Not So Boring Bacteria

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Science  31 Jan 2003:
Vol. 299, Issue 5607, pp. 627
DOI: 10.1126/science.299.5607.627b

Spiroplasms are minimalist (in genetic terms) free-living forms of life, having about the same dimensions as a eukaryotic cilium. How they move about has been a puzzle, as they do not possess external flagella-powered rotary motors, yet are highly active within viscous environments. These tiny bacteria do not have a rigid cell wall like their larger cousins. Instead, they possess an internal contractile cytoskeleton consisting of protein subunits with no known homologs.

Gilad et al. and Trachtenberg et al. show that the cytoskeleton is organized as a flat but twisted ribbon of seven paired fibrils attached at points to the inner side of the cell membrane, which makes the entire cell adopt a helical shape. The fibrils can rapidly change length by swapping different ring-shaped subunits, which alters the geometry of the ribbon. Changes in helical dimensions cause cell shape changes to drive a linear motor; standing waves can be propagated for forward movement and boring through viscous media. Direction can be reversed easily by switching the handedness of the helix. Generating a “kink” seems to be the favored mode of propulsion, and spiroplasms can also twitch and bend to aid reorientation and rotation. The next question is how spiroplasms decide which direction to take. — CA

Mol. Micobiol.47, 657; 671 (2003).

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