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Extracellular and Intracellular bacteria and their preferred growth phase within the host

Pathogenic bacteria can be grouped into three categories on the basis of their invasive properties for eukaryotic cells.

  1. Extracellular bacteria
  2. Facultative intracellular bacteria
  3. Obligate intracellular bacteria

Extracellular bacteria: Extracellular bacterial pathogens do not invade cells and proliferate instead in the extracellular environment which is enriched with body fluids.  Some of extracellular bacteria even don’t penetrate body tissues (e.g. V. cholerae) but adhere to epithelial surfaces and cause disease by secreting potent toxins. 

Although bacteria such as E. coli and P. aeruginosa are termed noninvasive, they frequently spread rapidly to various tissues once they gain access to the body.  Extracellular bacteria do not have the capacity to survive the intracellular environment or to induce their own uptake by most host cells.

Predominantly extracellular bacteria are:

1.       Bacillus anthracis
2.       Enterotoxigenic Escherichia coli
3.       Haemophilus influenza
4.       Mycoplasma spp
5.       Pseudomonas aeruginosa
6.       Staphylococcus aureus
7.       Streptococcus pyogenes
8.       Vibrio cholerae

Intracellular Bacteria: Intracellular pathogens commonly caused “granulomatous lesions”. It is divided into two groups-

Facultative Intracellular Bacteria and their survival mechanism :

Facultative intracellular bacteria invade host cells when it gives them selective advantage in the host. Bacteria that can enter and survive within eukaryotic cells are shielded from humoral antibodies and can be eliminated only by a cellular immune response. However these bacteria must possess specialized mechanisms to protect them from the harsh environment of the lysosomal enzymes encountered within the cells.

Other facultative intracellular bacteria/fungi are:

  1.  Invasive Escherichia coli
  2. Neisseria spp
  3. Brucella spp
  4. Histoplasma capsulatum (yeast)
  5. Shigella spp

Obligate intracellular bacteria

This group of bacteria can’t live outside the host cells. For e.g. Chlamydial cells are unable to carry out energy metabolism and lack many biosynthetic pathways, therefore they are entirely dependent on the host cell to supply them with ATP and other intermediates.  Because of this dependency Chlamydiae were earlier thought to be virus (All viruses are obligate intracellular parasites). Obligate intracellular bacteria cannot be grown in artificial media (agar plates/broths) in laboratories but requires viable eukaryotic host cells (eg. Cell culture, embryonated eggs, susceptible animals).

Other obligate intracellular bacteria are:

  1. Mycobacterium leprae cannot be cultured in vitro; it is an obligate intracellular parasite.
  2. Coxiella burnetti: The metabolic activity of Coxiella burnettii is greatly increased in the acidic environment of the phagolysosome.
  3. Ricekettsia spp