Intracellular parasites are microparasites that are capable of growing and reproducing inside the cells of a host. Intracellular parasites can be either facultative or obligate in nature. 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 cellto live and reproduce. Many of these types of cells require specialized host types, and invasion of host cells occurs in different ways.
Types of Parasites
There are two main types of intracellular parasites: Facultative and Obligate.
Facultative
Facultative intracellular parasites are capable of living and reproducing either inside or outside cells. Bacterial examples include:
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:
The mitochondria in eukaryotic cells may also have originally been such parasites, but ended up forming a mutualistic relationship. 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-feverpathogenCoxiella burnetii to grow in an axenic culture and suggested the technique may be useful for study of other pathogens.
Invasion
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 a few different cell types. The entrance of these host cells will differ between intracellular parasites. Not all intracellular parasites will enter a cell the same way. Some will work with specific components in or on the host cell, an example being Trypanosoma cruzi. This parasite will attach itself to the host cell while increasing the intracellular calcium, which in turn disrupts the actin at the site of attachment, causing the host cell to create a lysosomal-barrier around the disruption. The parasite will take advantage of this membrane and produce a vacuole into the host cell. Other intracellular parasites have developed other ways to enter a host cell that are not requiring a specific component or action from within the host cell. An example of this is that some intracellular parasites use a method called gliding motility. This is the use of an actin-myosin motor that is connected the intracellular parasites cytoskeleton.
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.
Susceptibility
People with T cell deficiencies are particularly susceptible to intracellular pathogens.
