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Bacterial Capsules

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I am trying to understand the purpose of this feature that bacterial possess. Based in what I have read, they are typically found in gram neg bacteria and protect against desiccation and phagocytosis. Is it present all of the time on the bacterial surface or only in times of profound stress? Now when one does a gram stain, does the capsule absorb the gram stain since it is the outer layer of the cell wall or does it seep through it? If so why-- does it have to do with the molecule composition of the capsule structure?

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  • 1 month later...

I'm not sure, but there is correlation between a bacterium being Gram positive/negative and the structure of its cell wall. Usually Gram positive have just a petidoglycan cell wall and a plasma membrane, and Gram negative have those and an outer membrane consisting of lipopolysaccharides and protein. This extra thickness and complexity contributes to it's ability to resist taking up Gram stain.

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If the OP is asking about bacterial capsules, they are generally exopolysaccharides. Most bacteria (also quite often in Gram+) produce them, but only if they produce enough of it that it becomes a recognizable, discreet structure it is being referred to as capsule. The production can be quite different depending on species, some just throw them out all the time, others have a low base production but ramp it up under a variety of conditions (sometimes under stress, sometimes in presence of a lot of food, or presence of other bacteria etc.). They do have a lot of additional functions, again depending on the organism, but also include adhesion, infection, biofilm formation.


A note on the Gram stain, the reason why Gram- are not stained is not due to resistance to staining via the additional layer, but rather due to the inability to retain stain as their peptidoglycan layer is so thin. In fact, crystal violet treatment generally stains Gram- as well as Gram+ bacteria, only the destaining step really allows you to distinguish those two.

As a rule of thumb, the shell of Gram- is weaker than those of Gram+, who often have additional protective, sometimes crystalline, layers on top of their thick peptidoglycan.

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  • 3 months later...

I agree with CharonY.


I would like to note, however, that the purpose of the bacterial capsule may be very diverse for different species, and it might be difficult to give a single answer to the question of its function. The purpose of the Gram stain is to characterize bacteria based on its cell wall composition, but deeper than characterization/identification (ie. function), you would have to study more into the behavior of the bacteria.


Also, to answer your second question about how the Gram stain works. As CharonY mentioned, the crystal violet is unable to be adsorbed by the gram- bacteria. This observation is likely because crystal violet contains three N atoms, including a charged group, which would result in high affinity for the sugar structures in both the peptidoglycan and lipopolysaccharide layers. However, when the decolorizer step is applied, the lipopolysaccharides appear to fall off of the cell wall, thus causing the bacteria to lose the capsule. The counterstain works much the same way as the crystal violet, but if the cells were already stained blue/violet on the peptidoglycan level, then the stain won't show up as much. Does that make sense?

Edited by iRNAblogger
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