bacterium

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is a singular noun and its plural is bacteria. Erroneous uses of bacteria as a singular noun are regrettably common in newspapers:

A common gut bacteria may be a major cause of rheumatoid arthritis—Independent, 1991.

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Van Leeuwenhoek and A Microscope

The first person to observe and describe bacteria, protozoans and spermatozoa was the Dutch naturalist Anton van Leeuwenhoek. Van Leeuwenhoek, born on this date in 1632, made lenses, assembling over the course of time nearly 250 different types of microscopes. He ground and shaped hundreds of different lenses, some magnifying objects up to 270 times. Able to see minute particles, he observed muscle fibers, microbes, and blood flow in capillaries, with his work leading to the establishment of cell biology.

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From our Archives: Today's Highlights, October 24, 2005

IN BRIEF: A single-celled organism.

A scientist may be able to see a single bacterium through a microscope.

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(pl. bacteria)

any of a vast and ubiquitous group of prokaryotic microorganisms that exist as single cells or in clusters or aggregates of single cells. Most authorities now place them in the exclusively prokaryotic kingdom Monera, along with the cyanobacteria (blue-green algae). The majority of bacteria possess a rigid cell wall; those lacking this feature are termed Archaea. Bacteria, Archaea, and Eukarya constitute the three primary Kingdoms (domains).
—bacterial adj.

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Pl. bacteria [L.] any prokaryotic microorganism. Bacteria are single-celled microorganisms that differ from all other organisms (the eukaryotes) in lacking a true nucleus and organelles such as mitochondria, chloroplasts and lysosomes. Their genetic material consists of a single double-stranded DNA molecule, whereas the genetic material of eukaryotes consists of multiple chromosomes, which are complex structures of DNA and protein.
Bacteria reproduce by binary fission and generally have a very high rate of population growth and mutation. Genetic material can be transferred between bacteria by three processes: transformation (absorption of naked DNA), transduction (transfer by a temperate bacteriophage), and conjugation (transfer by independently replicating DNA molecules, called plasmids). Some bacteria can also form spores, dehydrated forms that are relatively resistant to heat, cold, lack of water, toxic chemicals and radiation.
Most bacteria have a rigid cell wall outside of the cell membrane primarily composed of a dense layer of peptidoglycan, a network of polysaccharide chains with polypeptide cross-links. Some antimicrobial agents, the penicillins and cephalosporins, act by interfering with peptidoglycan synthesis.
Bacteria can have any of three types of external structures: flagella, which are rotating locomotor organelles; pili or fimbriae, which are minute filamentous appendages; and a capsule, which is a layer of gelatinous material around the cell. Large pili called sex pili are involved in conjugation while other pili are involved in adherence of bacteria to mucosal surfaces. The capsule is associated with virulent strains of bacteria and protects the bacterium from phagocytosis. See also bacteria.

• acid-fast b. — one that, because of wax-like composition of the cell wall, is not readily decolorized by acids after staining, especially Mycobacterium spp.
• coliform b. — particularly found in the gut (colon) of animals. See aerobacter aerogenes, escherichia and paracolobactrum.
• hemophilic b. — microorganisms of the genera Haemophilus and Bordetella, which have a nutritional requirement for fresh blood or whose growth is significantly stimulated on blood-containing media.
• lactic acid b. — bacteria that, in suitable media, ferment carbohydrates to form lactic acid.
• lysogenic b. — any bacterial cell harboring in its genome the genetic material (prophage) of a temperate bacteriophage and thus reproducing the bacteriophage DNA in each cell division; occasionally the prophage becomes nonintegrated (induced), replicates, lyses the bacterial cell, and is free to infect other cells.

For a list of words related to bacteria, see:

• Diagnostic Terminology - bacteria: pl., sing. bacterium; primitive microorganisms, usu. unicellular with unique cell wall composition, that reproduce by simple division and often cause disease
• Physiology - bacteria: pl., sing. bacterium; ubiquitous, unicellular organisms appearing singly or in chains in spherical, spiral, or rod-shaped form
• Bacteria - bacteria: pl., sing. bacterium; ubiquitous, unicellular organisms appearing singly or in chains in spherical, spiral, or rod-shaped form, forming major phylum of kingdom Monera

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