Obligate intracellular parasites

Intracellular parasites are microparasites that are capable of growing and reproducing inside the cells of a host. Some parasites can cause disease.

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

Bacterial examples include:

Fungal 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 in eukaryotic cells may also have originally been such parasites, but ended up forming a mutualistic relationship (endosymbiotic theory).

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.

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.

People with T cell deficiencies are particularly susceptible to intracellular pathogens.

...
Wikipedia