Use the word bacterium when you're talking about a single-celled organism. Your uncle's bad case of pneumonia might be caused by a bacterium.

You're more likely to be familiar with the plural form of bacterium, bacteria. A bacterium is frequently the cause of diseases, specifically those known as bacterial illnesses. In biology, a bacterium is simply a microorganism, or a very tiny living thing, with cell walls but no distinct nucleus. The word itself comes from the Greek word bakterion, "small stick or rod," which describes a bacterium's shape when seen under a microscope.

Definitions of bacterium
  1. noun
    (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
    synonyms: bacteria
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    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
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