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.
a tiny, single-celled microorganism, commonly known as a germ; some bacteria, called pathogens, cause disease