Stories about patients demanding antibiotics to treat cold and flu symptoms are all too common, highlighting the confusion many people have when it comes to understanding the difference between a viral and bacterial infection. It seems that a lot of people think viruses and bacteria are the same thing, and can therefore be treated with the same drugs. This post will give an introduction to viruses and the treatment of viral infections, and my next post will do the same for bacteria. Hopefully this will save a few GPs from arguments with sneezing patients!
Structure/Shape Viruses are classified as obligate intracellular parasites, meaning their simple structure requires a host cell for replication. They are between 20 and 300 nm in diameter, therefore much smaller than bacteria. Viruses are composed of an RNA or DNA genome surrounded by a protective protein coat called the capsid. The capsid is made of proteins coded for by the viral genome, and the shape of the capsid determines which morphological classification the virus belongs to. The four morphological classes are:
- Helical: the protein subunits form a helical shape around the RNA or DNA, which is also arranged in a helix.
- Isocahedral: The protein subunits form a shell around the RNA or DNA.
- Envelope: The virus is enveloped by a cell membrane of the host cell, which is studded with viral proteins.
- Complex: Viruses that are not entirely helical or isocahedral, with extra structures such as protein tails.
Genome Viruses can have either an RNA or DNA genome. 70% of viruses have RNA genomes, which can be single (ssRNA) or double stranded (dsRNA). ssRNAs can be sense or antisense strands; the sense strand can function as an mRNA, whereas the complementary antisense strand cannot. There are also ssRNA retroviruses (ssRNA-RTs), which are first transcribed into circular DNA (cDNA) before being integrated into the host cell’s genome and transcribed into RNA, which is translated into proteins. Most DNA viruses are double stranded, but members of the Parvovirus and Circovirus families have ssDNA, which is circular in Circoviruses. There are also dsDNA-RTs, which replicate using an intermediate RNA.
Replication The simple structure of viruses means that they need a host cell in order to replicate. Each replication cycle can produce 100s or 1000s of new virus particles. The replication stages are as follows:
- Attachment: surface proteins on the virus attach to complementary receptors on the host cell.
- Penetration: Non-enveloped viruses are translocated across the cell membrane, while enveloped viruses are enter the host cell by fusion of the viral envelope with the host cell membrane.
- Uncoating: The capsid is removed or degraded by enzymes, releasing the viral RNA or DNA
- Replication: mRNA is synthesised (if the virus is not a positive sense RNA virus) along with viral proteins. DNA viruses replicate using host cell polymerases in the nucleus. RNA viruses replicate in the cytoplasm, positive sense RNAs can be accessed by host ribosomes, but negative sense RNA must be transcribed into a complementary positive sense strand in order for the ribosome to attach to it.
- Assembly: the capsid forms around the RNA or DNA.
- Release: the mature virus exits the host cell.
Prevention and Treatment Vaccines are the best way of preventing viral diseases. There are three types of vaccine: live attenuated, inactivated, and recombinant-produced antigens. Live attenuated vaccines contain weakened viruses that are able to multiply in the body without causing disease. In some rare cases, such as in people with weak immune systems, live vaccines can cause infection, but advances in recombinant technology mean scientists are able to modify or remove genes that could cause disease. The MMR and seasonal flu vaccines are two examples of live attenuated vaccines. Inactivated vaccines contain viruses or viral components that have been chemically or physically inactivated, so they are unable to cause disease. The capsid proteins remain intact, so they are recognised by the immune system. Unlike live attenuated vaccines, they do not confer lifelong immunity, so booster injections are required because they virus does not replicate in the body. The polio vaccine contains an inactivated form of the poliovirus. Recombinant antigen vaccines are made of components such as surface proteins, but do not contain genetic material, so they cannot cause infection. The viral components used are ones that cause an antibody response, resulting in immunity. This technique is used to make the hepatitis B vaccination. Antiviral drugs are used to treat viral infections. The aim of antivirals is to disrupt replication at certain replication cycles stages that do not occur in uninfected cells or involve virus-specific enzymes. This is because the virus replication is linked to cell metabolism, so the replication cycle must be disrupted in a way that won’t affect normal metabolism. The penetration and uncoating stages of replication are targeted by antiviral drugs as they only occur in infected cells. Transcription of positive sense RNA to DNA in retroviruses is catalysed by reverse transcriptase, a viral enzyme. Examples of antivirals include amantidine for influenza, zidovudine for HIV, and vidarabine for herpes simplex virus.
References Medical Microbiology. 4th Edition – S. Baron
I hope you know a little more about viruses now, come back soon for an overview of bacterial infections.