Bacterial Cell Structure and Components Deep Dive

Cell Walls

A rigid structure is attached to the outer surface of the plasma membrane, which completely surrounds the cell. It is a structure common to all bacteria, with the exception of mycoplasmas, a group of intracellular parasites.

Functions of the Cell Wall

• It maintains the shape of the cell components.
• It has antigenic properties.
• It regulates exchanges with foreign substances, mainly the outer membrane called the lipopolysaccharide (LPS) layer in Gram-negative bacteria.
• It provides a negatively charged cellular surface.

External Envelopes

Some bacteria are covered externally to the cell wall with a mucous layer composed of polysaccharides and sometimes proteins. These are called capsules (thicker and firmly attached to the cell) and mucous layers (thinner).

Cytoplasm

The cytoplasm is composed of a gelatinous matrix, the protoplasm, with a high water content and granular appearance. It contains proteins, enzymes, 70S ribosomes, and structures characteristic of these cells.

Ribosomes

They consist of two subunits composed of RNA and proteins. They are involved in the synthesis of cellular proteins. These are the only organelles found in all types of cells.

Nucleoids

In the prokaryotic cell, the genetic material is found in the nucleoid, an area situated in the central region of the cytoplasm. It has a fibrillar appearance and is not protected by a nuclear membrane.

Genetic Material in Bacteria

In bacteria, the genetic material consists of:

• A single chromosome consisting of double-stranded, circular, and supercoiled DNA.
• Between one and several plasmids, which are small DNA molecules.

Flagella

They are the organs of locomotion; their number and arrangement vary from one bacterium to another. This is one of many criteria for classifying prokaryotic cells.

Flagella Components

Each flagellum comprises:

• A rigid, curved filament comprising a protein called flagellin.
• A hook that connects the filament to the cell surface.
• A basal structure comprising a series of rings.

Fimbriae and Pili

Fimbriae and pili are external appendages that are not involved in bacterial movement.

Fimbriae

Fimbriae are short, thin, and numerous in some bacteria; they have an adhesive role.

Pili (Hairs)

Pili are the longest, few in number, and are involved in the union of two cells during bacterial conjugation.