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Bacteria

Bacteria are single-celled microorganisms which can exist either as independent (free-living) organisms or as parasites (dependent upon another organism for life). They are about 0.5 to 2 micrometres in size. A grain of sand is 2 millimetres in size. This means you could fit 1000 bacterial cells into a single grain of sand. This means we can only see bacterial cells through the microscope.