Extracellular and Intracellular Bacteria

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

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

Extracellular bacteria

Extracellular bacterial pathogens do not invade cells instead, they proliferate in the extracellular environment which is enriched with body fluids. Some extracellular bacteria even don’t penetrate body tissues (e.g. Vibrio 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:

Intracellular Bacteria

Intracellular pathogens commonly cause “granulomatous lesions”. They are divided into two groups-

  • Those that can be cultured in microbiologic media in the laboratory (facultative) or
  • Those that required living cells/animals (obligate).

Facultative Intracellular Bacteria

Facultative intracellular bacteria invade host cells when it gives them a selective advantage. 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.

  • Legionella pneumophila: It prefers the intracellular environment of macrophages for growth. Legionella induces its own uptake and blocks lysosomal fusion by an undefined mechanism.
  • R. rickettsii destroys the phagosomal membrane with which the lysosomes fuse.
  • Mycobacterium tuberculosisM.tuberculosis survives intracellularly by inhibiting phagosome-lysosome fusion.
  • Listeria monocyotogenes: Listeria quickly escapes the phagosome into the cytoplasm before phagosome-lysosome fusion.
  • Salmonella spp: Very resistant to intracellular killing by phagocytic cells.

Other facultative intracellular bacteria are:

  1.  Invasive Escherichia coli
  2. Neisseria spp
  3. Brucella spp
  4. Shigella spp

Histoplasma capsulatum and Cryptococcus neoformans are facultative intracellular fungi.

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 a 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).

Pneumocystis jiroveci is an obligate intracellular fungi.

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. Rickettsia spp

Toxoplasma, Cryptosporidium, Plasmodium, Leishmania, Babesia, and Trypanosoma are obligate intracellular parasites.

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. As an asst. professor, I am teaching microbiology and immunology to medical and nursing students at PAHS, Nepal. I have been working as a microbiologist at Patan hospital for more than 10 years.

12 thoughts on “Extracellular and Intracellular Bacteria

  1. I have been working on a bacteria and I found it to be intracellular . TEM micrographs suggests that these bacteria forms vacuoles and tend to multiply within the vacuolated cups when infected with RAW macrophages cells.

    My query: Can anyone suggest a paper wher they prove a particular bacteria to be intracellular. want a ref paper.TQ

  2. Why the immune response to intracellular pathogens is different from extracellular pathogens?

    1. Dear Manish
      Thank you for your query. Did you studied Immunology? Your question requires understanding of various immunological concepts. If you have please look the chapters of Antigen processing and presentation. You will get answer there.

  3. Tankeshwar Acharya, thank you for your blog explaining intracellular and extracellular bacteria. I am doing some desktop research on chronic urinary tract infections and found your description helpful. Did you know there are recent findings that incriminate intracellular bacteria and/or biofilm bacterial infections in chronic lower urinary tract symptoms (LUTS). Scientists in the US (Wolfe, A & Braubaker, L) and UK (Malone-Lee, J & Rohn, J) have recently discovered through the use of molecular technology, that urine in not at all sterile and up to 450 different bacterial species are found living in the bladder of healthy people. They believe these bacterial communities could be working in the same way that gut bacteria do by protecting the bladder from invading pathogenic bacteria. But in some people, this protective mechanism stops working for some reason, and a pathogenic bacteria takes over and invades cells or forms biofilm on the surface of cells (Hultgren, S). Being a microbiologist, I thought you’d be interested in this topic.

    Renjan, this paper talks about intracellular bacterial communities: http://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.0040329

    1. Dear Friend thank you so much for your comment and this useful information, I will go through this article. Generally we regard Blood, Urine and other body fluids are sterile; after availability of molecular techniques and newer diagnostics, scientist/researcher are claiming that Blood is not sterile either. For routine diagnostics; so far we believe and practice that these specimen are sterile or organisms if present in these specimen do not grow on our routine culture media. I am hopeful that with the use of novel diagnostic techniques, researcher/scientist will discover and validate new facts/findings.

  4. The above link was about intracellular bacterial communities, which I shared because someone above asked for a paper that had evidence of bacteria being intracellular.

    In relation to my comments above about urine not being sterile, you might be interested in reading the Wolfe & Baubaker paper. It’s hot science. I agree, I hope new technology and techniques provide medical solutions to a lot of unanswered questions. I hope you find it interesting.

  5. Mr Acharya
    I am biomedical science student
    how does immune system switch between cell mediated and humeral in relation to intra or extra cellular bacteria..or both work along

  6. Thank you prof for you great explanitation, but I have a query, does treponema as a jenus considered one of the obligate intracellular bacteria? I really need your help

  7. Please what are the factors responsible for the predilection of intra-cellular bacteria? Thank you

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