Bacteria are microscopic living organisms, usually one-celled, that can be found everywhere. They can be dangerous, such as when they cause infection, or beneficial, as in the process of fermentation (such as in wine) and that of decomposition.

In 1676, Anton Van Leeuwenhoek first observed bacteria through a microscope and called them “animalcules.” In 1838, the German Naturalist Christian Gottfried Ehrenberg called them bacteria, from the Greek baktḗria, meaning "little stick." An apt word, as the first observed bacteria were shaped like rods, although bacteria can also be spiral or spherical in shape. A grammar note: The word bacteria is the plural form of "bacterium" and so should be written as plural, as in "Many bacteria are harmless."

Definitions of bacteria

n (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

show 49 types...
hide 49 types...
acidophil, acidophile
an organism that thrives in a relatively acid environment
probiotic, probiotic bacterium, probiotic flora, probiotic microflora
a beneficial bacterium found in the intestinal tract of healthy mammals; often considered to be a plant
a rodlike bacterium (especially any of the rod-shaped or branched bacteria in the root nodules of nitrogen-fixing plants)
eubacteria, eubacterium, true bacteria
a large group of bacteria having rigid cell walls; motile types have flagella
Calymmatobacterium, genus Calymmatobacterium
a genus of bacterial rods containing only the one species that causes granuloma inguinale
Francisella, genus Francisella
a genus of Gram-negative aerobic bacteria that occur as pathogens and parasite in many animals (including humans)
Neisseria gonorrhoeae, gonococcus
the pus-producing bacterium that causes gonorrhea
Legionella pneumophilia, legionella
the motile aerobic rod-shaped Gram-negative bacterium that thrives in central heating and air conditioning systems and can cause Legionnaires' disease
any of the bacteria in the soil that take part in the nitrogen cycle; they oxidize ammonium compounds into nitrites or oxidize nitrites into nitrates
penicillin-resistant bacteria
bacteria that are unaffected by penicillin
pus-forming bacteria
bacteria that produce pus
any rod-shaped bacterium
Gram-positive bacteria usually occurring in pairs
a strain of bacteria that is resistant to all antibiotics
B, bacillus
aerobic rod-shaped spore-producing bacterium; often occurring in chainlike formations; found primarily in soil
cocci, coccus
any spherical or nearly spherical bacteria
a bacterial cell intermediate in morphology between a coccus and a bacillus; a very short bacillus
spirilla, spirillum
any flagellated aerobic bacteria having a spirally twisted rodlike form
clostridia, clostridium
spindle-shaped bacterial cell especially one swollen at the center by an endospore
Clostridium botulinum, botulinum, botulinus
anaerobic bacterium producing botulin the toxin that causes botulism
clostridium perfringens
anaerobic Gram-positive rod bacterium that produces epsilon toxin; can be used as a bioweapon
blue-green algae, cyanobacteria
predominantly photosynthetic prokaryotic organisms containing a blue pigment in addition to chlorophyll; occur singly or in colonies in diverse habitats; important as phytoplankton
phototrophic bacteria, phototropic bacteria
green and purple bacteria; energy for growth is derived from sunlight; carbon is derived from carbon dioxide or organic carbon
bacteria usually producing greenish fluorescent water-soluble pigment; some pathogenic for plants and animals
bacteria producing yellow non-water-soluble pigments; some pathogenic for plants
nitric bacteria, nitrobacteria
soil bacteria that convert nitrites to nitrates
nitrosobacteria, nitrous bacteria
soil bacteria that oxidize ammonia to nitrites
small rod-shaped bacteria living in sewage or soil and oxidizing sulfur
spirally twisted elongate rodlike bacteria usually living in stagnant water
vibrio, vibrion
curved rodlike motile bacterium
any species of the genus Corynebacterium
any species of the genus Listeria
enteric bacteria, enterics, enterobacteria, entric
rod-shaped Gram-negative bacteria; most occur normally or pathogenically in intestines of humans and other animals
endospore-forming bacteria
a group of true bacteria
any of a group of very small rod-shaped bacteria that live in biting arthropods (as ticks and mites) and cause disease in vertebrate hosts; they cause typhus and other febrile diseases in human beings
coccoid rickettsia infesting birds and mammals; cause infections of eyes and lungs and genitourinary tract
any of a group of small parasitic bacteria that lack cell walls and can survive without oxygen; can cause pneumonia and urinary tract infection
nitrate bacterium, nitric bacterium
any of the nitrobacteria that oxidize nitrites into nitrates
nitrite bacterium, nitrous bacterium
any of the nitrobacteria that oxidize ammonia into nitrites
any bacteria (some of which are pathogenic for humans and animals) belonging to the order Actinomycetales
soil-inhabiting saprophytes and disease-producing plant and animal parasites
mycobacteria, mycobacterium
rod-shaped bacteria some saprophytic or causing diseases
any of various rod-shaped Gram-negative bacteria
gliding bacteria, myxobacter, myxobacteria, myxobacterium, slime bacteria
bacteria that form colonies in self-produced slime; inhabit moist soils or decaying plant matter or animal waste
a Gram-positive rod-shaped bacterium that produces lactic acid (especially in milk)
Lactobacillus acidophilus, acidophilus
a bacterium that is used to make yogurt and to supplement probiotics
Diplococcus pneumoniae, pneumococcus
bacterium causing pneumonia in mice and humans
strep, streptococci, streptococcus
spherical Gram-positive bacteria occurring in pairs or chains; cause e.g. scarlet fever and tonsillitis
spirochaete, spirochete
parasitic or free-living bacteria; many pathogenic to humans and other animals
Type of:
micro-organism, microorganism
any organism of microscopic size

Sign up, it's free!

Whether you're a student, an educator, or a lifelong learner, can put you on the path to systematic vocabulary improvement.