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Why do different bacteria have different shapes? Is it only related to their function?

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Due to multiple evolutionary forces acting over long periods of time (along with chance events, heterogeneous environments etc)? I really don't see this as a constructive question. You could just as well replace with "Why do birds have different colours?" or "Why do fish have different sizes?" –  fileunderwater Oct 10 '13 at 8:49
    
@fileunderwater: I see it as a very interesting biological question. I would rather compare it to "why do birds have differently shaped beaks?" –  nico Oct 10 '13 at 16:01
    
@nico I see that question as too broad as well - too many possible answers and reasonable mechanisms to be answerable in less than a book. –  fileunderwater Oct 10 '13 at 20:18

3 Answers 3

up vote 11 down vote accepted

Reasons for Bacteria with different shapes as given in Wikipedia/Bacteria:

The wide variety of shapes is determined by the bacterial cell wall and cytoskeleton, and is important because it can influence the ability of bacteria to acquire nutrients, attach to surfaces, swim through liquids and escape predators.

There is an article based on research by Kevin D. Young Bacterial morphology: Why have different shapes? which highlights 3 main points which defines the shape of Bacteria:

  1. Nutrient uptake: Cell shape, in and of itself, affects nutrient acquisition and argues that other nutritional situations may create conditions that favor one bacterial shape over another.
  2. Motility: Motility imposes a heavy selective pressure on cell shape. Fast cells are better off as rods with a certain length-to-width ratio, chemotactic cells must adopt shape ratios in line with their environments, and cells that forage near surfaces or navigate viscous environments may do best if they are slightly curved or spiral.
  3. Predation: In their struggle against being eaten, bacteria have adopted morphological defenses that may have produced the wealth of shapes we now observe.

These factors play a role in shaping bacteria. Further he explains what in general gives the shape to bacteria:

Bacterial morphology is affected by a combination of selective pressures - access to nutrients, cell division, attachment/dispersal, predation and motility (among others). Among these different factors, our understanding of the relationship between motility and cell shape is most complete; for example, highly motile bacteria are usually rods with a specific (optimal) shape and size, and movement through viscous fluids seems to a favor spiral shape. But since multiple selective forces are always in play, there is at the moment no way to predict cell shape based on the environment or vice versa.

Shape Matters: Why bacteria care how they look - This article highlights Kevin's work explaining why shapes matter for bacteria and they take it seriously for survival.

So adaptations to various environments could be possible but depending upon the environment we can not predict the shape of bacteria because some other factor might have played major role in giving shape to it. But the above mentioned 3 main factors gives some explanation.

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As for how certain bacteria achieve their shapes, the cell wall plays an important role. Almost all bacteria have a cell wall made of a substance called peptidoglycan, a mesh of peptides and saccharides that provides rigidity and strength to the bacteria. This wall is produced through a complex series of steps that starts inside the cell, where the peptide and sugar units are attached to a lipid to make a compound called Lipid II. However, Lipid II must be flipped to the outer layer of the cell membrane to deliver the peptides and sugars to the growing cell wall. The proteins responsible for flipping the Lipid II are believed to be FtsW, RodA, and SpoVE. FtsW is considered the main enzyme for most cell growth. RodA is found in rod shaped bacteria and is considered necessary for cell elongation. SpoVE is used to build the thick cell walls of bacterial spores. By controlling how Lipid II is delivered to the growing cell wall, these proteins play a role in controlling cell shape.

References:

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From the first reference:

The first issue to get settled is that the shape of a bacterium has biological relevance. One argument favoring this assertion is that even though bacteria have a wide variety of shapes, any one genus typically exhibits a limited subset of morphologies, hinting that, with a universe of shapes to choose from, individual bacteria adopt only those that are adaptive. Another clue is that some bacteria can modify their morphology in response to environmental cues or during the course of pathogenesis, suggesting that shape is important enough to merit regulation.

Two evolutionary arguments also support the utility of bacterial shape. First, shape has a vector through evolutionary time – rod-like organisms having arisen first and coccoid forms being derivatives at the ends of evolutionary lines. Progressive development of a trait implies that selective forces are operating. Secondly, prokaryotes with different genealogies may converge morphologically, indicating that a similar shape may confer advantages in certain environments. So, for example, although they have a non-peptidoglycan-based cell wall, the Archaea exhibit a range of morphological forms similar to that of the Bacteria. The simplest conclusion is that morphological adaptation serves an important biological function.

References: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2169503/ http://www.scienceclarified.com/As-Bi/Bacteria.html

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You should not copy and paste entire paragraphs for your answer without indicating that you did so. The second reference doesn't seem related to the question. –  Mike Taylor yesterday
    
Can you please expand your answer beyond a mere citation? –  Chris yesterday
    
I would debate Young's claim that "any one genus typically exhibits a limited subset of morphologies, hinting that ... individual bacteria adopt only those that are adaptive" because that neglects entirely the phylogenetic history of the genus. The species in a genus may have a particular shape solely because their common ancestor had that shape. There may have been no selective pressure to change the shape of species within the genus. –  Mike Taylor 8 hours ago

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