bacteria

Etymology 1

Irregular plural of bacterium from New Latin bactēria.

noun

1. plural of bacterium

noun

1. (US) A type, species, or strain of bacterium.
   Anaerobic bacteria function in the absence of oxygen, where as aerobic bacteria require sunlight and also oxygen. Both these bacterias are capable of breaking down the organic matter […] 2002, A.C. Panchdhari, Water Supply and Sanitary Installations, 2nd edition, page 177
2. (US, proscribed) Alternative form of bacterium.
3. (derogatory, slang) Lowlife, slob (could be treated as plural or singular).

Etymology 2

From New Latin bactēria, from Ancient Greek βακτηρίᾱ (baktēríā, “rod, stick”).

noun

1. (dated, medicine) An oval bacterium, as distinguished from a spherical coccus or rod-shaped bacillus.