Without obvious cytoplasmic transport mechanisms, the scope of cell sizes for most bacteria is quite narrow, sufficiently small to accommodate diffusion-limited processes but large enough to house the necessary macromolecules, such as ribosomes and chromosomal DNA, to maintain and duplicate the cell. One key physical constraint is diffusion limitation. Other selective pressures on shape include motility and resistance to predation. crescentus stalk is an extreme example of increasing surface area for nutrient foraging in nutrient-poor environments. One important selective pressure is competition for nutrients, which can be optimized by increasing the surface area relative to total cell volume. Why have such morphological diversity? The answer is partly because of selective pressures and in part from physical constraints. More detailed information about these species and their shapes can be found in a recent comprehensive review. Even bacteria without walls, such as the mycoplasmas, have their own distinctive shapes organized by an internal cytoskeletal scaffold. All this diversity does not even take into account the multicellular structures that many bacteria can form, such as the long chains of Anabaena or the flower-shaped fruiting bodies of Stigmatella aurantiaca. Others are hundreds of microns long and over 20 microns wide, such as Epulopiscium fishelsoni. Some bacteria are tiny rods, such as Bdellovibrio bacteriovorus, which hunts and invades other bacteria and grows in their cytoplasm. While they generally have similar shapes and sizes as bacteria, either round or rod-like, one extreme thermophile ( Haloquadratum walsbyi) forms flat squares. cholerae but with tapered ends and a polar stalk at one end. Then there are even more interesting ones, including Streptomyces, which form fungi-like mycelial mats and aerial hyphae Caulobacter crescentus, which forms a curved rod like V. coli and Bacillus subtilis are straight rods Vibrio cholerae is a curved rod the Borrelia burgdorferi spirochete is a flat wave, an elongated and iterated version of the curved rod Spiroplasma species are helix-shaped Mycobacterium tuberculosis is a pleomorphic rod that often branches or swells at the poles Staphylococcus aureus is a sphere, and Streptococcus pneumoniae is ovoid, with pointed polar caps. For example, of the relatively straightforward shapes, E. Here I review what is known about the mechanisms that determine the shape of prokaryotic cells.ĭespite usually being constrained by a cell wall, the shapes of bacteria are highly diverse, reflecting their large phylogenetic range. Other bacteria that lack MreB homologs or even cell walls usedistinct cytoskeletal systemsto maintain their distinct shapes. Round cocci, on the other hand, have lost their MreBcables and instead mustgrowmainly via their division septum, giving them their characteristic round or ovoid shapes.

The overlap and interplay between these two systems and the peptidoglycan-synthesizing enzymes they recruit are the major driving forces of cylindrical shapes. In rod-shaped bacilli, helical cables of actin-like MreB protein stretch along the cell length and orchestrate elongation of the cell wall, whereas the tubulin-like FtsZ protein directs formation of the division septum and the resulting cell poles.

This cytoskeleton-directed cell wall patterning is strikingly reminiscent of how plant cell wall growth is regulated by microtubules. Recent advances in cytological methods for bacteria have shown that distinct cytoskeletal filaments composed of actin and tubulin homologs are important for guiding growth patterns of the cell wall in bacteria, and that the glycan strands that constitute the wall are generally perpendicular to the direction of growth. Because of their relative simplicity, bacterial cells are excellent models for how genes and proteins can directly determine morphology. Prokaryotes come in a wide variety of shapes, determined largely by natural selection, physical constraints, and patterns of cell growth and division.

Diagrams of bacteria shapes rod shaped