Bacteria are single-celled organisms that are prokaryotic, which means that they do not have their genetic material stored in a nucleus or other membrane-bound organelles.

Some of these are aerobic and others are anaerobic. They are often classified in two broad categories, Gram-positive or Gram-negative, according to the composition and structure of their cell envelopes, which is reflected in the way they react to the Gram stain. Diseases caused in humans by these organisms can be treated by antibiotics and antibacterials however the phenomenon of resistance to these drugs in bacteria is increasing.