I had a heavy cold and went to see the doctor, but she didn’t give me antibiotics

…and she was right!

Patients are often advised not to expect a prescription for an antibiotic from their doctor if they are suffering from an infection that is likely caused by a virus. Sore throats, coughs and colds (collectively known as URTI – upper respiratory tract infections) are good examples of common viral infections.

Many people know that the reason for not taking antibiotics for a viral infection is that antibiotics don’t kill viruses; but why is that the case?

 

Rhinovirus: commonest cause of the common cold...an untouchable by antibiotics

Well, viruses are completely different in their structure to bacteria. Nearly all bacteria have a cell wall and many antibiotics, for example penicillin, target the bacterial cell wall. As bacteria grow and divide they need new make more and more new cell wall. Antibiotics disrupt this process and damage the cell wall to such an extent that the bacterium literally disintegrates. Other antibiotics target the production of new protein in the bacterial cell. Without the ability to make protein the bacterium is unable to function and either stops growing or dies.

Viruses don’t have a cell wall and they don’t make protein nor do they have any of other targets that antibiotics hit – this is why antibiotics don’t work in viral infections.

This doesn’t mean to say that you can’t treat viral infections:  a limited number or infections are treatable. For example if you have bad ‘flu you can be given a drug called oseltamivir (Tamiflu);  herpes virus infections including chicken-pox and shingles can be treated with aciclovir (Zovirax) and patients with HIV can be given one of several drugs called anti-retrovirals.

But the number of anti-viral drugs is much, much smaller that antibiotics. Why? It’s because viruses are not capable of an independent existence. A virus can only replicate inside a fully functioning cell whether it is plant, animal or human – it doesn’t have the capability of doing this on its own. A virus has to infect a cell and hi-jack the machinery inside the cell to make more virus. So the processes that would normally go on inside a cell, production of protein, for example, are taken over by the virus for its own needs.

Killing a virus inside a cell is a risky business. If we target the processes that are being used to make new virus, we are also targeting the same ones that are vital to continued functioning of the cell.  In fact, several of the very first anti-viral drugs had very nasty side effects because of the damage they caused to patients’ cells and organs.

So, it is better to hit the virus before it gets inside the cells but even here the opportunities are limited and there are only a few examples of drugs which act in this way, for example oseltamivir and some anti-HIV drugs

When pets pounce, Pasteurella may not be far behind

A belated Happy New Year to all Microbioloblog’s readers!

We grew a bacterium in the lab recently which was identified as Pasteurella multocida. Without looking at the patient’s specimen I could tell with confidence that it would be a swab from an infected animal bite, mostly likely a cat or a dog. This is because this bacterium is mostly likely to be found living in the mouths of these (and other) animals. When they bite, it is introduced into the skin or deeper tissues of the victim and can then go on to cause infection.

It turned out that our patient had been bitten by her cat but as there was hardly anything to see, she had merely cleaned the wound as best she could with an antiseptic. A few days later, however, her hand had begun to swell and it became really painful. A visit to our Casualty Department (Emergency Room) for treatment and a prescription for an antibiotic called co-amoxiclav resolved the problem.

One of the reasons why cat bites are much more likely to become infected than those inflicted by man’s best friend is that dog bites often result in crushing or tearing of the skin. As well as being painful, crushed or torn skin looks pretty bad and patients often visit the Casualty Department shortly after being bitten for attention. This means that the wound is cleaned up at an early stage and a preventative (prophylactic) course of antibiotics can be given.

Infected cat bite cause by Pasteurella multocida (Credit Pediatric Supersite)

Patients often tend not to seek medical attention after a cat bite. Why? This because cats’ teeth are thin and sharp, so when they bite they tend to cause “in and out” puncture wounds. There is often very little to see and patients think that there is little need to have the bite looked at by a doctor or nurse, so they don’t receive prophylactic antibiotics and infection is free to develop.