The genus Bacterium was a taxon described in 1828 by Christian Gottfried Ehrenberg. The type species was later changed from Bacterium triloculare to Bacterium coli (now Escherichia coli) as it was lost. In 1951 and then in 1954 it was recognised as a nomen generum rejiciendum, which means a generic name to be rejected, this also applied to its family Bacteriaceae.
This genus included non-spore forming rods whose relation to other species was obscure (a "taxonomy dumping group"). This is different from the genus Bacillus, whose members were spore forming rods (sensu Cohn 1872).
Many species were placed under the genus. Given that the genus was abolished in the process of forming the Bacteriological Code there is no such thing as an official list of species present. These are those accepted by Breed and Conn in 1935:
- Bacterium radiobacter, then Agrobacterium tumefaciens, now Rhizobium radiobacter
- Bacterium aerogenes, now Aerobacter aerogenes
- Bacterium violaceum, now Chromobacterium violaceum
- Bacterium amylovorum, now Erwinia amylovora
- Bacterium zopfii, now Kurthia zopfii
- Bacterium monocytogenes, now Listeria monocytogenes
- Bacterium pneumoniae, now Klebsiella pneumoniae
- Breed, R.; Conn, H. (1936). "The Status of the Generic Term Bacterium Ehrenberg 1828". Journal of bacteriology 31 (5): 517–518.
- "Reiection of the Generic Name Bacterium" (pdf). International Bulletin of Bacteriological Nomenclature and Taxonomy 4 (142). October 15, 1954. Retrieved 8 July 2013.
- Bacteriological code Appendix 5 J.O. 4.
- COHN (F.): Untersuchungen über Bakterien. Beitrage zur Biologie der Pflanzen Heft 2, 1872, 1, 127-224.