Monday, July 5, 2010

Molecular Structure of Bacteria

Molecular Structure of Bacteria

1- Cell Membrane:

It is very important structure in the bacterial cell, also called cytoplasmic membrane or sometimes protoplasmic membrane. Chemically cytoplasmic membrane is composed of bilayers of phospholipids.

Basic functions of cell membrane are :

1\ to control the movement of substances.

2\ to secrete hydrolytic enzymes.

3\ responsible for secretion of transport protein and proteins involved in cell wall synthesis.

2- Cytoplasm: which contains:

3- Genetic Material: a single piece of DNA without nuclear membrane.

4- Ribosomes: which are known as protein making organelle.

5- Mesosomes: Which are attached to the cell membrane and is thought that it is associated in cell division (binary fission).

6- Cell Wall: is another component of bacterial cell which is outside the cell, it is a rigid organelle and also it gives the bacteria it's shape and also prevents expansion of cell membrane (protects the cell against osmotic pressure). The other name of the cell wall is known as peptidoglycan which is composed of: polysaccharides: the main polysaccharides which are found in the composition are:

- N – acetyl glucose amine.

- N – acetyl muramic acid.

and this peptidoglycan is only found in the cell wall of bacteria. It is a target for antibiotics to act on. Some antibiotics act on the cell wall e.g. penicillins and cephalosporins. This is known as selective toxicity.

Differences between G+ve and G-ve bacteria:

Gram +ve

Gram –ve


Cell wall is thick

Cell wall is thin


Cell wall is compact

Cell wall is less compact.


Cell wall almost is peptidoglycan which includes 90%

Cell wall is peptidoglycan but only 10%


Contains a component of teichoic acid in the cell wall

No teichoic acid.


Presence of lipoteichoic acid

No lipoteichoic acid


Cell wall cotains No lipo polysaccharide

Cell wall contains lipo polysaccharides


The surface is made of protein

The surface is made of: lipo polysaccharides, lipo protein, phospholipids.

Defect or obstruction of the cell wall will give:

1- Protoplast

2- Spheroplast

3- L-forms

Specialized products outside the cell wall:

1- Capsule:

this is a layer of loose slime material which surrounds some bacterial cells. The capsules are composed of mainly of polysaccharides or peptides. They resist phagocytosis and so their presence on a bacterium is associated with virulence. They are identified by negative staining due to their low affinity for simple staining.

* e.g. of capsulated bacteria:

Klebsiella pneumoniae (polysaccharide)

Bacillus anthraces (poly – D – glutamic acid)

2- Flagella:

These are filaments that originate from the cytoplasm. They function as organs of motility. They are therefore seen only in organisms that are motile. They are made of protein (Flagellin).

The functions of flagella are:

1- motility (mainly)

2- attachment to site of infection e.g. stomach.

3- Diversity of antigens, this is used for diagnosis and this is mainly in Salmonella typhi (the causative agent of typhoid fever).

4- Invasion.

5- Colonization (form colony).

3- Pili:

Pili are divided into two groups:

1- Sex pili, referred to as sex pili because of their role during conjugation when genes are transferred from one cell(donor) to another cell (recipient).

2- Common pili, or Fimbriae: these are thought to be the organs of adhesion that help bacteria to attach to the host cells (Virulent Factor).

4- Spores:-

These are dense structure produced by some bacteria, e.g. the Bacilli and Clostridia groups, that enable them to survive adverse environmental conditions. They develop within and at the expense of the vegetative cell. The spore comprises the chromosomal material surrounded by several walls layers.

Chemically, endospore has a large amount of Ca++ and less number of enzymes. Dpiclonic acid is also presents in spore.

Spores are resistant to heat, stains, desiccation, chemicals and disinfectants.

Each spore germinates to produce a vegetative cell during favorable conditions.

The location and shape of the spore in the cell may be of diagnostic assistance, e.g. the spores of Clostridium tetani are terminal, and the diameter is greater than that of the parent cell, so that they are characteristically of drum stick appearance.

The positions of the spores are described as:

Terminal , Sub terminal , or Central

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