Exploring Spiroplasma Biology: Opportunities and Challenges

Abstract

Spiroplasmas are cell-wall-deficient helical bacteria belonging to the class Mollicutes. Their ability to maintain a helical shape in the absence of cell wall and their motility in the absence of external appendages have attracted attention from the scientific community for a long time. In this review we compare and contrast motility, shape determination and cytokinesis mechanisms of Spiroplasma with those of other Mollicutes and cell-walled bacteria. The current models for rod-shape determination and cytokinesis in cell-walled bacteria propose a prominent role for the cell wall synthesis machinery. These models also involve the cooperation of the actin-like protein MreB and FtsZ, the bacterial homolog of tubulin. However the exact role of the cytoskeletal proteins is still under much debate. Spiroplasma possess MreBs, exhibit a rod-shape dependent helical morphology, and divide by an FtsZ-dependent mechanism. Hence, spiroplasmas represent model organisms for deciphering the roles of MreBs and FtsZ in fundamental mechanisms of non-spherical shape determination and cytokinesis in bacteria, in the absence of a cell wall. Identification of components implicated in these processes and deciphering their functions would require genetic experiments. Challenges in genetic manipulations in spiroplasmas are a major bottleneck in understanding their biology. We discuss advancements in genome sequencing, gene editing technologies, super-resolution microscopy and electron cryomicroscopy and tomography, which can be employed for addressing long-standing questions related to Spiroplasma biology.

Keywords: MreB; Spiroplasma; Spiroplasma genetics; bacterial cytoskeleton; bacterial division; fibril; motility; shape determination.

FIGURE 1

Growth, cell division and motility mechanisms in bacteria. (A) Spherical, cell-walled bacteria grow and divide by insertion of peptidoglycan in FtsZ-dependent manner. Alternatively, some spherical bacteria divide by constriction. (B) Some of the rod-shaped, cell-walled bacteria such as E. coli grow uniformly along the length by uniform insertion of peptidoglycan facilitated by MreB patches and divide to produce daughter cells. The dark blue and light blue colors represent patches of MreB, and associated peptidoglycan synthesis machinery shown as red and pink dots, on the membrane at the front and back of the cell, respectively. (C) M. smegmatis, a cell-walled bacterium exhibits MreB-independent polar growth by positioning the peptidoglycan synthesis machinery (orange dots) with the help of DivIVA (pink) at the cell poles. (D) Caulobacter crescentus attains crescent shape by asymmetric growth using crescentin polymers (yellow line) that prevent peptidoglycan insertion at the site of their location. The crescentin polymers are positioned by MreB and the former prevents insertion of new peptidoglycan in its vicinity. The MreB patches present at locations away from crescentin facilitate cell growth, thus leading to crescent shape. (E) The most predominant mechanism of cell division by formation and constriction of FtsZ ring at the mid-cell region in cell-walled bacteria. The FtsZ-assisted insertion of peptidoglycan at the mid-cell region results into septum closure and separation of daughter cells. (F) Well studied motility mechanisms using appendages in cell-walled bacteria include the (i) flagella dependent swimming and (ii) type IV pili-based twitching motility. See Miyata et al. (2020) for a detailed review on motility mechanisms in bacteria.

FIGURE 2

Schematic representations of open questions in Spiroplasma biology. Open questions in Spiroplasma biology include- (A) Organization of cytoskeletal filaments and cell polarity determinants. A Spiroplasma cell showing its characteristic helical shape, distinct poles (one is tapered while the other is blunt) and cytoskeletal ribbon (blue) connecting the two poles through the shortest path along the cell body is shown. (B) Factors conferring spherical, rod and helical shapes, determinants of helical pitch and cell lengths in Spiroplasma. (C) Kinking-based motility in Spiroplasma. The kink is introduced at one of the poles by change in handedness of the cell and traverses through the cell body. Portions of the cell on either side of the kink have opposite handedness. (D) Modes of cell division (cross sectional and Y-shaped) and factors favoring them. The bulleted text in the figures summarizes the open questions.