MadSci Network: Microbiology

Re: why can't antibiotics kill viruses

Date: Wed May 11 19:11:36 2005
Posted By: Mark Sullivan, 2nd Year Internal Medicine Resident
Area of science: Microbiology
ID: 1113567287.Mi

Hi Mohammed,

Good question, and an important one. Antibiotics is a broad term used to describe medications that we use to kill or slow down microbial organsims that cause infection. In this term we have several types: Antibacterials, Antifungals, Antiparasitic and Antivirals. As you know we can be infected by any one of these types of microbes, some cause serious disease, others not so bad. Often when a person is ill enough, and the type of organism could be deadly we use an antibiotic to help the body fight these invaders off.

Your specific question is interesting for several reasons. First, most antibiotics work by preventing a bacteria, fungus or parasite from being able to do a certain activity. This includes things like building up it's cell wall, or making proteins necessary for either duplicating itself or performing a vital job. Penicillin is perhaps the most famous antibiotic, discovered by Alexander Flemming in 1929, and changed the face of medicine as we know it. Before that discovery people died of infections which are otherwise today very easy to treat! For example Strep throat is caused by a bacteria that is killed by Penicillin, but before this was responsible for causing many deaths. What makes an antibiotic like Penicillin useful is that it is very specific to bacterial functions and not to people. All organisms share similarities in DNA and RNA as well as proteins, but all species are different enough that sometimes you can target specific actions. In this case Penicillin prevents good strong buildup of a bacterial cell wall, which limits growth and ultimately leads to death.

So why don't antivirals work? Well, they do, but compared to bacteria it is not so easy. Viruses are different from all other forms of life in that they generally do not have any "internal machinery" of their own. A bacterial cell is able to carry out it's own functions, all it needs is a nutrition source and it can make copies of itself, get around, make and synthesize proteins and replicate DNA using only what it has. A virus lacks all of these things. A virus that is just sitting outside of a cell does virtually nothing until it encounters a cell to infect. Unless it infects a cell, it cannot make proteins, cannot duplicate itself, can not move around. But once inside a cell it uses the "hosts" internal machinery to do all these things and make more of itself. It essentially "hijacks" the host cell!

Now, remember how I said that antibiotics try to works on just the target and not anything else? Well, we do have certain antiviral drugs that are as specific as we can get to prevent a virus from either infecting a cell, or preventing the "hijacking" of other cell functions. Because of this there is still controversy in whether viruses can be classified as "alive" since they duplicate themselves without being able to infect a host cell. But, even viruses have some specific proteins that are made once a cell is infected, that can be targeted for antivirals. And most virused replicate very quickly, so by the time disease is detected, it is almost useless to start treatment.

Take for example the Flu. A very common virus, people get it every year. This virus mutates very easily, which is why making a vaccine is very tough. We can detect when someone has the flu virus, and if we detect this early enough (within 48 hours from the onset of symptoms), then we can give an antiviral which will help someone get over the flu much faster. But it doesn't cure the flu, cause like I said, once you get it, you got it! But after a while our immune system is able to fight this virus off and we get better.

HIV is a different kind of virus which can cause AIDS, and people can be infected for years without it ever actually causing symptoms. It actually attacks cells that make up the immune system, slowly wearing it down. We know that when it gets bad enough, people can get very sick and die of what are called oppotunistic infections. We have lots of anti-HIV medications, some work better than others. But none will get rid of the virus all together because the most important part of the virus, it's genome, inserts right into our own and stays hidden. This is only done by certain types of viruses called "Retroviruses". Now all DNA looks the same chemically, but the sequence is important for determining what type of protein is made, or what function is carried out. We don't have the ability to remove foriegn DNA from inside our own DNA yet.

More than this, most antibiotics can be overcome if a mutation occurs at the target site. Some types of infections that could once easily be treated are now resistant to some antibiotics. We are always trying to make new versions though to stay ahead of the game. But it is not easy since we work relatively slow at this and most microbes can duplicate and mutate within mintues to hours. So you can see that is quite challenge!

So, there are antibiotics for lots of things, however not all infections are treatable. The best prevention against infection is not to get infected in the first place. Hope that is not too much, this is kind of hard to explain without good pictures. The website, called "All the Virology on the WWW" is a good place to start. Some of the information is pretty technical though, so if you search around a little more I am sure you'll find something.

Good luck to you.


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