Wednesday, March 01, 2006

Protection of the cell membrane

Protection of the cell membrane in Gram positive and Gram negative bacteria is a important topic that is discussed very well in Chapter 2 of Schaechter 2006.

How do these two different types of bacterial outer cell layers (G+ versus G-) exclude toxic hydrophobic compounds?

How do hydrophilic compounds less than 700 molecular mass enter Gram negative cells?


Blogger Archeak9 said...

Hydrophobic exclusion:

“The cell wall (sacculus) of gram-positive bacteria, consististing of several layers of murein, forms a thick barrier that hinders the passage of hydrophobic compounds because its phosphates, sugars and charged amino acids are highly polar.” (Microbe pp25)
“external hydrophilic polysacharides(LPS)…on the outer membrane” (microbe pp27)

Hydrophilic penetration of small molecules in G- bacteria:
Inner membranes are penetrated by transporters and channels.
Outer membrane penetrated via porins (triads of proteins forming hydrophilic channels size restrictive to 600Da. (microbe pp27).
The triads = homotrimer of 30-50kda subunits each of which forms a 8 to 22 stranded beta barrel pore 7-12Angstrom (Biochemistry 3ed Voet&Voet pp399-401F )(see figure 401F or fig 2 of article below)
E. coli is known to produce three trimeric porins (OmpF, OmpC, and PhoE) each of which has different channel characteristics. Their expression is regulated dependent on their environment OmpF favoured in open water, OmpC favoured in the gut, and PhoE on the basis of phosphate shortage. (Microbiol Mol Biol Rev. 2003 December; 67(4): 593–656)

March 02, 2006 10:45 am  
Blogger Microbe Pundit said...

Wow. Really good stuff

March 03, 2006 10:42 am  
Anonymous Anonymous said...

Its worth noting the charges on the G+ wall come from NAcMuramic acid in the murein and phosphate groups in the teichoic acid. The charges and hydrophilicity of the outer-surface of G- come from the lipopolysaccharide layer.

Further suggestions:
I think it should be said clearly that the "channels" referred to in the text are the porin chanels in the outer membrane of G-ves.

I cannot think of a good example of an inner membrane channel that is well studies.(But always, I"m happy to learn I'm wrong)

Microbe Pundit

March 03, 2006 5:17 pm  
Blogger Archeak9 said...

Pundit seems to be correct. Although eukaryotic membranes are full of channels, good examples of G- bacteria inner membrane channels are not documented. A search yeilded about five examples but on closer examination all are associated with GTPases, ATPases (much like Tim/Bip)or result in lysis and hence are not good examples of channels. My guess is that there are two contributing reasons: 1/bacteria cannot rely upon the external environment to be consistent, 2/since cytoplasmic concentrations are higher than in Eukaryotic cells diffusion is much less likely to be benificial.

March 06, 2006 9:35 am  
Blogger ahilah said...

This comment has been removed by the author.

May 17, 2007 12:59 pm  
Anonymous imalda said...

OK from what i have learnt via the lecture notes and other material is this.

In regards to protection of the cell membrane in gram positive and gram negative bacteria, both the cell walls have different compositions which have evolved to protect the bacteria from harmful compounds.

imalda devaparanam

If we look at gram positive cell walls firstly, we know that gram positive bacteria have a very thick and multilayered peptidoglycan layer- and this peptidoglycan is in the form of murein in bacterial cells. Peptidoglycan is a polymer composed of sugars and amino acids to which a short peptide is connected. Peptidoglycan forms an thick barrier around the gram positive cells and helps protect the cell from high internal turgor. So in other words, this layer helps protect the cell in environments where the osmotic pressure is less than that of the cytoplasm. In the absence of peptidoglycan in such environments, the cell membrane would burst and the cell would lyse. In addition, the polar nature of the murein layer helps hinder the passage of hydrophobic compounds into the cell. This is why gram positive bacteria are are able to withstand hydrophobic compounds- such as bile salts in our intestines.

If we think about the other features of the gram positive cell wall other than just the murein layer, one must remember the presence of other polymers, such as techoic acids. These are polymers of sugar and alchohol which are linked via a phosphodiester bond. These polymers help in pathogenesis by promoting adhesion of the organism to the host.

If we look at gram negative bacteria, the cell wall of gram negatives have a very thin peptidoglycan layer, which is mainly single layered. So how are gram negatives protected? Gram negatives have an extra outer membrane that gram postives do not have. This outer membrane helps protect the cell from the activity of harsh chemicals. In addition, to this outer membrane, you would find the presence of lipolysaccharides. These are not present in gram postive cell walls. These lipopolysaccharides are composed of three parts: Lipid A, to which a polysaccharide core is attached and finally a highly immunogenic O-antigen. Lipid A attaches the LPS to the external leaflet of the cell wall, and upon release of the lipid A from the outer membrane, it can results in fever and shock in verterbrates. Hence LPS is considered an endotoxin. The hydrophillic nature of the O antigen helps exclude hydrophobic compounds from the cell, thereby protecting the cell. Gram negative bacteria seem to allow the diffusion of hydrophillic compounds by the presence of porins, which act as channels, and are found in the outer membrane of the cell. These porins allow the entrance of hydrophillic amino acids, sugars and certain ions that are essential to the cells survival.

in an exam situation, this is probably what i would have written, i hope this helps :S!

May 17, 2007 1:01 pm  

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