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

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.

The Food Standards Agency has produced some helpful advice to make sure that your seasonal celebrations aren’t spoiled by some unwelcome guests at Christmas

• If you buy a frozen turkey make sure it’s completely thawed before you cook it by checking there are no ice crystals in the cavity. Also, use a fork to test the thickest parts of the meat. A larger turkey can take up to two days to defrost entirely.
• Don’t wash your turkey before cooking. Washing is more likely to splash food bugs on to worktops, dishes and other foods. Proper cooking will kill bugs.
• Plan your cooking time properly. Follow the cooking times and other instructions on the label or, as a general guide, for a turkey weighing less than 4.5kg, allow 45 minutes per kg plus 20 minutes; for one weighing between 4.5kg and 6.5kg, allow 40 minutes per kg plus 20 minutes; and for those over 6.5kg, 35 minutes per kg plus 20 minutes. All turkeys should be cooked at 180ºC, 350ºF, Gas Mark 4 (after the oven has preheated).
• Check the turkey is cooked properly by cutting into the thickest part of the meat. None of the meat should still be pink and any juices that run out should be clear. Finally, the meat should be steaming hot all the way through.

This is the last post until January, so merry Christmas to all Microbioloblog’s readers and all good wishes for 2011.

It’s derived from the Italian word for “influence”. This is because mediaeval physicians believed that this was a disease which was caused by inauspicious astrological circumstances: influenza della stella “under the influence of the stars”. Later medics ditched the theory that diseases were linked to astrology and began to have more sensible ideas, for example, that cold weather was associated with the disease: influenza del freddo.

An astrological chart - not the best way of predicting a flu epidemic (Credit commons.wikimedia.org)

Influenza has also had several other names including the grippe. This is not to be confused with The Devil’s Grippe which sounds like a very nasty disease indeed. This is an infection caused by Coxsackie B virus. Patients with this illness – also known as Bornholm Disease – can have very severe pain in the rib cage, which can be so bad that they are unable to take deep breaths. Although this is alarming for the patient, the good news is that they will make a complete recovery.



This was the response I got when I offered to buy a colleague a drink at a Christmas outing recently.

Many people think that if you are taking any antibiotic you should avoid your favourite tipple.

Nice...but not with metronidazole (Credit commons.wikimedia.org)

In fact, this advice really only applies to the antibiotic, metronidazole. This is used to treat a number different of infections including dental infections in patients who are allergic to penicillin. If you knock back some alcohol while taking metronidazole you may experience palpitations;  facial flushing; (which may lead your drinking companions to think you’ve gone a bit too far); nausea and vomiting (ditto) or a splitting headache (which may lead you to think that you’ve gone too far).

This is an infection which is fairly common and elderly patients are most at risk. Antibiotics wipe out the “good” bacteria which normally live in the large bowel, but don’t kill off Clostridium difficile. The bacterium can then grow in the bowel and produce toxins which damage the lining of the intestine. It is this damage which results in the diarrhoea. The infection is easily treated with antibiotics (such as metronidazole) which kill the Clostridium difficile. If treatment is not started promptly the damage to the bowel can become more severe and this can lead to nasty inflammation of the bowel known as colitis. In the most extreme cases, bowel perforation can occur.

Diarrhoea in an elderly patient being treated with antibiotics could be caused by Clostridium difficile. Courtesy Public Health Image Library

So, why were the nurses so confident about their diagnosis? The smell of the patient’s motions! Several studies have shown that nurses are quite accurate in diagnosing Clostridium difficile by using their noses. Scientists are now working on technology which will detect certain substances in the odour given off by diarrhoeal stools with much greater accuracy.

Were the nurses correct this time? Yes, they were! The stool specimen we received in the lab tested positive for Clostridium difficile. The patient was started on metronidazole and is now recovering well.

I was called recently about a young patient who had been admitted to the hospital with a chest infection. The patient had been treated with an antibiotic called amoxicillin (a type of penicillin). After a few days of treatment the patient, didn’t seem to be improving and had also developed a rash. The doctor on the ward suspected that this was a penicillin allergy and asked me for an alternative. This was a reasonable explanation for this rash, but there was also another explanation. Might it be the infection and not the antibiotic?  A chest infection in a young patient with a rash, especially occurring during winter time, could be caused by a bacterium called Mycoplasma pneumoniae.

This could also explain why the patient was not getting better with amoxicillin treatment as Mycoplasma isn’t affected by penicillins, like amoxicillin. (This is because penicillins attack bacteria by preventing them from making new cell wall. Mycoplasma, is a very unusual bacterium – it doesn’t have a cell wall and so it isn’t affected by penicillins).

A blood test confirmed our suspicion – the patient did have a Mycoplasma infection. We changed the amoxicillin for another antibiotic (clarithromycin) and the patient is now on the mend.

Pneumococcus growing in the lab - the usual way of diagnosing pneumonia caused by this bacterium (Courtesy Public Health Image Library)

Well, we don’t culture the urine: we look for fragments of the bacteria causing the pneumonia which have been excreted by the kidneys into the urine. These fragments, or antigens, can be detected in the lab using technology which is similar to that used in home pregnancy testing kits. The test can detect antigens from pneumococcus (which is the commonest cause of pneumonia contracted outside hospital) and also Legionella pneumophila, which causes Legionnaires’ Disease. Using this test we were able to confirm a diagnosis of pneumococcal pneumonia which allowed us to “fine tune” the patient’s antibiotic therapy and hopefully speed his recovery.

Why? Well, because the hole was probably made by a bird pecking through the foil to enjoy some early morning refreshment. If the bird was a magpie or a jackdaw it may have had a bacterium called Campylobacter jejuni on its beak and as it pecks through the foil, the milk in the bottle can become contaminated with this bug – which is the commonest cause of bacterial food poisoning in the UK. If you can’t bear the thought of tipping the milk down the drain and use it, then you may be setting yourself up for a nasty stomach upset.

How does the Campylobacter get onto the birds’ beaks? It has to do with their (not very pleasant) feeding habits which see them foraging in all kinds of unsavoury places where this bacterium is found – including sewage plants. Yeuch.

He might spoil your breakfast (Credit Michael Nash)

So enjoy your “pinta” delivered by your local milkman or milklady and produced on your local dairy farm, but be careful with bottles with pecked tops!

Hand foot and mouth disease is an infection which is only seen in humans. Infection is commonest in toddlers and young children and outbreaks can occur in nurseries and primary schools. One of the signs of the disease is the appearance of small blisters – vesicles – on the hands, feet (usually the soles and palms) and around and inside the mouth. Fortunately, this is a mild infection which soon clears up.

Coxsackie virus – the cause of hand, foot and mouth disease

(It’s called Coxsackie after a town in New York State)

(Credit: Public Health Image Library)

Today I took a call from a GP about a pregnant woman who’d had contact with a girl with hand, foot and mouth disease. The patient was concerned about the possible effects on the baby if she were to catch the disease from the girl.

I’m often asked about cases like this and was happy to reassure the doctor and her patient that there were would be no risk to the baby even if Mum were to come down with the disease. I think I get called in situations like this because there is confusion between hand, foot and mouth and another infection which is associated with a rash – slapped cheek syndrome – as this can, under certain circumstances, have implications for the pregnancy.