When it comes to growing apples, no state dominates like Washington. It accounts for roughly 6 in 10 of all the apples grown in the U.S. One of the major threats facing this top crop is fire blight. The bacterial disease attacks apple and pear trees and can ruin an entire harvest, costing roughly $100 million annually in losses for the U.S. apple industry. The fire blight bacteria can also develop resistance to the antibiotic orchardists have typically used to protect their fruit trees.
But that resistance may have met its match in a compound that Washington State University microbiologist and associate professor Cynthia Haseltine calls “the universal assassin” for its ability to kill not only fire blight bacteria but also other harmful pathogens. Haseltine has spent nearly a decade developing this compound that is derived from a microbe found in extreme environments like volcanic vents and hot springs.
The compound is now being field tested for the first time in central Washington. It’s also shown promise in the lab at killing listeria bacteria which can grow on equipment in fruit packing plants.
Haseltine joins us to share details of the grant she was recently awarded to expand production and real-world testing of this novel approach to protecting Washington apples.
Note: The following transcript was transcribed digitally and validated for accuracy, readability and formatting by an OPB volunteer.
Dave Miller: From the Gert Boyle Studio at OPB, this is Think Out Loud. I’m Dave Miller. Could a microbe found in acidic hot springs help apple growers keep their crops disease-free? