MB0902
092184Q
Composition & Characteristics
Component:
-Petidoglycan
Peptidoglycan, which is also known as murein, is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of bacteria, forming the cell wall. The sugar component consists of alternating residues of β-(1,4) linked N-acetylglucosamine and N-acetylmuramic acid residues. Attached to the N-acetylmuramic acid is a peptide chain of three to five amino acids. The peptide chain can be cross-linked to the peptide chain of another strand forming the 3D mesh-like layer. Some Archaea have a similar layer of pseudopeptidoglycan or pseudomurein, where the sugar residues are β-(1,3) linked N-acetylglycosamine and N-acetyltalosaminuronic acid That is why the cell wall of Archaea is insensitive to lysozyme.
Functions:
- Maintains characteristic shape of cell
- Prevents cell from bursting
- Virulence-ability to cause disease
- Differentaiting between types of bacteria (Gram positive and negative)
Gram-positive bacteria
- cytoplasmic lipid membrane
- thick peptidoglycan layer(60%to 90%)
-teichoic acids and lipoids are present, forming lipoteichoic acids which serve to act as chelating agents, and also for certain types of adherence.
- capsule polysaccharides (only in some species)
- flagellum (only in some species)
-if it is present, it contains two rings for support as opposed to four in Gram-negative bacteria because Gram-positive bacteria have only one membrane layer.
Gram-negative bacteria
- Cytoplasmic membrane
- Thin peptidoglycan layer (10% to 20%)
- Outer membrane contains lipopolysaccharide (LPS, which consists of lipid A, core polysaccharide, and O antigen) outside the peptidoglycan layer
- Porins which are proteins that permit small molecules exist in the outer membrane
- Periplasmic space: A space between the layers of peptidoglycan and the secondary cell membrane (contains digestive enzymes and transport proteins)
- No teichoic acids or lipoteichoic acids are present
- Lipoproteins are attached to the polysaccharide backbone.
- Most do not sporulate (Coxiella burnetii, which produces spore-like structures, is a notable exception)
Differences between Gram postive and Gram negative Bateria Cell Walls
Peptidoglycan -Thick layer in Gram positive; thin layer in Gram negative
Teichoic acid - Present in Gram positive; Absent in Gram negative
Lipids -Very little in Gram positive; LPS in Gram negative
Outer membrane - Absent in Gram positive; Present in Gram negative
Periplasmic space - Absent in Gram positive; Present in Gram negative
Gram Staining
It is use to differentiate Gram-positive and Gram-negative bacteria based on the chemical and physical properties of their cell walls. Gram staining is not used to classify archaea, since these microorganisms yield widely varying responses that do not follow their phylogenetic groups.
Reagants:
- Crystal violet (Primary Stain)
- Iodine (Mordant)
- Alcohol (Decolourizer)
- Safrain ( Counter Stain)
Crystal violet (CV) dissociates in aqueous solutions into CV+ and chloride (Cl – ) ions. These ions penetrate through the cell wall and cell membrane of both Gram-positive and Gram-negative cells. The CV+ ion interacts with negatively charged components of bacterial cells and stains the cells purple. Iodine (I – or I3 – ) interacts with CV+ and forms large complexes of crystal violet and iodine (CV – I) within the inner and outer layers of the cell. When a decolorizer such as alcohol or acetone is added, it interacts with the lipids of the cell membrane. A Gram-negative cell will lose its outer membrane and the peptidoglycan layer is left exposed. The CV – I complexes are washed from the Gram-negative cell along with the outer membrane. In contrast, a Gram-positive cell becomes dehydrated from an ethanol treatment. The large CV – I complexes become trapped within the Gram-positive cell due to the multilayered nature of its peptidoglycan. The decolorization step is critical and must be timed correctly; the crystal violet stain will be removed from both Gram-positive and negative cells if the decolorizing agent is left on too long (a matter of seconds).
After decolorization, the Gram-positive cell remains purple and the Gram-negative cell loses its purple color. Counterstain, which is usually positively-charged safranin or basic fuchsin, is applied last to give decolorized Gram-negative bacteria a pink or red color.
Atypical Cells Walls
1. Archaea
- Extreme environmental conditions: methanogens, halophiles, thermophiles
- May lack cell wall
- Walls : No peptidoglycan, have pseudomurein
2. Mycoplasma
- Smallest bacteria (pass through filters for bacteria)
- Very little or no cell wall (very weak to ouside environment)
- Can withstand osmotic lysis
(plasma membrane:sterols, carotenoids, lipoglycans) =>confers strength
References:
http://en.wikipedia.org/wiki/Peptidoglycan
http://en.wikipedia.org/wiki/Gram-positive_bacteria
http://en.wikipedia.org/wiki/Gram-negative_bacteria
http://en.wikipedia.org/wiki/Gram_staining
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