Wednesday, November 22, 2006

Tuesday, November 21, 2006

So what is this Campylobacter thing?

Campylobacters are spiral-shaped bacteria (see picture above). They can infect both humans and animals, including birds. About 99% of the Campylobacter infections of humans are caused by just one species - Campylobacter jejuni. The micro-organisms appear to grow well in birds, which have a slightly higher body temperature than humans. Though chickens are often carriers, they show no symptoms of infection, so it's not possible to tell that the bird is infected without doing microbiological testing. C. jejuni does not grow below about 30C, so the presence of high numbers on chicken are unlikely to be the result of temperature abuse.

In humans the disease usually takes the form of abdominal pain and diarrhoea, which may contain blood, fever (typical of the body response to an infection), and vomitting may also occur. The symptoms usually last for about a week and usually no special medical intervention is required, but sometimes the patient requires hospitalization. There are some potential long-term consequences of infection - arthritis or Guillain-Barré Syndrome may follow infection. This is an auto-immune disease caused by the body's own defence system being triggered to attack the nerves. The incidence of Guillain-Barré Syndrome is estimated to be about 1 per 1000 cases.

Human infection is probably caused mainly by handling raw poultry, though the organisms may also be carried by farm animals and can be found in water supplies that have been contaminated by farm run-off. Though consumption of undercooked poultry has been suggested as a cause, in my opinion cross contamination in the kitchen (see "Should I eat the chicken?") is more likely.

The bacterium is easy to destroy by cooking. It cannot tolerate drying and can be killed by oxygen, so microbiologists have to take special precautions to handle and culture it - we grow Campylobacter in special containers where we can reduce the amount of oxygen present, replacing oxygen with carbon dioxide.

Friday, November 17, 2006

Should I eat the chicken?

A recent publication in New Zealand caused a lot of interest by claiming that sale of raw chicken should be banned, in favour of frozen chicken. The basis for the claim was that chicken meat is routinely contaminated with Campylobacter. The freezing process would kill the Campylobacter and render the meat safe.

Was this claim justified?

First we need to understand something about chicken farming, chicken processing and, not least, Campylobacter.

Let's start with Campylobacter. Campylobacter is a bacterium commonly found in animals and the environment. It can get into food via many different contamination vectors (carriers), including untreated water, birds, insects and animals. It is often found in the farm environment, which may be contaminated by animals.

The number of Campylobacter we need to eat to become infected is very low; various estimates put the "infecting dose" at between 6 and 800 cells. This means that even low levels of contamination in food can cause a disease if the food is consumed. Even contact with infected animals, for example in petting zoos, can put sufficient bacteria onto the fingers to give an infecting dose if they are put into the mouth (a problem with little children) or used to prepare food without first washing them carefully.

"But surely" you are asking "we cook chicken before we eat it". Well, yes we do. Assuming that we cook it properly, the Campylobacter cells will be killed and the chicken is safe to eat. But supposing that we handle the raw chicken and then prepare a salad; or we use the same chopping board to prepare the raw meat and then cut the salad on the same board, using the same knife? If we don't wash and sanitize the knife and board, we can "cross contaminate" the food from the chicken, via the knife and board.

The suggestion that raw chicken should be banned, and only frozen chicken sold now has some attraction. "Freezing kills Campylobacter, so that will make the chicken safe and we won't get cross contamination, right?" Well, it goes some way to reducing the contamination. But, as with most things to do with bacteria, it's not black and white. Again, estimates vary; freezing may destroy tens of millions of Campylobacter, but equally may have little effect. FAO/WHO data suggest a hundred-fold reduction if the product is frozen and held for a minimum of 3 weeks. A reduction of a hundred-fold sounds a lot, but in microbiological terms this might still leave millions of bacteria on the product. And as we have seen, this is plenty to give an infecting dose.

So, to go back to the title of this posting, should we eat the chicken? My personal opinion is a qualified "Yes". The caveat is that we must make sure to handle the raw chicken safely, preventing cross contamination of utensils, surfaces and other foods in the kitchen. Cook the meat properly and prevent recontamination.

