Some like it cold

Many of the bacteria which cause infections in humans grow well at 37 degrees C. In fact when we are trying to grow bacteria from a patient’s specimen we incubate the Petri dishes at this temperature. Some microbes, however, prefer a slightly lower temperature. For example we use an incubator set to 30 degrees C to grow many fungi including those which cause common skin infections such as athlete’s foot.

Some pathogenic (disease-causing) bacteria can grow at much lower temperatures. Listeria monocytogenes is one of these. This bug causes meningitis and blood stream infections in patients whose immune systems are functioning well. It will grow quite happily at temperatures as low as 4 degrees C. This is the temperature of a typical domestic refrigerator: a fact that has added signficance given that the usual way in which listeriosis (the infection caused by Listeria monocytogenes) is transmitted which is by eating food contaminated with the bacterium.

Listeria monocytogenes

(credit NHS Wales)

 
Listeria spends most of its time outside of the human body in the environment. In the environment it doesn’t produce any of the disease-causing (virulence) factors which result in the symptoms of listeriosis. After all, why should it? It’d be a waste of time and energy producing virulence factors which it couldn’t use. So the bacterium makes the virulence factors only when it needs to – when it’s inside a human. And how does it know when it’s inside a human? The answer is temperature. In the environment temperatures are low, but if the bacterium senses the temperature has risen to 37 degrees C then the chances that the temperature is at this level are because it is inside a human (or animal) and that it’s now time to turn on the virulence factors. Neat.

The patient with appendicitis…who didn’t have appendicitis

Every medical student learns the signs of appendicitis. One of these is pain in the abdomen. Early on in the condition this tends to be periumbilical – around the tummy button. Later this moves to the lower abdomen on the right hand side. Examination of  abdomen is usually very uncomfortable for the patient. Typically abdominal tenderness is greatest at “McBurney’s point”

McBurney’s point (Number 1 on the line)

(credit: wrongdiagnosis.com)

Occasionally, however, a patient with all the typical signs of appendicitis (including tenderness over McBurney’s point) is operated on to remove an inflammed appendix but is found to have a normal appendix. So what is responsible for the patient’s “appendicitis”?

The anwer lies with a bacterium, Yersinia entercolitica. This microbe, which is a distant cousin, of the bacterium which causes bubonic plague, is usually acquired through contaminated food or water. It can cause a bowel infection in which there is inflammation of the lymph glands of the digestive tract and it is this inflammation which can produce symptoms very like those of appendicitis.

In cases of Yersinia entercolitica infection when surgeons find these swollen lymph glands and a normal appendix, they still remove the healthy appendix. The patient may not have appedicitis at the time of the operation but in taking out the appendix they make sure that the patient will never have appendicitis later in life.

Cellulitis is infection of cellulite?

The quick answer to this question is… no. Disease with names which end in –itis are inflammatory conditions, which are often caused by infection. Arthritis is inflammation of a joint; bronchitis is inflammation of the bronchi (the tubes which carry air into the lungs) and meningitis refers to inflammation of the meninges – membranes which cover the brain and spinal cord. So you’d think that a condition called cellulitis would describe infection of cellulite. In actual fact cellulitis is an infection of the skin and the layers of tissue underneath it. It can affect the skin of any part of the body, but it is usually seen on the feet and legs as well as the arms and less commonly the face.

 

Cellulitis of the foot. The affected skin is marked with a pen to see if the infection is continuing to spread or is responding to antibiotics

Credit: emedicine @ medscape

So cellulitis hasn’t anything to do with cellulite. Some cases follow a scratch, a cut or an insect or animal bite, but often there appears to be no reason for the infection to have started. People prone to swelling of the legs – oedema- are at greater risk of cellulitis.

 

Today we were called about a patient with cellulitis of the leg. It was not the first time the patient had developed cellulitis. In fact it was the third time this year. I was asked if there could be any reason for this, especially as the patient said that there had been no trauma such as bumping the leg against furniture or cuts before any of these episodes. My answer? Check for athlete’s foot! The splits in the skin between the toes that you get with athlete’s foot provide an easy route of entry into the skin for the bacteria which cause cellulitis. If you don’t examine the feet of a patient with cellulitis of the leg fully, it’s an easy thing to miss. So we’ll give our patient antibiotics to treat the cellulitis and some antifungal medication to clear up the athlete’s foot and hopefully the patient won’t be back for a fourth time.

Take some advice…from a hedgehog

Today is European Antibiotic Awareness Day. One of the reasons why we have to contend with multi-drug resistant “superbugs” is because we overuse antibiotics. Health professionals are often too quick to prescribe antibiotics and patients perhaps are too quick to ask for them.

So take a tip from a hedgehog…

…by clicking on the link below.

http://www.channelplayer.tv/ecdctv2009/default.asp?p=1&lang=en-GB&assetid=4183&cid=108

When talking about over-use of antibiotics we can’t ignore the fact that human consumption only accounts for about half of all the antibiotics used. The other half is used in agriculture. In some parts of the world antibiotics are added to the food of animals or poultry. This makes the animals reach market weight much quicker and increases production. This is goof news if you are a consumer in that your food will be cheaper because of the antibiotics used in this way. The bad news if you are a patient suffering from an infection is that it might be more difficult to treat because the bacterium which is causing might be resistant to the antibiotics you have been prescribed.

