Bacterial Cell Structure

A typical bacterial cell shows the following structural features:

Capsule and Slime Layers

A lose gelatinous sheath called slime layer is usually deposited around the bacterial cell wall. It is usually composed of polysaccharides. In some bacteria, the slime layer becomes thick due to the presence of some nitrogen compounds and forms a capsule. A capsule is particularly common in the parasitic forms.

Cell Wall

It is the outer tough covering around the cell, which provides specific shape and protection to the bacterium. It prevents the cell from swelling and bursting due to osmotic changes. It is composed of a polysaccharide called murein or peptidoglycan. It consists of polysaccharide cross linked with short amino acid chains.

The cell wall of gram positive bacteria is much thicker and contains less lipids compared to that of Gram negative bacteria. The cell wall can be dissolved by the enzyme lysozyme.

Plasma Membrane

It is a thin, elastic, selectively permeable membrane found internal to the cell wall. It is composed of phospholipids, proteins and polysaccharides. The plasma membrane is the site of most of the metabolic pathways. Certain structures are found associated with plasma membrane.

Mesosomes

These are invaginations formed by the plasma membrane. They are spherical or elongated structures which bring about a functional compartmentalisation of the cell. They have significant roles in cell division and replication. They are more common in Gram positive bacteria.

Flagella

These are long, slender thread-like structures, which help in locomotion. A bacterial cell may have one to many flagella. The flagella are entirely composed of a protein called flagellin. The basal body of each flagellum is directly attached to the plasma membrane. It is not covered by any sheath.

Pili (or Fimbriae)

These are extremely minute hair-like structures found mostly in male cells. They are composed of protein subunits called pilin. They take part in the formation of conjugation tube. They are therefore called sex pili.

Cytoplasm

It is a semi-fluid ground substance enclosed by the plasma membrane. It appears granular due to the presence of large number (as many as 20,000) ribosomes. The ribosomes may occur singly or in clusters called polyribosomes. Membrane-bound cell organelles like mitochondria, lysosomes, golgi complex, vacuoles and endoplasmic reticulum are absent.

fig. 8.3 - A Typical Bacterial Cell

Nucleoid (or Bacterial Chromosome)

Since the bacterial cell is prokaryotic, a true nucleus is absent. The nuclear material is represented by DNA which is not associated with histones. It is identified as a nucleoid or bacterial chromosome. It is a circular ring. It is attached at a point to the plasma membrane.

Plasmids

Apart from the nucleoid, certain bacterial cells contain additional rings of DNA called plasmids. The plasmid DNA replicates independently. It has certain genes like fertility factor (F-factor), resistance factor (R-factor), nitrogen fixing genes (Nif-genes). Some plasmids may temporarily become associated with nucleoid DNA and are known as episomes.