bacteria

bacteria /baktir″ē·ə/ sing. bacterium [Gk, bakterion, small staff] , a domain of life existing as small unicellular microorganisms. The genera vary morphologically, being spheric (cocci), rod-shaped (bacilli), spiral (spirochetes), or comma-shaped (vibrios). The nature, severity, and outcome of any infection caused by a bacterium are characteristic of that species.