viscous media by Sasajima Y and Miyata M (2021) Prospects for the Mechanism switching handedness (Shaevitz et al., 2005; Wada and Netz, 2009). of Spiroplasma Swimming. The genus Spiroplasma belongs to the phylum Tenericutes, composed of the class Mollicutes, Front. Microbiol. 12:706426. which evolved from the phylum Firmicutes represented by Bacillus and Clostridium. Mollicutes doi: 10.3389/fmicb.2021.706426 have some of the smallest genome sizes among all culturable organisms and lack a peptidoglycan Frontiers in Microbiology| www.frontiersin.org 1 August 2021| Volume 12| Article 706426 fmicb-12-706426 August 23, 2021 Time: 14:50 # 2 Sasajima and Miyata Spiroplasma Swimming (PG) layer, unlike other bacteria (Razin et al., 1998; Grosjean progresses like a corkscrew (Figure 1B)(Wada and Netz, 2009). et al., 2014; Miyata et al., 2020). These unique characteristics When the cell swims in one direction, the distributions of helix were established during the evolutionary process of Mollicutes handedness, generation, and traveling of the kink in a cell can from Firmicutes. Many bacterial species, including Escherichia be presented as shown in Figure 1C (Nakane et al., 2020). Note coli and Bacillus subtilis, can grow in the L-form, which does not that the cell architecture is composed of proteins, and that the synthesize the PG layer under stresses inhibiting peptidoglycan structures and behaviors have intrinsic chirality not in mirror maintenance (Claessen and Errington, 2019). Mollicutes may images (Sasajima et al., 2021). have survived in the L-form due to their ability to escape the No other motility system is driven by switching the innate immune system of their hosts by halting the synthesis of helical handedness of a cell (Miyata et al., 2020; Nakamura, PG, a major target of natural immune system of hosts (Claessen 2020). Although Spirochetes, which are a phylum of Gram- and Errington, 2019). During the evolution from a Firmicutes- negative bacteria, are also known as helical swimming bacteria, like ancestor to extant Mollicutes, they established stable their swimming system is completely different from that of parasitism with the acquisition of adhesion ability, modulation Spiroplasma (Figure 1D)(Liu et al., 2009; Charon et al., 2012; of antigenic properties, and reduction in metabolic pathways. Nakamura, 2020). Spirochetes rotate and propel the cell body by In the absence of the PG, the flagella-based motility common rotating their flagella that are aligned along the cell axis in the in Firmicutes was lost because the machinery is anchored to periplasmic space. the PG layer (Miyata et al., 2020). Hence, Mollicutes may have evolved new motility systems because motility is beneficial for Cell Architecture in Swimming parasitic life. Interestingly, in addition to Spiroplasma swimming, The characteristic helical cell shape of Spiroplasma is maintained Mollicutes have two types of unique gliding motilities, even by an internal ribbon approximately 150 nm wide, which though they are a small group, as discussed previously (Miyata runs the whole cell length (Figure 2A)(Kürner et al., 2005; and Hamaguchi, 2016a,b). The mechanism of Spiroplasma Trachtenberg et al., 2008; Liu et al., 2017; Sasajima et al., 2021). swimming has attracted many researchers in the fields of The ribbon, composed of sheets of cytoskeletal filaments, is mycoplasmology, motility, and structural biology. Although the aligned along the innermost line of the helical cell structure. mechanism has been discussed in some aspects, a model for It can be divided into the central zone, 30 nm wide, and the whole image has not emerged (Kürner et al., 2005; Shaevitz outer zones based on electron microscopy (EM) (Kürner et al., et al., 2005; Trachtenberg et al., 2008; Wada and Netz, 2009; 2005). In the outer zone, “fibril” filaments are suggested to Roth et al., 2018; Harne et al., 2020a,b; Nakane et al., 2020). In be aligned horizontally with 10-nm periodicity (Kürner et al., this perspective review, we suggest a working model to explain 2005). Fibril protein is specific to the Spiroplasma genus and has the swimming mechanism and its evolution based on currently been the focus of research since 1980 (Figure 2B)(Townsend available information and ideas. et al., 1980; Trachtenberg et al., 2008, 2014). Recently, the critical role of fibril filaments has been suggested based on its structure determination using EM (Sasajima et al., 2021). MAIN TEXT The structure was consistent with previous works but did not suggest contraction and extension, as expected previously Swimming Scheme (Kürner et al., 2005; Cohen-Krausz et al., 2011). Instead, it Spiroplasma swimming has been analyzed for three species, suggested roles as the determinant of cell helicity. The fibril Spiroplasma melliferum, Spiroplasma citri, and Spiroplasma filament is composed of oval rings with dimensions of 11 and eriocheiris (Shaevitz et al., 2005; Wada and Netz, 2009; Liu 6 nm, a cylindrical connecting part, and aligned with an 8.7- et al., 2017; Roth et al., 2018; Harne et al., 2020a; Nakane nm unit length (Figure 2B)(Sasajima et al., 2021). The units et al., 2020; Sasajima et al., 2021). They are characterized as a do not disassemble or change its length; however, each unit helical cell 2–10-mm long with a tapered end (Figures 1A,B) is twisted either side relative to adjacent ones. The side view (Supplementary Video 1). They swim up to 5 mm/s in viscous of the fibril filament showed that the cylindrical part formed media by dynamically switching their handedness. The cells have a positive curvature (Figure 2B). In a cell, the fibril filaments different handedness simultaneously localizing along the cell axis, are aligned beneath the membrane with this curvature, forming reversing the handedness from the tapered end, making a kink at parts of the ribbon (Figure 3A)(Kürner et al., 2005; Sasajima the boundary of the axis (Shaevitz et al., 2005; Wada and Netz, et al., 2021). The isolated fibril filaments showed half pitches 2009; Nakane et al., 2020). When the kink travels along the cell around 350 nm similar to those of cells, suggesting that the axis, the helical structure on one side of the kink rotates in one cell