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Introduction to bacteria

Bacteria are prokaryotic organisms. They are found in almost every habitat on Earth.

Bacterial cells are smaller than eukaryotic cells. They measure about $$ 2-5\text{ }\mu\text{m}$$ (micrometres) in diameter, compared with $$10-100\text{ }\mu\text{m}$$ for eukaryotic cells. Bacteria are typically rod shaped or spherical.

Bacteria lack membrane-bound organelles, so all their metabolic processes occur in the cytoplasm. A bacterial cell does not have a nucleus.

Bacteria do, however, have ribosomes where protein synthesis takes place. These ribosomes are smaller than the ones found in eukaryotes (80S) and are called 70S ribosomes.

Bacteria have cell walls that are often made of peptidoglycan. This is a carbohydrate that can be stained, as in the image.

These anthrax bacteria have been stained.
These anthrax bacteria have been stained.

Bacteria can be divided into two categories depending on the arrangement of their cell walls.

The cell walls are made from peptidoglycan. This is a carbohydrate that forms cross-links with peptides (short polymers of amino acids). Peptidoglycan turns blue or violet in a Gram stain.

  • Gram-positive bacteria have cell walls outside the cell membranes. The cell walls are made up of about 30-40 layers of peptidoglycan.
  • Gram-negative bacteria have cell walls each sandwiched between two cell membranes. These cell walls generally consist of just a few sheets (2-3) of peptidoglycan.
These anthrax bacteria have been stained in a gram stain.
These anthrax bacteria have been stained in a gram stain.

The categories Gram-positive and Gram-negative refer to how bacteria respond to Gram staining. This is a common method of differentiating between the two types of bacteria.

The Gram stain dyes peptidoglycan blue or violet.

Gram-positive bacteria are stained blue or violet by the dye. This is because they have thick peptidoglycan cell walls positioned outside the cells that are hence stainable by dye.

Gram-negative bacteria are not stained blue or violet by the dye. This is because they have thin peptidoglycan cell walls that cannot hold the die after the staining.

A counterstain is usually applied after the initial staining to give the gram-negative bacteria a pink or red colour.

These anthrax bacteria have been stained.
These anthrax bacteria have been stained.