Intracellular parasite

Intracellular parasites are

There are two main types of intracellular parasites: Facultative and Obligate.[2]

Facultative intracellular parasites are capable of living and reproducing in or outside of host cells. Obligate intracellular parasites, on the other hand, need a host cell to live and reproduce. Many of these types of cells require specialized host types, and invasion of host cells occurs in different ways.^[2]

Facultative

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

Bacterial examples include:

Fungal examples include:

Obligate

Obligate intracellular parasites cannot reproduce outside their host cell, meaning that the parasite's

All viruses are obligate intracellular parasites.

Bacterial examples (that affect humans) include:

• Chlamydia, and closely related species.^[14]
• Rickettsia
• Coxiella
• Certain species of Mycobacterium such as Mycobacterium leprae, that survive in phagocytes
• Anaplasma phagocytophilum^[b][15]

Protozoan examples (that affect humans) include:

• Apicomplexans (Plasmodium spp., Toxoplasma gondii and Cryptosporidium parvum^[16]
  )
• Trypanosomatids (Leishmania spp. and Trypanosoma cruzi
  )

• Pneumocystis jirovecii^[17]

The

When an intracellular parasite goes to enter a host cell, it is particular about the type of host cell. This is because most intracellular parasites are able to infect only a few different cell types.[21]

• Viruses use a number of host receptors to gain entry to the cell, usually by causing endocytosis.^[7] See viral entry for more on this well-studied topic.
• Bacteria are also generally small enough to be engulfed by endocytosis, which they trigger with adhesins. Unlike viruses, they can and often do manipulate the cell's behavior beforehand, by injecting effector proteins into the cytosol.^[7]
• Protists are generally too big to enter through endocytosis; they use alternate ways.^[22]
  • Plasmodium and Toxoplasma gondii are
    apicomplexans, named for the fact they have a "apical complex", used for gaining entry into the cell. The apicomplexan first moves on the cell looking for an ideal receptor. When the receptor is found, it re-orients itself so the apical complex points at the cell. It then secretes a number of proteins to form a moving junction, through which it gains entry.^[22]
  • plasma membrane repair. All nucleated cells use calcium concentration as a signal for membrane damage. T. cruzi attaches to the target cell then increases the calcium concentration inside, disrupting the actin network and triggering the repair mechanism. Lysosomes are recruited to this disruption and release their contents to the extracellular side, as a way to replenish the plasma membrane. T. cruzi take advantage of the excess membrane to form a vacuole in the host cell, gaining entry.^[21] Because this repair mechanism is universal to all cells with a nucleus, T. cruzi is not picky about the target cell type. Leishmania also uses this mechanism.^[22]
  • Leishmania can also trigger phagocytosis. It is able to withstand the degradation process the cell carries out following phagocytosis.^[22]
  • Microsporidians, which are tiny protozoans related to fungi, seems to form "polar tubes" that poke into the target cell.^[22]