 

 

 

What made Milwaukee famous…

…is the title of a song recorded by Rod Stewart. The song refers to beer. At one time Milwaukee had four breweries including Schlitz and Milller. For microbiologists Milwaukee is famous, or rather infamous, for another reason. In 1993 the city witnessed the largest ever outbreak of an infection called cryptosporidiosis which affected an estimated 400,000 people.

Rod Stewart – singing about beer and not cows’ milk

Cryptosporidiosis is caused by a parasite – Cryptosporidium parvum. Most infections result from drinking water contaminated with the parasite and the Milwaukee outbreak was thought to have occurred because of a failure of a drinking water treatment plant which served the city.

Most people who become infected experience an unpleasant diarrhoeal illness which usually gets better without any specific treatment. Patients whose immune systems aren’t functioning correctly, such as those with HIV/AIDS, however, can have a very severe form of the disease. Infections in immune-deficient people can be very difficult to treat and at one time bovine colostrum (the first milk produced by a cow after the birth of her calf) was one of the few treatment options. Fortunately a new drug – nitazoxanide – is making things easier, although the drug is not yet widely available in the UK.

Attack – the best form of defence

Humans have evolved a series of highly sophisticated defence mechanisms to protect us against infection. A key weapon in our armoury is the white blood cell. These cells are produced in the bone marrow and released into the blood stream. They are continually on patrol. If they are alerted to the presence of an invading bacterium they are quickly on the scene hunting down the intruder – a process known as chemotaxis. Once they have located it, they then engulf the bacterium: this is called phagocytosis. The bacterium inside the white blood cell is rapidly destroyed by a number of highly toxic substances.

White blood cell attacking bacteria (Credit: Denis Kunkel)

Some bacteria, however, can counter-attack before they are phagocytosed. Some strains of Staphylococcus aureus produce a toxin called PVL which destroys white blood cells before they can engulf them, thus neutralising an important line of defence. Recently we have seen an increasing  number of infectins caused by PVL strains. They can cause nasty skin infections as well as a particularly serious form of pneumonia which often occurs in previously fit and well young patients. These infections can be difficult to treat so, if possible, we try to use antibiotics which “switch off” production of the PVL toxin by the bacteria to allow us to get on top of the infection more quickly.

Garlic – good for gladiators and good for us?

Apparently Roman gladiators were rubbed down with onion to firm up their muscles. However there might have been another reason for this unusual body rub. Garlic contains a substance called allicin which has antiseptic properties. So perhaps the garlic oil was also useful for preventing infection setting in to any wounds they received in combat.

 

Maximus (Russell Crowe) – garlic fan?

With the continuing emergence of antibiotic-resistant superbugs, we are increasingly looking towards natural products such as garlic oil to treat infections. So what was good for the gladiators all those years ago may be good for us some time soon.

Winter-time and the stomachs are queasy

We are entering the “winter vomiting disease” season. Winter vomiting disease is caused by norovirus

The infection comes on very quickly. One minute you feel fine, the next you are vomiting and have diarrhoea. The symptoms are unpleasant but don’t last for too long and within 12-48 hours most people feel better. For some patients, the elderly for example, the infection can be more serious.

The virus can be very easily spread. When a person with norovirus vomits, they can release many millions of infectious particles into the air. Some of these are particles are very small which means that they can remain suspended in the air for prolonged periods. Diarrhoeal bowel motions are also highly infectious.

Norovirus can spread very quickly in hospitals and the only effective control measure we have is to close an affected ward to new admissions until the outbreak has run its course. Hospitals are busier over the winter months than at other times of the year and closing wards because of outbreaks of winter vomiting disease can cause major problems.

Unless their visit is very urgent we ask people not to visit their friends or relatives in hospital if they have diarrhoea or vomiting and to delay their visit for at least 48 hours after their symptoms have settled as resolved as infectious virus is still present in the stools for a few days after diarrhoea has stopped.

Norovirus picture courtesy of the Public Health Image Library

What’s in a name – part 2

Sir Alexander Fleming named the first antibiotic, penicillin after Penicillium, the mould which produced the antibiotic. Other antibiotics were also named after the microbe from which the new antibiotic such as Streptomycin which came from Streptomyces.

Other antibiotics were named after places. For example, nystatin, an antifungal drug comes from New York State.

Probably the most unusually named antibiotic is rifampicin (rifampin in the US). The scientists who discovered this compound were such big fans of the cult French language film Le Rififi that they named the drug after it.

One of the highlights of the film is the thirty minute “heist” sequence which is completely silent.

I’m not having a flu jab because…

…it can give you a mild dose of the flu.

Flu can be a serious infection if you are an older person or have health problems such as heart or lung disease. Yet many people who would benefit from flu vaccination don’t have the jab because they have heard that having the vaccine can result in a dose of influenza.

This isn’t true! Unlike vaccines for measles, mumps and rubella, which contain “live” viruses, flu vaccines are “inactivated” vaccines. This means that they don’t have any active virus in them and so it is not possible to get the flu from the jab.