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I've read that the bacterial capsule protects the bacterium from phagocytes and prevents water (and nutrients, possibly?) leakage from the bacterium.

A less organized and less dense version is called a slime layer. It seems that the slime layer has similar functions, according to Wikipedia.

What are the differences between the effects a capsule and a slime layer bring to a bacterium?

I thought about the followings:

  • The capsule covers most part of the bacterium, so adhesion proteins and transport proteins on the plasma membrane do not work effectively, whereäs with a slime layer, more proteins on the plasma membrane are exposed to the exterior, making them work more effectively.
  • Maintenance of the capsule takes more energy than that of the slime layer simply because there are more glycoproteins in the capsule.
  • Bacteria with capsules are less likely to lyse than those with slime layers.

Is there anything else significant that goes on this list? Or are there any points I made that are not true?

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According to the Wikipedia section on bacterial capsules, they're made of the same thing:

When the amorphous viscid secretion (that makes up the capsule) diffuses into the surrounding medium and remains as a loose undemarcated secretion, it is known as slime layer.

Todar's Online Textbook of Bacteriology synonymizes "slime layer" and "biofilm":

A true capsule is a discrete detectable layer of polysaccharides deposited outside the cell wall. A less discrete structure or matrix which embeds the cells is a called a slime layer or a biofilm.

This makes sense with what I remember learning about slime walls--the main difference I saw was that the capsule was directly associated with/only protects a single bacterial cell, whereas the slime layer is associated with/protects multiple bacteria. I'm not sure that I'd call it a biofilm though, as those are notoriously difficult to remove; their removal in hospital and food processing sysyems is a current research topic.

Regarding your three points above, I might instead argue that the slime layer provides another layer of protection in addition to the bacterial capsule.

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  • $\begingroup$ So the slime layer protects multiple bacteria attached to each other due to the polar polysaccharides? If so, is this protection weaker than that of the capsule? Also, I don't understand how the polysaccharides can "diffuse into the surrounding medium". I thought the capsule / slime layer is the network formed by the polysaccharide parts of plasma membrane glycoproteins. How do these polysaccharides detach from their protein parts and diffuse into the surroundings? Or is it just that the polysaccharides have polar attractions with water (and possibly other polar substances)? $\endgroup$ – hello all Jun 16 '15 at 15:42
  • $\begingroup$ From Wikipedia: "The capsule... is a polysaccharide layer that lies outside the cell envelope of bacteria... which can be found in both Gram-negative bacteria and Gram-positive bacteria should not be confused with the second lipid membrane (or bacterial outer membrane), which contains lipopolysaccharides and lipoproteins and is found only in Gram-negative bacteria. $\endgroup$ – Luigi Jun 16 '15 at 15:56
  • $\begingroup$ So the capsule is not a network of polysaccharides of membrane glycoproteins but simply a network of polysaccharides. $\endgroup$ – hello all Jun 17 '15 at 4:01

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