The Results: What's in a sausage?

10 October 2017

Interview with

Ed Farnell, University of Cambridge

Remember the sausage we were genetically sequencing earlier? Let’s head back to the Cambridge Pathology lab where, joined by the butcher who made the sausage, Stevie Bain and Ed Farnell reveal what they’ve found...

Ed - What we’re looking at is a pie chart which shows the amount of reads from each animal as a percentage. Each individual read is a bit of the conserved region that the sequence sequenced.

Stevie - The percentage of reads we say okay, we know that this specific sequence looks like this in the cull. How many kinds is that present?

Ed - Exactly. I guess the good new is your sausage is 98.61% - according to this - pig, which is pretty good mostly. It’s pretty much all pig and then we’ve got a few trace amounts of cow or beef. A little bit of sheep, a little bit of chicken, and a trace amount of human as well.

Stevie - Hold on a second! You’ve found human DNA?

Ed - Yes.

Stevie - In some sausages.

Ed - We mix them up by hand so it could be just a tiny amount of a trace that wa, and also handling them. The way that we’ve prepared it in the lab as well, there's DNa in dust, there’s DNA on the bench surfaces.

Stevie - Going back to some of the other things that we found in there. A little bit of cow in there. We have 0.26% sheep, so that’s the percentage of read. How many times, as a percentage does the sheep genetic code show up in the sausage?

Ed - In our sequencing data, exactly.

Stevie - But that doesn’t necessarily mean that 0.26% of the sausage is sheep?

Ed -  Exactly. And there’s a couple of reasons why that might be. One is that we do amplify the region and we might get some biases where some sequences are more amplified than others just because of the type of sequence they are. The other issue we might have as well is that the mitochondria, which we’re actually sequencing, they’re in the muscles of these animals which is what goes into our sausages. And one of the interesting things is across all the different species, the pig, the cow, and the sheep, there are actually lots of different amounts of mitochondria. Say, for example, in a pig, in a single pig cell you might have a hundred mitochondria. Whereas, in a single cow cell you might have a thousand mitochondria or visa versa. So that can also bias how accurate our classification is.

Stevie - And that’s why this type of test it tells us what DNA is present in there but then the Food Standards Agency would use a more accurate test to quantify exactly how much of each animal is in the meat?

Ed - Exactly, yeah.

Stevie - So certainly high quality sausages then and no need to be concerned about those traces of human?

Ed - No, definitely not.

Stevie - Because I guess that’s the amazing thing about DNA, we’re leaving a tracing of ourselves touching the sample or chopping up the sausage it’s just going to show up?

Ed - The knife used to cut it - anything. Because we have sequencing data and we can actually see the variation in the different bits of sequence we get. We did find something quite interesting actually and what I wanted to ask you is were these sausages all made from one pig?

Butcher - No, they’re made from probably I would imagine two or three.

Ed - So that’s really cool because when we looked at the sequence data what we could see was probably three or four different pigs in there, just from the genetic variations. That’s really cool, so it matches up with what you guys do when you make the sausages.

Stevie - How do you feel about the results?

Butcher - I’m relieved, to be honest. Yes, my boss will be pleased, put it that way.

Stevie - Well, now that that’s sorted, the only thing left to do is cook them up and tuck in.

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