Abstract
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 switch the handedness of helical fibril filaments. Cooperative changes of these filaments cause helicity to shift down the cell axis. Interestingly, unlike other motility systems, the fibril protein and Spiroplasma MreBs can be traced back to their ancestors. The fibril protein has evolved from methylthioadenosine/S-adenosylhomocysteine (MTA/SAH) nucleosidase, which is essential for growth, and MreBs, which function as a scaffold for peptidoglycan synthesis in walled bacteria.
Highlights
Specialty section: This article was submitted to Microbial Physiology and Metabolism, a section of the journal Frontiers in Microbiology
Spiroplasma poulsonii is known to disrupt the sex ratio of Drosophila species by killing males (Regassa and Gasparich, 2006; Harumoto and Lemaitre, 2018). Their successful survival may be supported by a unique swimming mechanism, which may be advantageous for translocation in the tissues of their hosts, because they do not stack due to high load to their appendages as do flagella and pili, which are widespread in bacterial motility (Miyata et al, 2020; Nakamura, 2020)
We suggest a working model to explain the swimming mechanism and its evolution based on currently available information and ideas
Summary
Specialty section: This article was submitted to Microbial Physiology and Metabolism, a section of the journal Frontiers in MicrobiologyReceived: 07 May 2021 Accepted: 12 July 2021 Published: 27 August 2021Prospects for the Mechanism of Spiroplasma Swimming.Front. Spiroplasma possesses helical cell morphology and swims in viscous media by switching handedness (Shaevitz et al, 2005; Wada and Netz, 2009). In addition to Spiroplasma swimming, Mollicutes have two types of unique gliding motilities, even though they are a small group, as discussed previously (Miyata and Hamaguchi, 2016a,b).
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