Bacteria (singular bacterium) are a type of biological cell. Each bacterium is a single cell. They constitute a large domain of prokaryotic microorganisms. The cell structure is simpler than that of other organisms as there is no nucleus or membrane-bound organelles. Instead, their control center containing the genetic information is contained in a single loop of DNA. Some bacteria have an extra circle of genetic material called a plasmid. The plasmid often contains genes that give the bacterium some advantage over other bacteria. For example, it may contain a gene that makes the bacterium resistant to a certain antibiotic.
Bacteria are classified into five groups according to their basic shapes: spherical (cocci), rod (bacilli), spiral (spirilla), comma (vibrios), or corkscrew (spirochaetes). They can exist as single cells, in pairs, chains or clusters.
Until recently, all bacteria were grouped into a single kingdom of prokaryotes, Monera, which included both eubacteria and archaebacteria. Eubacteria are distinguished by having very strong cell walls containing peptidoglycan. Archaebacteria lack peptidoglycan in their cell walls and their genes are more similar to those found in eukaryotes than are those of eubacteria. The differences are so great that most biologists now agree that archaebacteria and eubacteria should be assigned to separate kingdoms. In the new taxonomic scheme, eubacteria, including cyanobacteria, make up the Kingdom Monera, while archaebacteria, redesignated archaea, comprise the Kingdom Archaea.
Bacteria reproduce by binary fission. In this process, a single bacterial cell called the “parent,” makes a copy of its DNA and grows larger by doubling its cellular content. The cell then splits apart, pushing the duplicated material out and creating two identical “daughter” cells.