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Bacterium definition

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.