Staying in Shape: the Impact of Cell Shape on Bacterial Survival in Diverse Environments

Abstract

Bacteria display an abundance of cellular forms and can change shape during their life cycle. Many plausible models regarding the functional significance of cell morphology have emerged. A greater understanding of the genetic programs underpinning morphological variation in diverse bacterial groups, combined with assays of bacteria under conditions that mimic their varied natural environments, from flowing freshwater streams to diverse human body sites, provides new opportunities to probe the functional significance of cell shape. Here we explore shape diversity among bacteria, at the levels of cell geometry, size, and surface appendages (both placement and number), as it relates to survival in diverse environments. Cell shape in most bacteria is determined by the cell wall. A major challenge in this field has been deconvoluting the effects of differences in the chemical properties of the cell wall and the resulting cell shape perturbations on observed fitness changes. Still, such studies have begun to reveal the selective pressures that drive the diverse forms (or cell wall compositions) observed in mammalian pathogens and bacteria more generally, including efficient adherence to biotic and abiotic surfaces, survival under low-nutrient or stressful conditions, evasion of mammalian complement deposition, efficient dispersal through mucous barriers and tissues, and efficient nutrient acquisition.

FIG 1

Cell wall composition and architecture in different bacterial classes and shapes. (A) Monomeric and cross-linked peptidoglycan (PG) structure depicting the N-acetylglucosamine (G)–N-acetylmuramic acid (M) disaccharide with a cross-link mediating meso-diaminopimelic acid (m-Dap) in the third position of the pentapeptide stem. (B) Glycan strands (repeating PG disaccharide subunits) are generally arranged circumferentially around the cell in both Gram-positive and Gram-negative bacteria. Gram-positive bacteria have a thick layer of PG (shown in purple), while Gram-negative bacteria have a thin layer between the inner (black outline) and outer (gray outline) membranes. (C) Model for the generation of curvature and twist in curved and helical bacteria whereby asymmetric wedges or zones of PG synthesis and/or modification may promote increased or decreased PG incorporation on one side or region of the cell.

FIG 2

Model for how the curved shape of Caulobacter crescentus enhances surface colonization under flow conditions. The curved shape of the bacterium allows the dividing cell to reorient the new daughter cell pole closer to the surface, thus increasing the likelihood of attachment. The dividing cell also increases the probability of daughter cell attachment by retracting the pilus right before complete division. Cells lacking curvature cannot orient the new daughter cell pole closer to the surface, resulting in the release of the swarmer daughter cell instead of surface attachment and colonization. (Adapted from reference by permission from Macmillan Publishers Ltd.)

FIG 3

Helicobacter pylori intraspecies cell shape heterogeneity. (A) Phase-contrast images of three clinical strains of Helicobacter pylori (188–190). (B) Quantitative Celltool scatterplot analysis (19) showing cell side curvature versus axis length for clonal populations of the indicated strains of H. pylori (>100 cells/strain).

FIG 4

Schematic of bacterial cellular appendages discussed in this review.

FIG 5

Model for the physiological role of stalk elongation in Caulobacter crescentus. Stalk elongation may serve to elevate cells away from attached surfaces to increase nutrient availability and to distance cells from other competitors. (Adapted from reference by permission of Taylor & Francis LLC.)