bacteria

single-celled organisms that underpin all life-sustaining processes. GRAM'S STAIN is a test used to distinguish between the two types (Gram positive and Gram negative). They are also identified by shape: spiral, (spirilli), rod-like (bacilli), spherical (cocci), comma-shaped (vibrios) and the spirochaetal, which are corkscrew-like. Bacteria are the key agents in the chemical cycles of carbon, oxygen, nitrogen and sulphur.