Bacterial Cell Structure and Functions

Bacterial Characteristics & Cell Shapes

Bacteria are prokaryotic cellular organisms. Their structure is very simple and lacks a nucleus. Most bacteria, with the exception of mycoplasmas, have a cell wall that is more or less rigid, but with a very diverse structure and chemical composition, different from those of plants. Based on cell shape:

• Cocci are small, spherical cells. They group together in pairs (diplococci), in irregular groups or clusters (staphylococci), in rows (streptococci), and in packets (sarcinae).
• Bacilli are short, cylindrical, rod-shaped cells. They may occur as pairs or chains.
• Vibrio are comma-shaped cells.
• Spirilla are rigid, spiral-shaped cells. Spirochetes are long, thin, spiral-shaped cells, which are more flexible than spirilla.

Bacterial Cell Wall & Capsule

The cell wall is a rigid structure due to a layer of murein, a peptidoglycan which gives the bacterial cell its shape. There are two types of walls:

• Gram-positive: During crystal violet staining, the dye is retained inside the bacteria due to the large thickness of the peptidoglycan layer (appears violet-blue). It is approximately 90% peptidoglycan, plus teichoic acids.
• Gram-negative: During crystal violet staining, the dye is not retained due to its low proportion of peptidoglycan (it fades, and adding safranin dye reveals a red color). It is approximately 20% peptidoglycan, plus lipopolysaccharides.

Cell Wall Functions:

• Mechanical and osmotic protection.
• Exhibits cellular recognition signals.
• In some cases, it houses enzymatic processes involved in cellular metabolism.

Bacterial Capsule Functions:

The bacterial capsule has several functions:

• Protection against antibodies and medicines.
• Protection against desiccation from the environment.
• Fixation of bacteria on different kinds of substrates.

Bacterial Cytoplasm & Inclusions

The cytoplasm has a granular appearance due to the ribosomes and contains most of the bacterial DNA. Plasmids are circular DNA fragments scattered throughout the cytoplasm; they contain non-essential genes but often provide an advantage to the cell and are easily transmissible from one cell to another. The plasma membrane is rich in proteins; the membrane forms mesosomes, which are involved in the division of the cytoplasm during cell reproduction.

Inside the cytoplasm, various cytoplasmic inclusions can be found, such as:

• Small vacuoles that accumulate gas (gas vesicles).
• Chlorosomes that store photosynthetic pigments in green photosynthetic bacteria.
• Glycogen granules that constitute a carbon energy reserve.
• Polyphosphate granules.

Only in mycoplasmas are there protein granules.