(microbiology) single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission; important as pathogens and for biochemical properties; taxonomy is difficult; often considered to be plants
"The microscope revealed millions of bacteria floating in the water sample, highlighting their role as microscopic pathogens."
plural of bacterium
"The laboratory analysis identified the pathogen as an oval bacterium rather than a spherical coccus or rod-shaped bacillus."
An oval bacterium, as distinguished from a spherical coccus or rod-shaped bacillus.
In plain English: Bacteria are tiny single-celled organisms that live everywhere and can either help us stay healthy or make us sick.
"Washing your hands helps kill harmful bacteria and keeps you healthy."
Usage: Bacteria is the plural form of bacterium and refers to microscopic organisms that can be shaped like rods, spheres, or ovals; do not use it when referring to a single organism unless you are specifically discussing oval shapes in contrast to other forms. In everyday usage, treat bacteria as an uncountable mass noun rather than counting individual units with numbers.
The word bacteria comes from the New Latin term bactērium, which was borrowed from Ancient Greek. Originally, it meant "little rod."