bacterium Definition

any of a domain (Bacteria) of chiefly round, spiral, or rod-shaped single-celled prokaryotic microorganisms that typically live in soil, water, organic matter, or the bodies of plants and animals, that make their own food especially from sunlight or are saprophytic or parasitic, are often motile by means of flagella, reproduce especially by binary fission, and include many important pathogens; broadly : prokaryote

any of various bacteria (especially genus Thiobacillus) capable of metabolizing sulfur compounds