peptidoglycan
Ocak 31, 2019
Gram Negative Bacteria
Gram Negative Bacteria
Gram negative bacteria are bacteria that do not retain the crystal violet dye in the Gram stain protocol. Gram negative bacteria will thus appear red or pink following a Gram stain procedure due to the effects of the counterstain (for example safranin).
The Gram Stain
In microbiology, the visualization of bacteria at the microscopic level is facilitated by the use of stains, which react with components present in some cells but not others. This technique is used to classify bacteria as either Gram-positive or Gram-negative depending on their colour following a specific staining procedure originally developed by Hans Christian Gram. Gram-positive bacteria appear dark blue or violet due to the crystal violet stain following the Gram stain procedure; Gram-negative bacteria, which cannot retain the crystal violet stain, appear red or pink due to the counterstain (usually safranin).
The reason bacteria are either Gram-positive or Gram-negative is due to the structure of their cell envelope. (The cell envelope is defined as the cell membrane and cell wall plus an outer membrane, if one is present.) Gram-positive bacteria, for example, retain the crystal violet due to the amount of peptidoglycan in the cell wall. It can be said therefore that the Gram-stain procedure separates bacteria into two broad categories based on structural differences in the cell envelope.
Cell Envelope of Gram Negative Bacteria
The Gram negative cell envelope contains an additional outer membrane composed by phospholipids and lipopolysaccharides which face the external environment. The highly charged nature of lipopolysaccharides confer an overall negative charge to the Gram negative cell wall. The chemical structure of the outer membrane lipopolysaccharides is often unique to specific bacterial strains (i.e. sub-species) and is responsible for many of the antigenic properties of these strains. Many species of Gram-negative bacteria are pathogenic. This pathogenicity is often associated with the lipopolysaccharide (LPS) layer of the Gram-negative cell envelope.
Characteristics of Gram Negative Bacteria
Gram-negative bacteria have a characteristic cell envelope structure very different from Gram-positive bacteria. Gram-negative bacteria have a cytoplasmic membrane, a thin peptidoglycan layer, and an outer membrane containing lipopolysaccharide. There is a space between the cytoplasmic membrane and the outer membrane called the periplasmic space or periplasm. The periplasmic space contains the loose network of peptidoglycan chains referred to as the peptidoglycan layer.
- Acinetobacter
- Actinobacillus
- Bordetella
- Brucella
- Campylobacter
- Cyanobacteria
- Enterobacter
- Erwinia
- Escherichia coli
- Franciscella
- Helicobacter
- Hemophilus
- Klebsiella
- Legionella
- Moraxella
- Neisseria
- Pasteurella
- Proteus
- Pseudomonas
- Salmonella
- Serratia
- Shigella
- Treponema
- Vibrio
- Yersinia
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6 Ekim 2018 Cumartesi
peptidoglycan
Ekim 06, 2018
PEPTIDOGLYCAN
PEPTIDOGLYCAN
Peptidoglycan consists of carbohydrate backbone (glycan chain) composed of alternating units of N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) molecules. Glycan chains are connected by short peptides.Attached to each of the muramic acid molecules are a tetrapeptide consisting of both D- and L- amino acids, the precise composition of which differs between bacteria. Teichoic acid and lipoteichoic acid which are polymers of a sugar alcohol (ribitol or glycerol) are embedded in it.
Special Amino Acids Found in Peptidoglycan Layer:
➧ Diaminopimelic acid: Unique to bacterial cells.
➧ D- alanine: Involved in the cross links between tetrapeptides and in the action of penicillin.
Functions of Peptidoglycan Layer
- It provides rigid support to bacterial cells and maintains the characteristic shape of the cell.
- Allows bacterial cell to withstand media of low osmotic pressure, such as water.
- Peptidoglycan is a good target for antibacterial drugs. Eg. Penicillins, cephalosporins etc inhibit transpeptidase reaction which makes cross-links between the two adjacent tetrapeptides.
- Lysozyme enzyme present in human tears, mucus, and saliva cleave peptidoglycan backbone breaking its glycosyl bonds.
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