Shaping and moving a spiroplasma

Area of Science:

• Microbiology
• Cell Biology
• Structural Biology

Background:

• Mollicutes represent the simplest known free-living cells, evolved from Gram-positive bacteria via genome reduction.
• They possess minimal genomes, lack cell walls and flagella, yet are motile and chemotactic.
• Unlike other bacteria, Mollicutes lack homologs for typical bacterial motility and cytoskeleton genes.

Purpose of the Study:

• To investigate the unique helical cell geometry and motility mechanisms of Spiroplasmas.
• To elucidate the structural basis of Spiroplasma motility and its relation to their minimal genome.
• To quantify Spiroplasma structure and dynamics using microscopy and biophysical calculations.

Main Methods:

• Analysis of Mollicute phylogeny and genome reduction.
• Structural characterization using light and electron microscopy.
• Biophysical modeling to correlate structure with helical dynamics and motility.

Main Results:

• Spiroplasmas possess a unique helical cell structure composed of a dynamic membranal tube and an internal helical cytoskeletal ribbon.
• The cytoskeleton, formed by fib gene product filaments, acts as a linear motor, driving helical dynamics through fibril length changes.
• Cellular helical dynamics enable nonreciprocating movements and directional swimming, with flexing allowing for changes in direction.

Conclusions:

• Spiroplasma's helical geometry and contractile cytoskeleton are key to their unique motility and structural simplicity.
• This helical structure allows for analytical quantification and provides insights into minimal cell mechanics.
• The findings reveal novel mechanisms of motility in minimal organisms, distinct from typical bacterial systems.