Intracellular Parasites

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Protozoa Microorganism. Citations Publications citing this paper. Musyoka , Aquillah M. Kanzi , Kevin A. Modulation der Apoptose in Raw The effects of the abortifacient parasite, Neospora Caninum, on bovine foetuses in early and late gestation Patrick Sylvester Craig. The effects of nitric oxide on the immune system during Trypanosoma cruzi infection.

Multiphoton imaging of ultrashort pulse laser ablation in the intracellular parasite Theileria. Viruses are classified by the type of nucleic acid they contain, and the shape of their protein capsule.

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Examples of several viruses are shown in Figures Figure 1. Structure of a variety of viruses. Figure 2. The Adenovirus top and papillomavirus bottom. The Adenovirus is a DNA virus that causes colds and "pink eye". Structure of the adenovirus left , and transmission electron micrograph of the virus.

The Papillomavirus is a DNA virus that causes warts. These infectious particles are small, about 15 nm in diameter. Structure left and electron micrograph color added of the paillomavirus. Figure 3. Transmission electron micrograph of the influenva virus. The Influenza virus causes the flu. All viruses have at least two parts. An outer capsid , composed of protein subunits. The viral genome is at most several hundred genes. In contrast, a human cell contains over thirty thousand genes.

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A viral particle may also contain various proteins, especially enzymes e. Several different replication cycle types are known for viruses.


Some viruses simuilar to that shown in the animated GIF in Figure 4, attach to the outside of the host cell. Figure 4. Animated GIF of the attachment of a virus to its host. Figure 5. Top: T4 bacteriophage DNA virus. TEM x55, Based on an image size of 1 inch in the narrow dimension.

The virus is in a large clump in the corner. SEM x3, Based on an image size of 1 inch in the narrow dimension. These images are copyright Dennis Kunkel at www. Viruses gain entry and are specific to a particular host cell because portions of the capsid or spikes of the envelope adhere to specific receptor sites on host cell plasma membrane.

Their nucleic acid then enters the cell, where the viral genome codes for production of protein units in the capsid.


Bacteriophages invade the host cell, take over the cell, and begin replicating viruses, eventually lysing or bursting the host cell, releasing the new viruses to infect additional cells. Figure 6 illustrates a typical T bacteriophage, as well as several of these viruses being assembled in a host. Figure 6. Left: Several of the T bacteriophage viruses. Right: Structure of a T bacteriophage virus.

Image from Purves et al.

Under certain conditions the viral DNA can detach and direct replication of new virus, eventually killing the host cell. Once inside the cell, the nucleic acid follows one of two paths: lytic or lysogenic, as shown in Figure 7. Virus may have genes for a few special enzymes needed for the virus to reproduce and exit from the host cell. A virus in essence takes over the metabolic machinery of the host cell when it reproduces. Figure 7. Replication cycle of a bacteriophage virus.

Images from Purves et al. The lytic cycle is one in which the virus takes over operation of the bacterium immediately upon entering it, with the production of new viruses and their subsequent release destroying the bacterium. The lysogenic cycle is seen when the virus incorporates its DNA into that of the bacterium, with some delay until the production of new viruses.

Survival of protozoan intracellular parasites in host cells.

When this occurs, the phage is latent, and the viral DNA is called a prophage. This prophage is replicated along with host DNA, so all subsequent cells produced by the infected but latent cell lysogenic cells carry a copy of the prophage. Certain environmental factors for example, ultraviolet radiation induce the prophage to enter the biosynthesis stage of the lytic cycle, followed by maturation and release. Still other viruses invade animal cells and replicate without killing the host cell immediately.

New viruses are released by budding off the host cell's plasma membrane, turning the host cell for a time into a viral factory. Animal viruses replicate very muck like bacteriophages do, although with modifications. If the virus has an envelope, glycoprotein spikes first adhere to plasma membrane receptors.

The entire virus not just the viral nucleic acid is then taken into the host cell by endocytosis. Once inside the host cell, the virus loses its envelope and capsid. The viral nucleic acid, now free of its covering, proceeds with biosynthesis.

Microbiology lecture 6 - Obligate intracellular parasites - Rickettsia, Chlamydia bacteria

Newly assembled viral particles are released not vial cell lysis, but rather by budding. During this process, the viral particles pick up their envelopes on host cell membrane. Components of viral envelopes are obtained from the plasma membrane as the viruses leave the cell. Budding does not necessarily result in the death of the host cell. This process is shown in Figure 8.


Once the retroviral RNA and reverse transcriptase are inside the host cell, as shown in Figure 9, the enzyme reverses transcription by making a single stranded DNA from the retroviral RNA. It remains in the genome and is replicated whenever the host DNA replicates. If viral DNA is transcribed, new viruses are produced by biosynthesis, maturation, and release by budding.

Retroviruses include HIV and also cause certain forms of cancer.

Redirection of Host Vesicle Trafficking Pathways by Intracellular Parasites