What is a Bacterium (singular) Bacteria (plural)???
"A unicellular prokaryotic microorganism that usually multiplies by cell division and has a cell wall that provides a constancy of form; may be aerobic or anaerobic, motile or nonmotile, and free-living, saprophytic, parasitic, or pathogenic"
[Source: Dirckx, J.H., Ed. 2001. Stedman's Concise Medical Dictionary for the Health Professions. Lippincott Williams & Wilkins]

Characteristics of a Bacterium
1. Prokaryotic, i.e., a living cell without a nucleus and membrane-bound organelles
2. Enclosed in a cell wall; note however that there are some species that lack a cell wall (e.g., Mycoplasma pneumoniae; some Archaea)
3. Generally reproduce by means of binary fission
4. Most derive nutrition from organic compounds, though some are photosynthetic or chemotrophic (derive energy from redox reactions)
5. Many bacteria are harmless, though others have a devastating effect on humans and other species
6. Microscopic, typically between 0.2 and 10 micrometers (10E-6 m) long

Classification of Bacteria
This is an interesting subject. For one, microbiologist believe that only about 1% of all bacteria have been identified. Also, the criteria used to identify bacteria (below) do not necessarily reflect evolutionary relationships among the species. Molecular techniques (e.g., DNA sequencing) are providing more insight into the evolutionary relationships among bacteria. The field is still young!

Some Criteria Used to Identify Bacteria

Cell Morphology
Cell Type
Phage Type
Nucleic Acid Profile

Some Examples of Cell Morphology

spherical or ovoid
cocci that divide and remain paired
grapelike cluster of cocci
Staphylococcus aureus
chainlike arrangement of cocci
Streptococcus pyogenes
rod shaped
Janthinobacterium lividum
chainlike arrangement of bacilli
corkscrew shaped
Borrelia burgdorferi

Image Sources: Janthinobacterium lividum - Cedar Crest College
Borrelia burgdorferi -
Staphylococcus aureus and Streptococcus pyogenes -