Ultra Structure of Bacteria :

Under electron microscope, the structure of bacterial cell is look like a capsule. It has following components----

Capsule:

1. It is the outer layer of the bacteria cell.

2. Depending on the chemical nature capsules are thick or thin, and rigid or flexible.

3. It is most important virulence factor of bacteria which is visualized by negative staining technique.

4. Only some bacterial species possess capsule.

5. Capsule is usually made of polysaccharide (e.g. pneumococcus), occasionally polypeptide (e.g. anthrax bacilli) and hyaluronic acid (e.g. streptococcus).

6.  A few capsules are polypeptides like Bacillus anthracis, is composed entirely of a polymer of glutamic acids.

There are two types of capsules,

 1. Macrocapsule: Thickness of 0.2 µm or more, visible under light microscope. 

 2. Microcapsule: Thickness is less than 0.2 µm, visible under Electron microscope. 

Functions: 

 1. Preventive nature: Capsular polysaccharide bind significant amount of water making cell resistant to drying and also prevent attachment of bacteriophage on cell surface. 

 2. Protection: It protect from mechanical injury, temperature, drying etc. 

 3. Attachment: Capsule helps in attachment on the surface. For example: Streptococcus mutants that cause dental carries attach on teeth surface by its capsule. 

 4. Anti-phagocytic: Capsule resist phagocytosis by WBCs. 

 5. Source of nutrition: It acts as a resource organ of nutrition during insufficient nutrient supply. 

 6. Repulsion: Same charge capsulated bacteria repel each other. 

Cell wall:

1. Cell wall is the rigid wall situated outside of the plasma membrane.

2. It provides shape of the bacteria as well as provides protection from osmotic lysis.

3. Cell wall is made up of peptidoglycan. 

4. Peptidoglycan is a polymer consists of two sugar molecules viz. N-acetylglucosamine and N-acetylmuramic acid. Apart from that it also contains several amino acids.

Flagella:

 1. Flagella are slender and rigid hair like structures.

 2. The eukaryotic flagellum is a long, rod-like structure, surrounded by an extension of the cell membrane like a sheath.

3. They are about 20 nm cross and up-to 15 to 20 µm long.

4. The bulk of the structure is a filament called an axoneme.

5.  Monotrichous bacteria have one polar flagellum whereas amphitrichous bacteria have either singe or duster of flagellum at both pole.

Fimbriae and Pili:

1. Many Gram negative bacilli contain short, fine, hair like, non-flagellar appendages called fimbriae.

2. Pili are similar appendages which are about 1-10 cells.

3. The filament of pilus is straight and diameter is 7 nm to 10 nm, larger than diameter of fimbriae.

4. It is made up of pilin protein. 

5. Molecular weight is 17,000. 

6. Pili are non-motile but adhesive structure.

7. They are genetically determined by sex factor or conjugative plasmid and hence they help in conjugation of male bacteria, in the attachment of pathogenic bacteria to their host cell.

Spinae: 

Some Gram-positive bacteria have tubular unicellular and rigid appendage of single protein moiety, known as spinae. They help the bacterial cell to tolerate environmental stress such as salinity, pH and temperature etc. 

Cell Membrane: 

 1. Cell or plasma membrane is a thin structure that completely surrounds the cell.

2. It is about 8 nm thick.

3. This structure is critical barrier separating the inside of cell from environment.

4. The cell membrane is also highly selective barrier enabling the cell to concentrate a specific metabolite and excrete waste material.

5. The main function of cell membrane is selective permeability and transport of solute, electron transport and oxidative phosphorylation in aerobic species, excretion of hydrolytic exoenzymes.

Inclusion bodies: 

 1. Various types of inclusion bodies organic and inorganic granules are present in the cytoplasmic matrix.

2.  They are mainly used for storage of energy and reduce osmotic pressure by tying up molecules in particulate forms like polyphosphate granules, glycogen granule etc.

3. Some of the membrane enclosed inclusion bodies are like glycogen-sulfur granules, carboxysomes etc.

Ribosomes: 

1. Ribosomes are loosely attached to the plasma membrane (70S ribosomes).

2. They are made up of both protein and ribonucleic acid.

3. They are the site of protein synthesis.

4. Plasma membrane ribosomes make proteins for transport to the outside.

Nucleoid:

1. Prokaryotic chromosome is located in an irregularly shaped region called nucleoid.

2.  Nucleoid is composed of 60% DNA, 30% RNA and 10% protein by weight.

3.  Prokaryotes contain single circle of double stranded DNA but some are have linear DNA chromosome.

Mesosome:  

1. Mesosomes are present in the cell membrane of bacterial cells that fold inward.

2. They play a major role in cellular respiration.

3. They appear as a pocket form by the cytoplasmic membrane, filled with vesicles, tubules or lamellae.

Difference between Gram Positive and Gram Negative Bacteria-----