Obligate parasites need a host to survive and complete their life cycle, including viruses, bacteria, fungi, and animals. They can be difficult to study and guard against, and some, like Chlamydia, cause diseases. Even some protozoa are obligate intracellular parasites, and mitochondria may have evolved from intracellular parasites.
An obligate parasite is a parasite that must stay with its host, or it dies. Obligate parasites depend on the presence of a host to complete their life cycle. Obligate parasites are common. There are parasitic plants, fungi, bacteria and animals. The inverse of an obligate parasite is a facultative parasite, one that can complete its life cycle independently of a host.
One of the most common obligate parasites are viruses. Viruses are fragments of genetic material covered in a protein sheath, capable of hijacking cells’ protein synthesis machinery and using them to pump out copies of the virus. Due to their inability to reproduce on their own, viruses have sometimes been excluded from the realm of life, although this definition of “life” may be inappropriate because there are a number of more complex obligate parasites. Guarding against viruses may have been one of the earliest evolutionary imperatives of bacteria and eukaryotes, and both evolved a variety of genetic error-checking and response mechanisms to slow down viral invaders.
There are other obligate intracellular parasites besides viruses. These include bacteria such as Chlamydias and Rickettsia, among the smallest viruses with the least complex genomes. The Chlamydia bacterium is responsible for the sexually transmitted disease no. 1 in the world, chlamydia, which is also the leading cause of infectious blindness. Because obligate intracellular parasites lack a tractable genetic system and cannot be grown in a conventional artificial nutrient environment and require tissue culture, they can be difficult to study. Historically, these bacteria were thought to be something between a virus and a bacteria.
Even some protozoa (eukaryotes, cells much more complex than bacteria) are obligate intracellular parasites, in particular Plasmodium, of which at least ten species infect humans. These are thought to be descended from dinoflagellates, photosynthetic protozoa, which eventually lost their photosynthetic ability as their parasitic lifestyle increased in emphasis. Interestingly, it is thought that mitochondria, the powerhouses found in every human cell, may have begun their evolutionary journey as intracellular parasites, but later became so integrated into the host that they actually became part of it.
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