Bacterial Classification

Overview
  • Categorizations of bacteria can highlight any number of structural, biochemical, or life-cycle aspects of bacteria. Here we discuss important classificaitons of bacteria, the titles can be used as interlinks to list bacteria within this class.
Cell Wall
  • Overview
    • Bacteria are often classified by the structural features of their Cell Wall. Please refer to Bacterial Cell Wall to review the biochemical features of the different types of cell walls. The major Cell Wall-based classfications of bacteria are mentioned below.
  • Classes
    • Gram Positive Bacteria
    • Gram Negative Bacteria
    • Acid-fast Bacteria
    • Cell Wall Absent Bacteria
Metabolism
  • Overview
    • Bacteria use a wide variety of biochemical strategies to derive energy from nutrients. Some bacteria possess an electron transport chain and so can use oxygen to power generation of ATP whereas others generate ATP purely through fermentation of sugars. However, oxygen can also be toxic and some organisms possess enzymes such as Catalase and Superoxide Dismutase to prevent oxygen toxicity. The relationship of bacteria with oxygen is one important way by which bacteria can be classified.
  • Subcategories
    • Obligate Aerobes: Require the presence of oxygen for growth to power their electron transport chains and thus possess catalase and dismutase
    • Facultative Anaerobes: Can grow in the presence or absence of oxygen by either using their electron transport chains or employing fermentation all while possessing catalase and dismutase
    • Microaerophilics: Use only fermentation for energy but possess low amounts of dismutase and so can tolerate small amounts of oxygen in the environment
    • Obligate Anaerobes: Use only fermentation for energy and possess no dismutase or catalase and so cannot tolerate oxygen in the environment.
Life Cycle
  • Overview
    • Most bacteria grow independently in host tissues and are thus exposed to potentially opsonizing antibodies. However, some bacteria have developed strategies to grow within human host cells, thus being protected from antibodies.
  • Facultative Intracellular Organisms:
    • Most facultative intracellular bacteria are phagocytosed by immune cells but secrete factors which prevent their destruction once inside the phagosome. Remarkably, facultative intracellular organisms can replicate within the phagosome. This capacity exists in addition to the ability to replicate outside of host cells.
  • Obligate Intracellular Parasites:
    • Unlike facultative intracellular organisms, these bacteria can only replicate inside host cells. This is because these organisms cannot generate their own ATP and must parasitize this energy source from host cells. The forms of these organisms which exist outside of host cells are purely infectious and cannot replicate independently.