Why can’t I take an antibiotic for my cold: A primer on viruses.

This is another article I put up on my Medium.com account.

 

As I’ve mentioned in a prior post, and is likely noted in other posts on my blog (and numerous medicine related sites), a virus is genetic material — either DNA or RNA — encapsulated in a protein coat. In some cases there is also a lipid envelope around the protein coat.

This gets to the crux of why antibiotics don’t work on viruses. Viruses need a living cell to reproduce, and does so by co opting the metabolic pathways of the host cell it invades. Normal cells use DNA to pass genetic information from one (cell) generation to the next. The DNA gets transcribed into RNA which is then taken to the cytoplasm of the cell (in eukaryotic cells) or other parts of the cell (in bacteria/non eukarotic cells), where proteins and ribosomes/ribosomal RNA read the RNA and ‘transcribe’ it into a protein.

In some cases, viruses do code for novel (relative to cells) enzymes and/or proteins. EG, reverse transcriptase in HIV viruses (HIV is a RNA based virus, but it’s genetic code is converted to DNA for replication). Other viruses replicate the RNA directly for future virus particles. However, as I note above, the vast majority of the replication is done by the cell’s own proteins.

This is why antibiotics don’t work on viruses. Bacteria have have different types of cell walls than human cells. They require chemicals that our cells just don’t use. Penicillins and cephalosporins interfere with the cell wall construction as an example. Other antibiotics target pathways our cells don’t have (eg, trimethaprim-sulfamathoxazole targets folate production in bacteria. Our cells don’t produce folate so it doesn’t affect us). There are other examples that could be used as well that utilize a chemical’s affinity for a bacterial protein/enzyme more than for the equivalent in human cells.

Having said this, there are some antiviral drugs for some (not all) viruses. These target the proteins viruses code for that are novel to the specific virus/virus family.

Now at this point, you might be thinking “well, my great aunt _namehere_ was told she had a viral pneumonia, but when she got admitted to the hospital she was given antibiotics”. In some cases, a viral infection goes through its course and the person can develop a secondary infection that’s bacterial in nature. In that case, antibiotics might be warranted. Or she was given them to prevent the superinfection/secondary infection. This is true of any viral illness, even corona viruses, rhinoviruses and covid 19.

Coronavirus biology 101

This is a post I put up in my Medium.com account.

Initially I was going to write specifically about covid19 and how to prevent getting (perhaps — or at least decreasing the chances of getting) it. I realized there are more than enough articles already about that subject and can be found on the CDC website.

Instead I decided to write more generically about Coronaviruses and doing so, hopefully address questions some people might have, and that might not be answered by posts that deal with coronovid19. First of all a virus, unlike bacteria, algae, fungi and animal cells, is just a bunch of genetic material encapsulated by protien (and in some cases, a lipid coat). They don’t have any mechanism to move, metabolize, ingest nutrients, replicate genetic material etc. Viruses are entirely dependent on infecting cells, and co-opting their metabolic pathways to reproduce. With some, there are some genes that code for things that cells — be they bacteria or eukariots — don’t have. An example would be Reverse Transcriptase in HIV viruses (Reverse Transcriptase reads RNA from the HIV genome and encodes it into DNA for use in viral replication. The virus is still dependent on the cell for the actual copying and reproduction of said genetic material and of making new viruses).

In the case of Coronaviruses, the genetic material is single stranded RNA. There are other families of RNA based viruses, such as Rhabdoviruses (EG the virus that causes rabies), Orthomyxoviruses (one such causes Measles), and others. The virus that causes HIV also uses RNA as genetic material, but by some classifications isn’t considered an RNA virus because the RNA has to be coded back into DNA for replication.

Because viruses don’t have the proteins/cell parts/etc to replicate themselves, as I note above, they’re dependent on a host cell for replication. This is why antibiotics don’t work for the common cold (or other viral illnesses. Though in some cases, such as HIV and some viruses closely related to the chicken pox virus, there are antivirals that work). As they don’t have a cell wall that’s different from our on cells’ walls, antibiotics that affect the building of the bacterial cell walls won’t work. For example, penicillin and cephalosporins interfere with an enzyme and chemical specific to what bacteria use to make cell walls. Other antibiotics interefere with enzymes and other proteins that are different enough from human cell proteins that the antibiotics don’t affect us. Others (eg, bactrim, aka trimethaprim-sulfamethoxazole) interefere with bacteria specific metabolic pathways.

Having said that, some viruses do need a particular enzyme or protein to reproduce that human (and mammalian cells in general) don’t have and do provide a target for medications. These are different than antibiotics.

As for coronaviruses, many are respiratory pathogens. In humans some coronoviruses cause the common cold. As one might expect or know, transmission can be through inhaling droplets that have active virus in them (and is why sneezing and coughing can spread the common cold, COVID-19, and so forth — the virus particles can become aerosolized). They can remain infectious on surfaces. Though the amount of time they can remain viable and therefore infectious is different for each virus and varies as well due to environmental conditions.

Anyhow, I hope this helps people understand why for viral illnesses, doctors don’t just hand out antibiotics. For COVID 19 information, use the link I have at the start of this post (it will take you to the appropriate page at the CDC).