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The Collaborative International Dictionary of English v.0.48 (gcide)

Bacterium \Bac*te"ri*um\ (b[a^]k*t[=e]"r[i^]*[u^]m), n.; pl.
{Bacteria} (b[a^]k*t[=e]"r[i^]*[.a]). [NL., fr. Gr.
bakth`rion, ba`ktron, a staff: cf. F. bact['e]rie.] (Biol.)
A microscopic single-celled organism having no
distinguishable nucleus, belonging to the kingdom Monera.
Bacteria have varying shapes, usually taking the form of a
jointed rodlike filament, or a small sphere, but also in
certain cases having a branched form. Bacteria are destitute
of chlorophyll, but in those members of the phylum Cyanophyta
(the blue-green algae) other light-absorbing pigments are
present. They are the smallest of microscopic organisms which
have their own metabolic processes carried on within cell
membranes, viruses being smaller but not capable of living
freely. The bacteria are very widely diffused in nature, and
multiply with marvelous rapidity, both by fission and by
spores. Bacteria may require oxygen for their
energy-producing metabolism, and these are called {aerobes};
or may multiply in the absence of oxygen, these forms being
{anaerobes}. Certain species are active agents in
fermentation, while others appear to be the cause of certain
infectious diseases. The branch of science with studies
bacteria is {bacteriology}, being a division of
{microbiology}. See {Bacillus}.
[1913 Webster PJC]