- A former genus of the Sarcodina, now termed Amoeba.
- A genus of motile, peritrichous, non-spore-forming, aerobic to facultatively anaerobic bacteria (family Enterobacteriaceae) containing gram-negative rods; coccoid forms, large irregular involution forms, filaments, and spheroplasts occur under different culture conditions. The metabolism is fermentative, producing acid or acid and visible gas from glucose; lactose is not fermented, and they rapidly decompose urea and deaminate phenylalanine. Proteus occurs primarily in fecal matter and in putrefying materials. The type species is Proteus vulgaris.
[G. Proteus, a sea god, who had the power to change his form]