Modes of cell wall growth differentiation in rod-shaped bacteria

Abstract

A bacterial cell takes on the challenge to preserve and reproduce its shape at every generation against a substantial internal pressure by surrounding itself with a mechanical support, a peptidoglycan cell wall. The enlargement of the cell wall via net incorporation of precursors into the pre-existing wall conditions bacterial growth and morphology. However, generation, reproduction and/or modification of a specific shape requires that the incorporation takes place at precise locations for a defined time period. Much has been learnt in the past few years about the biochemistry of the peptidoglycan synthesis process, but topological approaches to the understanding of shape generation have been hindered by a lack of appropriate techniques. Recent technological advances are paving the way for substantial progress in understanding the mechanisms of bacterial morphogenesis. Here we review the latest developments, focusing on the impact of new techniques on the precise mapping of cell wall growth sites.

Figure 1. Schematics of different modes of cell wall growth in bacteria

A) E. coli (or B. subtilis) follow a disperse mode of growth during elongation followed by activation of zonal synthesis at the time of septation. Short MreB filaments direct local incorporation of nascent PG by the elongation biosynthetic complexes. B) C. crescentus elongates and divides by zonal insertion of nascent PG at a location slightly offset from the axis of symmetry. Subsequent septation will lead to distinct offspring cells. C) A. tumefaciens elongates by zonal-apical growth, followed by activation of a central site of zonal synthesis and symmetric septation. D,E) In cocci (i.e. S. aureus) and ovococci (i.e. S. pneumoniae) elongation and septation are essentially a single process mediated by zonal central PG incorporation. Dark blue, active PG biosynthetic complexes (regions); light blue, PG produced by zonal insertion; beige, PG produced by disperse insertion.

Figure 2. Different modes of growth in a natural population

A saliva sample was pulse-labeled successively with a blue, a red, and a green Fluorescent-D-amino acid (FDAA) with washing of excess dye between each labeling. The large area labeled in green is a eukaryotic cell that was fortuitously labeled by a degradation product of the green FDAA (FDAAs do not label eukaryotic cells). The labeling patterns on this image provides a chronological account of the areas of PG synthesis during each pulse-labeling. Cells (a) and (b) grow polarly but cell (a) grows at the same rate from each pole, while cells (b) probably switched growth from the blue (or blue-red) pole to the red-green pole during the experiment. Cells (c) and (d) grow by septal incorporation. Alternating planes of growth are evident in cell (d).