Pathogenic bacteria can be grouped into three categories on the basis of their invasive properties for eukaryotic cells.
- Extracellular bacteria
- Facultative intracellular bacteria
- Obligate intracellular bacteria
Extracellular bacteria: Extracellular bacterial pathogens do not invade cells. Instead they proliferate in the extracellular environment which is enriched with body fluids. Some of 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:
1. Bacillus anthracis
2. Enterotoxigenic Escherichia coli
3. Haemophilus influenzae
4. Mycoplasma spp
5. Pseudomonas aeruginosa
6. Staphylococcus aureus
7. Streptococcus pyogenes
8. Vibrio cholerae
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 and their survival mechanism :
Facultative intracellular bacteria invade host cells when it gives them 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 intracellular environment of macrophages for growth. Legionella induce its own uptake and blocks lysosomal fusion by undefined mechanism.
- R. rickettsii destroys the phagosomal membrane with which the lysosomes fuse.
- Mycobacterium tuberculosis: M.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:
- Invasive Escherichia coli
- Neisseria spp
- Brucella spp
- 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 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:
- Mycobacterium leprae cannot be cultured in vitro; it is an obligate intracellular parasite.
- Coxiella burnetti: The metabolic activity of Coxiella burnettii is greatly increased in the acidic environment of the phagolysosome.
- Ricekettsia spp
Toxoplasma, Cryptosporidium, Plasmodium, Leishmania, Babesia and Trypanosoma are obligate intracellular parasites.