Structure of bacterial cell
Bacterial cell have an essential structural components : cell wall, cytoplasm membrane , intra cytoplasm structure and cell surface appendages ( capsule , flagella , fimbriae , spore ) . The biochemical composition of these structures are macromolecules are arranged or sequenced in primary – structure of molecule in which the subunits are put together such as :
DNA , RNA -------- Nucléotides , Protein --------- amino acid
fatty acid -------- Phospholipids , sugars --------- Polysaccharide
A- Cell wall
The cell wall of a bacterium is an essential structure that protects the cell protoplast from osmotic lyses . The cell wall of bacteria consists of polymer of two sugar derivatives N- acetylglucosamine and N- acetylmuramic acid cross linked by short chains of amino acids (peptide), this molecule is a type of peptidoglycan called murein .
***Chemical composition of the cell wall
Bacteria are divided into two separated groups Gram positive and Gram negative based on staining properties . Gram stain developed in 1884 by Christian Gram ,the most widely employed in bacteriology lab .
A- Gram positive bacteria cell wall composed of :
*Peptidoglycan
This layer is very thick in G +ve bacteria constituting 50-80nm of cell wall and responsible for the rigidity of cell wall and retention of crystal violet dyes during the Gram stain procedure .
**Teichoic acid and thin layer of lipid
B- Gram negative bacteria cell wall composed of :
*Inner layer of peptidoglcan
This layer is thin constituting of (5-10) nm of cell wall which can not retain the crystal violet stain .
** Outer layer of lipopolysaccharides (LPS)
cotaining of lipid A (endotoxin) and polysaccharide (fig.).
Periplasmic space between the inner and outer layers ***
It is filled with gel and is crossed by lipoprotein molecules to link the peptidoglycan layer and LPS layer.
Functions of bacterial cell wall
1-It maintains the shape of bacterial cell.
2- It protects the bacterial cell from osmotic lyses .
3-It is responsible for endotoxic activity of G- ve bacteria and Gram stain reaction.
4- It plays a role in cell division .
Example on cell wall deficient bacteria
a- Mycoplasma
This is naturally deficient in cell wall so , M. is pleomorphic shape and not affected by penicillin treatment .
b- L- forms
Some of bacteria under certain condition are fail in synthesis of cell wall when the cells is subjected to penicillin drug or lysozymes.
Staining of bacteria
Stains : are chemical compounds with colored ions which react with the different constituents of the living cell to stain the transparent and minute cells of bacteria which are difficult to see by naked eyes .
The stain may be classified into simple and differential , the simple stain colored all parts of the cell e.g. methylene blue while the differential stain are so selected that react with specific groups of the different parts of the cell or of different genera and species of bacteria for example Gram stain helps in the identification of bacteria G+ ve and G-ve especially the pathogenic agents that causing disease in the lab. Many theories have been proposed to explain the observed difference in Gram staining , one of the most common theory is that based on variation in the chemical composition of bacterial cell wall . G+ve bacteria contain magnesium- RNA-protein – carbohydrate complex which form an insoluble substance with the crystal violet and iodine , this complex is not washed with alcohol , while the lipid content of the cell wall being 10 times in G-ve as much as in G+ve ones , this lead to the solubility of lipids in alcohol and increase of cell wall porosity in G-ve and crystal violet iodine –complex can be extracted , so , G-ve cells become colorless after alcohol washing and take the another color safranin (cell turn pink) as shown in the below figure .