The Microbiology of Viruses
From MyMCAT
Contents |
Introduction
By many classical definitions, viruses are not alive- they can not replicate on their own and die quickly without a host. But beyond this narrow view, viruses are essentially optimized replicating machines functioning with only the bare essentials .
Structure of Viruses
Viruses are much smaller than bacteria and are more comparable to the size of large protein aggregates. The most basic virus consists of genetic material coated by a protein complex known as the capsid. Their compressed genome, which may be RNA or DNA depending on the species, encodes only the information necessary to replicate: proteins for infecting new hosts, proteins to produce the capsid, and often proteins used to evade detection and destruction by their hosts.
In more complex viruses, typically those that infect animals, virus often have a second layer round them composed of a plasma membrane which is actually the product of budding out of an infected cell.
The Life Cycle of a Virus
Not every living cell can support a virus, thus viruses have protein receptors on their surface for binding only the target cells they are designed to infect. Upon successful attachment, viral proteins often undergo conformational changes which either trigger the enter virus to be endocytosed or trigger the injection of their infectious payload.
Once inside a cell, viruses may begin replicating immediately or wait in a latent state until a later activation event. Their replication is entirely dependent on host machinery and host metabolites. They do not synthesize their own proteins or nucleotides, but rather take advantage of the ribosomes and polymerases already inside the cell to begin synthesizing their own components.
Viruses that are surrounded by a plasma membrane typically acquire this by replicating capsids near the cell's plasma membrane or golgi and pinching through the lipid membrane such that they become wrapped by it. Viruses that are noneveloped on the other hand, typically replicate inside the cytoplasm of the cell and once a critical mass occurs and the cells dies, are simply released while the cell undergoes lysis. Either method in the end kills the host cell and releases hundreds of more identical infectious viruses.
Special Topics
Bacteriophages
Bacteriophages are specifically viruses that infect bacteria. They are almost always nonenveloped.
Many bacteriophages are also capable of entering a lysogenic cycle, rather a lytic cycle. In a regular lytic cycle, when infection occurs, the virus replicates until the host cell dies and once the host lysis, the viruses are released. In a lysogenic cycle, rather than replicating viruses right away, the viral genetic material integrates with the host's DNA and a dormant state occurs in which the bacteria is free to replicate. During these bacterial replications however, the viral DNA is also replicated and persist's in the host's genome. Upon a specific activation signal, the viral genome may activate returning the virus to a lytic cycle in which new viral particles are synthesized and the host cell dies.
Human Immunodeficiency Virus (HIV)
Human viruses are extremely specific to the types of cells in which they can replicate. For instance, Influenza replicates inside the epithelial cells of human lungs and Herpes replicates in the mucosal cells of their one's lips or genitals. HIV, an enveloped, RNA virus, is unique in that the cells which it infects are actually the immune cells that typically fight viral infections. It is the role of CD4+ T-cells to seek out infected cells and destroy them through recognizing foreign material presented in MHC complexes. In HIV infections, it is these CD4+ T-cells which are infected. Furthermore, HIV utilizes a unique set of enzymes to integrate itself into the cell's genome, effectively preventing itself from being destroyed once inside a cell.
Over time, the continued destruction of T-cells leads to a critical point at which the individual is no longer capable of mounting strong immune responses not only against HIV but any infection. At this point, the syndrome known as AIDS begins and marks a severe deterioration point for the health of the individual.
The term retrovirus is used to describe viruses which can convert their RNA genetic material back into DNA for integration into the host's genome.
