Morphology of Caulobacter crescentus and the Mechanical Role of Crescentin

Abstract

Bacterial cells exist in a wide variety of shapes. To understand the mechanism of bacterial shape maintenance, we investigate the morphology of Caulobacter crescentus, which is a Gram-negative bacterium that adopts a helical crescent shape. It is known that crescentin, an intermediate filament homolog of C. crescentus, is required for maintaining this asymmetrical cell shape. We employ a continuum model to understand the interaction between the bacterial cell wall and the crescentin bundle. The model allows us to examine different scenarios of attaching crescentin to the cell wall and compute the shape of the bacterium. Results show that if the sole influence of crescentin is mechanical, then the crescentin bundle is unrealistically rigid and must be attached to the cell wall directly. The model suggests that alternative roles for crescentin such as how it influences cell wall growth must be considered.

Figure 1

(A) The shape of the cell is described by the cell surface p′ and the crescentin bundle u. A full geometrical description is given in the Supporting Material. (B) The interaction between the crescentin bundle and the cell body is determined by how the bundle is attached to the cell wall. If the bundle is attached rigidly to the wall, then the twist of the bundle (described by e^f₁, e^f₂) is coupled to the surface normal, n′, of the wall. (C) If the bundle is anchored to the soft inner membrane, the cell wall is unaffected by twist in the bundle.

Figure 2

Typical shapes for normal-length cells. (A) The bundle is not physically anchored to the cell wall, and is attached in the fashion of Fig. 1C. (B) The bundle is anchored to the cell wall (Fig. 1B). There is a slight twist in the cell body. The dimensions of the axes are in microns.

Figure 3

Longer cells with a stiff crescentin bundle. (A) The bundle is anchored to the cell as in Fig. 1B. The pitch of the cell is ∼2.9 μm. The crescentin bundle is twisted with respect to its equilibrium shape. (B) If the torsion of the bundle is not coupled to the cell wall, the cell never adopts a helical shape.