Ma'anar ƙamus na kalmar "bakteriya" ƙananan ƙwayoyin cuta ce mai ɗabi'a guda ɗaya wadda yawanci tana da kwayar halittar DNA madauwari mai sauƙi kuma ba ta da tsakiya mai ɗaure membrane, mitochondria, ko wasu hadaddun gabobin. Ana samun kwayoyin cuta a wurare daban-daban, wadanda suka hada da kasa, ruwa, da halittu masu rai, kuma suna iya yin illa mai fa'ida da illa ga lafiyar dan Adam da muhalli.