Molecular mechanisms for the evolution of bacterial morphologies and growth modes

Abstract

Bacteria exhibit a rich diversity of morphologies. Within this diversity, there is a uniformity of shape for each species that is replicated faithfully each generation, suggesting that bacterial shape is as selectable as any other biochemical adaptation. We describe the spatiotemporal mechanisms that target peptidoglycan synthesis to different subcellular zones to generate the rod-shape of model organisms Escherichia coli and Bacillus subtilis. We then demonstrate, using the related genera Caulobacter and Asticcacaulis as examples, how the modularity of the core components of the peptidoglycan synthesis machinery permits repositioning of the machinery to achieve different growth modes and morphologies. Finally, we highlight cases in which the mechanisms that underlie morphological evolution are beginning to be understood, and how they depend upon the expansion and diversification of the core components of the peptidoglycan synthesis machinery.

Keywords: Asticcacaulis; Caulobacter; FtsZ; MreB; bacterial morphology; bacterial shape; peptidoglycan synthesis.

FIGURE 1

Phylogeny and morphology of bacteria exhibiting different modes of growth. rpoC sequences were aligned using MUSCLE v3.8.31 (Edgar, 2004). RAxML v7.0.4 (Stamatakis, 2006) reconstructed the maximum likelihood phylogeny using the JTT amino acid substitution matrix with rate variation among sites modeled by a four-category discrete gamma distribution and an additional invariant category. Note that the stalk of Planctomyces maris is composed of several parallel fibrils (Bauld and Staley, 1976), and is not bound by cell envelop layers like that of the Caulobacterales. Bacteria are not drawn to scale. Scale bar reflects numbers of substitutions per site.

FIGURE 2

Growth modes in rod-shaped bacteria. Several growth modes with the regions of active peptidoglycan synthesis are schematized. Colors indicate regions of active peptidoglycan synthesis. (A) Lateral elongation is well-studied in E. coli, B. subtilis, and C. crescentus, and often assumed for many rod-shaped bacteria. (B) A majority of bacterial families divide in a FtsZ-dependent manner. Notable exceptions are members of the PVC (Planctomycetes, Verrucomicrobia, and Chlamydiae) superfamily. (C) Polar elongation is characteristic of Actinobacteria, where it is bipolar. It has recently been demonstrated in Rhizobiales (Brown et al., 2012) where it is unipolar. (D) Budding occurs both at the ends of stalks and off the cell body in Alphaproteobacteria. (E) Stalks are wide-spread in Alphaproteobacteria. While the exact mechanism for growth may differ among families, all depend on zonal growth of the peptidoglycan at the cell-stalk junction. (F) Some species exhibit medial (pre-septal) growth, which appears to be peptidoglycan synthesis near the division plane before full assembly of the Z-ring.

FIGURE 3

Peptidoglycan synthesis machinery schematic. The divisome and elongasome both consist of cytoplasmic scaffolds (orange), membrane-spanning elements that include regulatory proteins as well as peptidoglycan precursor synthesis machinery (blue), and peptidoglycan-modifying enzymes (red). Each assembly has genetic components that are biochemically distinct despite retaining similar functions (Table). In general, (1) cytoplasmic scaffolds such as FtsZ or MreB direct the location of peptidoglycan synthesis and recruit various cytoplasmic and inner-membrane components. (2) Cytoplasmic and inner membrane enzymes synthesize peptidoglycan monomers (lipid II) in the cytoplasm [only (a) MraY and (b) MurG shown] and (3) a flippase flips them across the inner membrane. (4, 5) Various membrane proteins function to regulate or organize the divisome or elongasome protein assemblies. (6) Bifunctional or monofunctional peptidoglycan synthases (a) polymerize lipid II into glycan strands and (b) crosslink the peptide chains to form the sacculus. Various enzymes modify the peptidoglycan after synthesis: (7) carboxypeptidases trim peptide chains, (8) endopeptidases cleave crosslinks, (9) lytic transglycosylases cleave the glycan strand, (10) and amidases remove peptide chains from the glycan strand. Question marks in the table indicate that elongasome proteins responsible for the indicated activities remain to be identified. Some proteins essential to the divisome have been omitted: ZipA, ZapABCD, FtsQ, FtsL, and FtsB.

FIGURE 4

Cytoplasmic scaffold localization dynamics and growth modes in Caulobacter crescentus. (A) C. crescentus exhibits four growth modes over the course of its dimorphic cell cycle: lateral elongation (swarmer and stalked cells), stalk synthesis and elongation (stalked cells), medial elongation (stalked cells), and septation. Colors indicate regions of active peptidoglycan synthesis. (B) The activity and dynamics of various peptidoglycan synthesis machineries, which are regulated by polar regulators, underlie the different growth modes. In swarmer cells, MipZ localizes at the flagellar pole, driving FtsZ to the opposite pole, and MreB is dispersed along the length of the cell. During the swarmer to stalked cell transition, the flagellar pole undergoes remodeling to become the stalked pole. This transition involves the recruitment of bactofilin and peptidoglycan synthesis machinery to the stalked pole by the PopZ network to initiate stalk synthesis. During initiation of chromosome replication and origin duplication, some of the MipZ-ParB complex binds the new origin and migrates with it to the new pole, displacing FtsZ from the new pole to the division plane, where it forms the Z-ring. MreB condenses at the division plane before dispersing again during constriction due to regulation by TipN. After septation, FtsZ reassembles into the Z-ring quickly in stalked cells, making medial elongation the primary mode of elongation in this morphology.