Keep the Christmas dinner safe from merry microbes

In households around the country the countdown is on to the big dinner. But what about those uninvited guests? The bacterial ones that might be lurking in the turkey, duck or goose?

"Most people would be familiar with salmonella, but campylobacter is the bacteria that causes the most food poisoning in Ireland," says Dr Gary Kearney, director of Food Science with Safefood.

Campylobacter bacteria can live happily in the guts of poultry, where they seem to cause the bird no harm, but if humans eat the live bug it can cause illness. That's why it is important to store, prepare and cook the bird this Christmas in ways that reduce the chances of live Campylobacter making it into our fare.

“It’s all about planning,” says Kearney. He advises clearing out the fridge in mid-December to make room for the incoming raw poultry. “Get rid of the old sauces that have been open for months and move the trays around [so you] have enough room at the bottom of the fridge so it can thaw out.”

Thawing the bird or ham at the top of the fridge runs the risk of juices running on to items below, he says. “Then the hitch-hiker really is hitch-hiking and contaminating the other foods.”

Don’t wash the turkey

Many people might wash the defrosted and raw turkey, but Kearney says this is not a great idea, as water droplets can splash and carry campylobacter, contaminating other surfaces. “Research has shown that the droplets can travel up to 2ft, but generally about 1.5ft, and that can go on the draining board and [items] that are already washed,” he says.

The live bacteria can stay there for a while too, he adds. "It develops a biofilm [that] keeps itself safe from the atmosphere and everything else. It could stay for an hour."

He is also keen on colour-coded chopping boards to further minimise cross- contamination. Otherwise the campylobacter might end up on a board that had the poultry on it, and then move to another foodstuff being prepared on the board later. “Buy a pack of different chopping boards in different colours, and use the red one only for raw meat or poultry and don’t ever use any other ready-to-eat foods or raw salads on that,” he says.

No pink meat

Thorough cooking kills the bacteria, so the usual rules apply for the bird: no pink meat and be sure that the juices run clear before eating. After the festive meal, Kearney says that the leftover cooked meat should not be left languishing for too long.

“There is a temptation on Christmas day that, after dinner, everyone is on the couch half-dozing and the meat is still sitting out in the kitchen,” he says. “So when you finish the meal, someone should go out straight away and cut the meat off the bone [and put it in the fridge or freezer]. Don’t leave it out for longer than two hours because bacteria will grow.”

Reheating the leftovers offers ambient bacteria another chance to grow on the food, so those tasty morsels need to be fully reheated before eating. They should not be given a third spin of the wheel.

The bacteria on poultry or in the kitchen aren’t the only micro-organisms that can have a party over the festive period. Our gut microbiota, the trillions of bacteria that live naturally in our intestines, are affected by the types and amounts of foods that we eat, and that tends to change over the party season.

"Microbes can very quickly change their gene expression in response to the environment, and this applies too for our gut bacteria," says Dr Susan Joyce, a lecturer in the school of biochemistry and cell biology at University College Cork and a principal investigator at the APC Microbiome Institute.

“If your diet changes over Christmas, with lots more sugary and fatty foods than normal, this in turn changes the diet or substrates that your gut bacteria can use. They can respond within short periods of time, maybe a day or two, so that more growth of some types of bacteria can occur.”

Changes in microbiota

The impact of a sudden swell of massive dinners, chocolates, mince pies, Christmas pudding and alcoholic drinks can go beyond bacteria.

“If you eat a very fat-rich diet, the resulting changes in the microbiota can change the chemicals present in your gut to alter its health. Some of the effects could increase the gut permeability, and alcohol can increase gut leakiness too,” says Joyce.

“Your gut health can be affected, and this in turn can increase the burden of endotoxins circulating in your body – and in the longer term that could affect how some cells in your immune system work.”

It all sounds a bit apocalyptic, but the good news is that the short-term intestinal changes can quickly reverse when you go back to eating normal fare, says Joyce.

Meanwhile her advice to help mind your gut bugs over the festive season is to eat a variety of foods, include plenty of fibre, fruit and veg, drink plenty of water and get out for some exercise.

“It’s common sense,” she says.

FAIR AND FOWL: THE CAMPYLOBACTER CONUNDRUM

Does campylobacter cause illness? That can depend on where it is hanging out. The bacterium is well known to live in the intestines of poultry, where the bug seems to cause the fowl no foul. But if the live bacterium makes its way into the human intestine, it can trigger illness.

It's a conundrum that intrigues Dr Tadhg Ó Cróinín at University College Dublin, who is looking at virulence genes in campylobacter jejuni.

“What got me interested in this bacterium is that even though it can make us sick, it lives quite normally in chickens in large numbers and it causes them no obvious problems, so presumably it is a natural part of the chicken microbiome,” he says.

But in humans, campylobacter behaves differently, invading the epithelial cells that line the gut wall. “We can link this to changes in DNA topology, or coiling, and to the bacteria expressing different genes,” says Ó Cróinín, who works at the National Children’s Research Centre in Crumlin. “And we have identified some genes that seem to be involved in this process.”

There are also signals from the host, it seems. "When we put campylobacter in mucus from chicken, which it would encounter in the chicken intestine, it just swims around happily, but if it encounters human mucus it starts to change its behaviour," says Ó Cróinín of the work, which was funded by Science Foundation Ireland and the National Children's Research Centre. "There's something about human mucus that prompts the bacteria to act differently and become more virulent or disease-causing."

Understanding the complex interplay between the host environment and the bacterial virulence could point to new strategies to reduce campylobacter’s negative fallout, he says.

“The more you understand that the more ways you can come up with ideas, and if we can change its behaviour, we can affect its ability to cause disease.”