How does salmonella get into an egg?

29 April 2012

Question

Naked Scientists, Love your show & appreciate what you do.

I have a problem - I can't keep my fingers out of the raw cookie dough! I know food poisoning can come from raw eggs though. Does the bacteria come from the outside of the shell or can it really be passed on to offspring?

And how much of a risk is it really?

Thanks!
Jessica

Answer

Chris - I looked up the number of eggs that are thought to be contaminated with Salmonella in the UK where incidentally, we consume 24 million eggs every day. It's a very huge number. It's nearly 9 billion eggs per year just in the UK and the estimate is that about 0.3% of them are carrying Salmonella. The number of organisms that are in the egg in an infected egg is very low, 5 or 6, and proper storage of the eggs and making sure they're not kept too warm where the bacteria will multiply means that that number stays very low. And if the number stays low, then the chances of acquiring the infection is extremely low. You need a very high dose of Salmonella to make sure that someone will get infected. The number of cases of Salmonella we get every year are far and away too small to really show that this is having a major impact. We think there's probably under 10,000 cases of Salmonella infection in the UK every year.

How does the bug get into the egg in the first place though? Well, there's a number of routes. One of them is that when chickens catch Salmonella, unlike us, where we get symptoms, it doesn't cause a symptom for a chicken. It can pass from the chicken's ovaries where it can set up an infection straight into the egg and it gets inside the egg via that route. Another way is that chicken shells are relatively porous and so, for something as tiny as a bacterium, it is very easy for them to sneak from the outside, from the chicken faeces, into the egg. The third route is that some eggs, whilst they leave the chicken pristine, pick up the Salmonella from the environment whether that's the farmyard they're laid in or the factory they're processed in. And so, there's no guarantee that an egg is going to be safe or unsafe. The best bet is, if there might be a risk for you, just to use your common sense and cook it well.

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