Question

Which of the following microbes are considered obligate intracellular parasites?

A) Rickettsia rickettsii

B) Mycoplasma pneumoniae

C) Mycobacterium tuberculosis

D) viruses

E) Chlamydia trachomatis

Answer 1

Obligate intracellular parasites are the parasites that require hosts for survival. They cannot survive outside the host cellular environment because they lack one or several processes required for metabolism or replication etc.. These parasites use host machinery to replicate or obtain the nutrients and energy for their survival.

Rickettsia ricketsii are obligate intracellular parasites. They are spread by arthropod vectors and require both arthropod and mammal cells to complete their life cycle. They replicate by binary fission inside the host cells. The reason for their obligate parasitism lies in their permeable cell membrane. This makes their cytoplasm unstable. Hence they require the host cytoplasm to ensure survival.

Mycoplasma pneumoniae are not obligate parasites. They are the smallest free-living organisms. They require a medium rich in sterol and serum proteins for growth. Hence, they have the ability to survive with limited capacity outside the host.

Mycobacterium tuberculosis can survive outside the host. Hence they are not obligate parasites.

Viruses are simply nucleoprotein complexes. They lack organelles, enzymes and any form of energy reserves. Therefore, they can survive only inside the host cells. They cannot replicate outside the host. Thus, these are obligate intracellular parasites.

Chlamydia trachomatis lacks ATP-generating ability. Thus, it is completely dependent on its host for energy requirements. Hence it is an obligate-parastie.

Thus, from the given options, the obligate intracellular parasites are A, D and E.