Intracellular parasite

Intracellular parasites are parasitic microorganisms - microparasites that are capable of growing and reproducing inside the cells of a host.

Facultative

Facultative intracellular parasites are capable of living and reproducing either inside or outside cells.

Bacterial examples include:

• Bartonella henselae
• Francisella tularensis
• Listeria monocytogenes
• Salmonella typhi^[1]
• Brucella
• Legionella
• Mycobacterium
• Nocardia
• Rhodococcus equi^[2]
• Yersinia
• Neisseria meningitidis^[3]

Fungal examples include:

• Histoplasma capsulatum.^[4]
• Cryptococcus neoformans^[5]

Obligate

Two apicomplexans, Toxoplasma gondii, within their host cell. Transmission electron microscopy

Obligate intracellular parasites cannot reproduce outside their host cell, meaning that the parasite's reproduction is entirely reliant on intracellular resources.

Obligate intracellular parasites of humans include:

• Viruses
• Certain bacteria, including:
  • Chlamydia, and closely related species.^[6]
  • Rickettsia
  • Coxiella
  • Certain species of Mycobacterium such as Mycobacterium leprae
• Certain protozoa, including:
  • Apicomplexans (Plasmodium spp., Toxoplasma gondii and Cryptosporidium parvum^[7])
  • Trypanosomatids (Leishmania spp. and Trypanosoma cruzi)
• Certain fungi
  • Pneumocystis jirovecii^[8]

The mitochondria in eukaryotic cells may also have originally been such parasites, but ended up forming a mutualistic relationship (endosymbiotic theory).^[9]

Study of obligate pathogens is difficult because they cannot usually be reproduced outside the host. However, in 2009 scientists reported a technique allowing the Q-fever pathogen Coxiella burnetii to grow in an axenic culture and suggested the technique may be useful for study of other pathogens.^[10]

Nutrition

The majority of intracellular parasites must keep host cells alive as long as possible while they are reproducing and growing. In order to grow they need nutrients that might be scarce in their free form in the cell. To study the mechanism that intracellular parasites use to obtain nutrients Legionella pneumophila, a facultative intracellular parasite, has been used as a model. It is known that Legionella pneumophila obtains nutrients by promoting host proteasomal degradation. Self-degradation of host proteins into amino acids provides the parasite with its primary carbon and energy source.^[11]

Susceptibility

People with T cell deficiencies are particularly susceptible to intracellular pathogens.^[12]

References