ABSTRACT
Bacteria (singular, bacterium) are small unicellular organisms that are classiŠed taxonomically in the domain Bacteria (or Eubacteria), the kingdom Prokaryotae (or Prokaryota or Monera). The only other domain in the kingdom Prokaryotae covers Archaea (or Archaeobacteria for “ancient bacteria”). With sizes ranging from 10−7 to 10−4 mm, prokaryotes are bigger than viruses (10−8-10−6 mm), but smaller than eukaryotes (10−5-103 mm). While both bacteria and archaea may have evolved independently from an ancient common ancestor, eukaryotes may have arisen from ancient bacteria entering into endosymbiotic associations with the ancestors of eukaryotic cells (possibly related to the archaea) to form either mitochondria or hydrogenosomes. A subsequent independent engulfment by some mitochondria-containing eukaryotes of cyanobacterial-like organisms may have led to the formation of chloroplasts in algae and plants.
