Bacterias are unicellular prokaryotic micro organisms. They have no chlorophil and no true branching.

Unit of measurement used in bacteriology is micron. Limit of resolution with the unaided eye is 200 micron. Size of medically important bacteria is 0.2 - 1.5 micron. Leewenhoek first observed bacteria using hand ground lenses

Classification of bacteria based on shape

1. Cocci - They are spherical or oval shape
2. Bacilli - They are rod shaped.
3. Vibrio - They are comma shaped or curved rod shaped.
4. Spyrilla - Rigid spiral shape.
5. Spyrochetes - Spiral shape.
6. Actinomycetes - Branching filamentous shaped.
7. Mycoplasma - Cell wall deficient form.
8. Coco bacillus - Not round not rod shaped.

Different types of bacteria based on arrangement.

1. Cocci - single
2. Diplococci - pair.
3. Streptococci - chains.
4. Tetrad - groups of four
5. Sarcina - 3D arrangement
6. Staphylococci - grape like clusters.

Bacteria have a distinct cell structure. Bacterial cell structure includes cell envelope, cell appantages and cytoplasm.

Bacterias have a very rigid cell wall amd gives shape to the cell. Its function is to prevent cell expansion and bursting due to water uptake. Cell wall constitute a significant portion of the dry weight of the cell and it is essential for bacterial growth and division. It cannot be seen by direct light microscope. It do not stain with simple stains. It may be demonstrated by the following

• Microdissection
• Reaction with specific antibodies
• Mechanical rupture of the cell
• Differential staining
• Electron microscopy

Chemically the cell wall is composed of peptidoglycon. Peptide formed by N acetyl glucosamine and N acetyl muramic acid, cross linked by peptide chains. Embedded in it are polyalcohol called Teichoic acid. Some are linked to lipids and are called Lipoteichoic acid. Lipoteichoic acid link peptidoglycan to cytoplasmic membrane and the peptidoglycan gives rigidity. The functions of Teichoic acid are :- 

1. Gives negative charge
2. Major antigenic determinant
3. Transport ions
4. Anchoring
5. External permeability barrier

Outer membrane is found only in Gram negative bacteria, it functions as an initial barrier to the environment and is composed of lipopolysaccharide and phospholipids.

The lipopolysaccharide present on the cell wall of Gram negative bacteria account for their endotoxic activity and antigen specificity.

Flagella

Flagella are long hair like helical filaments extending from cytoplasmic membrane to exterior of the cell wall. They are of four types

1. Monotrichous - single flagella on one side. Eg: Vibrio cholerae
2. Lophotrichous - tuft of flagella on one side. Eg: Bartonella bacillifornis
3. Amphitrichous - single or tuft of flagella on both side. Eg: Spirillum serpens
4. Peritrichous - surrounded by flagella. Eg: Escherichia coli.

Spore

Some bacteria have the ability to form highly resistant resting stage called spores, which helps them to overcome adverse environmental conditions that are unfavourable for vegetative growth of cell.