The New Zealand Food Safety Authority (NZFSA) says that two of the most important ways of keeping food safe are to follow the 4Cs rule – Clean, Cook, Cover and Chill the food. Also follow the 20+20 hand wash rule before and after handling poultry and preparing food - wash your hands, using plenty of soap and warm water for at least 20 seconds, paying careful attention to finger nails and the webs between fingers. Rinse well and dry them for a further 20 seconds, using a clean dry hand towel or disposable paper towel. It has been shown that wet hands can transfer microbial contamination to utensils, refrigerator door handles, taps and other potential cross contamination points, so the drying is just as important as the washing.

Odd Spot: On 3 December 1988, Edwina Currie, the outspoken junior health minister in the UK Thatcher government was quoted as telling ITN, "Most of the egg production in this country sadly is now infected with salmonella." Sales of eggs plummeted 60 per cent overnight. The loss of revenue forced farmers to slaughter four million hens and 400 million unsaleable eggs had to be destroyed. The government had to provide a multi million pound rescue package for the poultry industry and the unfortunate Ms. Currie resigned on the 17th December. Undoubtedly, there was some substance to her claims. Vaccination of hens to protect against salmonella has now greatly reduced the levels of egg contamination to the present 0.3% reported by the latest FSA survey and since 1997 the incidence of salmonellosis in UK has fallen by a factor of about 4x. However, there is still a concern that eggs on sale in UK may contain Salmonella Enteritidis organisms, perhaps as a result of importation of eggs produced by unvaccinated hens.

Perhaps Edwina was right. Only time will tell if the advocates of frozen chicken are right too.

Thursday, November 16, 2006

Bacteria and food

By far the majority of food poisoning is caused by allowing bacteria to grow in foods to the point where they either make the food toxic (poisonous) by virtue of the by-products they secrete into the food, or because they grow to a population large enough to cause an infection in the consumer when the food is eaten.

In either case, the food has normally been "abused" in some way, perhaps leaving it in a car parked in the sun, or leaving food out on the kitchen table instead of putting it in the refrigerator.

Where do these bacteria come from? Well, bacteria are all around us - they help break down wastes, such as dead plant material or dead animals. If they didn't, life would soon become impossible on Earth, as all the organic matter would be locked up in these dead organisms. Bacteria are normal inhabitants of soil, the intestinal tract of man and animals and on the hides and hair.

Only a very small proportion of all the bacterial types on Earth are pathogens (organisms that can cause disease) and even fewer of those are transmitted in foods. In fact, the most important of these micro-organisms are members of just a few genera: Salmonella, Clostridium, Listeria, Escherichia, Campylobacter and Staphylococcus. There are others, but these will do for a start. Over time, this blog will contain lots of examples of these bacteria.

At first sight, the names of these groups are enough to put anyone off reading any further. But once you have seen them a few times, you'll recognize them (though only a microbiologist would think of calling them "Old friends"!)

I didn't really answer the question about where these things come from. Well, if they are in the environment, they'll get into food by many routes. They might get onto meat as the animal is slaughtered and butchered; careless food handlers might go to the toilet at break time and not wash their hands on their return - this could transfer bacteria from their own gut to the food when they handle it; flies and other insects may act as carriers or vectors to transfer bacteria from dirty places to food; vermin, such as rats and mice may also transfer bacteria around. Even birds may get into food premises and deposit droppings on surfaces, equipment or directly onto food. Sometimes the entry route is much less obvious, such as bird droppings on the roof getting washed by rain into a food premises through cracks in the roof.

All this comes down to keeping food clean, keeping it cold and not allowing it to be exposed to the environment for any longer than necessary. There is a lot to be said for cooking food and serving it straight from the pot.

Wednesday, November 15, 2006


I am a professional food microbiologist and I'm creating this blog to help ordinary folk and food manufacturers to understand food safety so that we can reduce the incidence of food mediated illness. I would like it to become a resource for teachers in schools.

I also want to dispell some of the myths about food safety. If readers send comments or questions, I'll try to incorporate them into future postings.

Obviously, some of the posts will be my own opinions, but I'll try to keep these comments based firmly in science.

I hope you find the site useful and perhaps even thought-provoking.

John D Brooks
November 2006