![]() ![]() ( 133) (K) Nocardia opaca (L) Chain of ratoon stunt-associated bacteria (M) Caulobacter sp. (I) transverse section of ratoon stunt-associated bacterium (J) Planctomyces sp. (A) Stella strain IFAM1312 ( 380) (B) Microcyclus (a genus since renamed Ancylobacter) flavus ( 367) (C) Bifidobacterium bifidum (D) Clostridium cocleatum (E) Aquaspirillum autotrophicum (F) Pyroditium abyssi ( 380) (G) Escherichia coli (H) Bifidobacterium sp. These same cells are included in smaller form in the dashed blue circle to compare their sizes to those of larger bacteria, which are drawn relative to the 10-μm line. Those in the dashed black circle are drawn relative to the 5-μm line. This collage of different cells, unless otherwise stated, is constructed from descriptions and illustrations given by Starr et al. As Zinder and Dworkin point out, our dogmatic fixation on rods, cocci, and spirals has “obscured the spectrum of enormous morphological diversity manifested by the bacteria” ( 380). And yet, amazingly, this short inventory barely begins to catalogue the known forms. The sizes of individual cells range over at least six orders of magnitude. Other organisms grow as branched or unbranched filaments, live in sheathed or unsheathed chains, or aggregate in primitive or highly organized multicellular composites. The prosthecate bacteria radiate extensions that create star-like constellations or bulbous whiskers, all of which, though seemingly irregular, replicate faithfully. One is a flat square, and another is a slim, coin-like circular disk. There are cells that look like lemons, teardrops, or oblong spheroids some are bent, curved, flat sided, triangular, bean shaped, or helical others are rounded, squared, pointed, curved, or tapered. A simple way to verify this is to take a leisurely stroll through Bergey's Manual of Determinative Bacteriology ( 133) or The Prokaryotes ( 65, 313), pausing to admire the surprising and bewildering riot of shapes, sizes, and aggregates, some of which are illustrated in Fig. Which is a shame, because the bacteria seem to care very much. To be brutally honest, few people care that bacteria have different shapes. From cradle to coffin, it's enclosure that defines us. Space must be cut off, shaped, defined, for us to inhabit. … We can't feel at home with the infinite sky above and around us. It's not in the open we feel comforted but in the shadows. Just as we are beginning to answer how bacteria create their shapes, it seems reasonable and essential that we expand our efforts to understand why they do so. Bacteria respond to these forces by performing a type of calculus, integrating over a number of environmental and behavioral factors to produce a size and shape that are optimal for the circumstances in which they live. Specifically, cell shape is driven by eight general considerations: nutrient access, cell division and segregation, attachment to surfaces, passive dispersal, active motility, polar differentiation, the need to escape predators, and the advantages of cellular differentiation. ![]() The aim of this review is to spell out the physical, environmental, and biological forces that favor different bacterial morphologies and which, therefore, contribute to natural selection. All of these imply that shape is a selectable feature that aids survival. Why do bacteria have shape? Is morphology valuable or just a trivial secondary characteristic? Why should bacteria have one shape instead of another? Three broad considerations suggest that bacterial shapes are not accidental but are biologically important: cells adopt uniform morphologies from among a wide variety of possibilities, some cells modify their shape as conditions demand, and morphology can be tracked through evolutionary lineages. ![]()
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