Viruses

What are viruses?

Viruses are not microbes, they are not properly microbes, but just an infection agent that contain a nucleic acid responsible for the infection of the host. Viruses are able to multiply and live only within host cells. These are parasites that contain nucleic acid and are responsible for example, of infection in the nose.

Viral components

One of the viral components is the nucleic acid, and the capsid: the nucleic acid is protected within the capsid, which is made by proteins and sometimes (just in some viruses) we can expect an outside structure of the capsid that has got some proteins. It is very important for the virus to recognize the specific receptor of the host cell. The spike proteins are present in the envelope, if the virus has got it, or they can just be stuck to the capsid. The capsid is made of proteins and the envelope is a double phospholipidic membrane, because it arrives from the host cells, acquired by the virus when it exists the host cells.

Virus life cycle

The virus goes in the host cell using different methods, based on the presence of the envelope. It starts to replicate, producing different proteins important for the assembly, using enzymes in the cell for the DNA replication (apart from retroviruses, because they need a specific enzyme that is able to replicate RNA from DNA, and it is not normally present in the cells). All the other cells divide and then, once the mitogenetic activity has been replicated, the proteins that compose the host cells are going to be reproduced, that is the assembly of the new virus. Once the virus is assembled, it is able to escape from the cells. And during this process, some of the virus acquire the envelope that is part of the cell. This is all the process of penetration, uncoated virus, maturation, release, and so on.

How can we grow viruses?

Remember that the only way to “catch” them is by using living cells. We can use eggs to grow viruses, and this is the way we did in the past, for diagnostic but also for research. Now the eggs are still used to grow them, also for the production of vaccines. Some vaccines against viruses are made by growing them in eggs, but sometimes the viruses outgrow the egg. Otherwise, we use a cell culture: primary, secondary or immortalized cell cultures. The immortalized cell cultures are the best system, because they grow constantly, it is one of their main characteristics. Instead, for primary and secondary cultures, we can grow cells for several days or months, but they will eventually die.