Structures external to the Cell Wall:

( glycocalyx, flagella, axial filaments, and pili).

1- Glycocalyx and Capsule:

Many bacteria synthesize large amounts of extracellular polymers when growing in their natural environments.

These polymers form capsules or glycocalyx. Its chemical nature may be

polysaccharide as in the Pneumococcus or polypeptide as in Bacillus

anthracis .

When these polymers closely surrounding the cell; it is called Capsule, But if

these polymers form a loose meshwork of fibrils extending outward from the

cell; it is called glycocalyx

Glycocalyx and Capsule:a

a

Function of capsule or glycocalyx is to protect bacterial cells from phagocytosis.Capsule and glycocalyx can be demonstrated by light microscope using of

capsule stain of bacterial smear or India ink wet mount.

2-Flagella and motility: Motile bacteria possess filamentous appendages known as flagella, which act as organs of locomotion.

The flagellum is a long, thin filament, twisted spirally in an open, regular waveform. It is about 0.02 μm thick and is usually several times the length of the

bacterial cell.According to the species, there may be one, or up to 20, flagella per cell.

a

Flagella may be classified according to there arrangement as follow:

1- Monotrichous (single polar flagellum).

2- Lophotrichous (tuft of polar flagella).

3- Perituichous (flagella distributed over the cell).

4- Amphitrichous (one flagellum at each side of cell).

Flagella are associated with chemotaxis process (chemical attraction)

of bacterial cells that contribute in disease pathogenesis.

Monotrichous flagellum Lophotrichous flagella

Peritrichous flagella

3- Axial filaments:Some types of bacteria have a flagellum that lie inside periplasmic space

(over cell wall peptidoglycan and under the outer membrane).

This flagellum called endoflagellum or axial filament.The endoflagella are more than half the length of the organisms and run

along the axial aspect of the spiral body. They are responsible for rotary motility of these organisms.

4- Pili and Fimbriae: Many bacteria possess filamentous appendages termed pili or

fimbriae.

These are more numerous than flagella (e.g. 100-500, being borne surrounding each cell).

They are from 0.1 to 1.5 μm in length and having a uniform width between 4 and 8 nm.

Pili are important in mediating adhesion between the bacteria

and host cells ( hemagglutination phenomenon).

Another type of pili called sex pili is longer and initiate the

process of conjugation ( genetic material exchange between

bacteria).

Pili and Fimbriae:a

The Bacterial Cell Wall: The cell envelope:The layers that surrounding bacterial cell are called the cell envelope.

The Gram-positive cell envelope: It is composed from three layers: the cytoplasmic membrane, a thick peptidoglycan layer and a variable outer layer called capsule.

The Gram-negative cell envelope: It is composed from inner membrane (cytoplasmic membrane), the outer membrane, the peptidoglycan sheet, and capsules if present. Periplasmic space separates outer and inner membrane.

The Gram-positive cell envelope: The Gram-negative cell envelope:

a

The cell wall:

It is the outer covering layer of the bacterial cell. It is a rigid structure consisting of two layers in Gram-positive bacteria and of three layers in Gram-negative.

In Gram-positive bacteria, it is composed from pepeidoglycan (thick) layer and the associated capsule if present.

Peptidoglycan is long polymers of two sugar derivatives, NAG (N-Acetyl Glucosamine) and NAM (N-Acetyl Muramic acid) with side chains of four alternating D and L amino acids attached to NAM.

The cell wall structure:a

Structures internal to the Cell Wall:The cytoplasmic membrane:

It is consisting of phospholipids bilayers and proteins (integral and peripheral protein).

The major functions of cytoplasmic membrane are: 1- Selective permeability and transport of solutes. 2- Electron transport by its electron transport chain. 3- Excretion of hydrolytic exoenzymes.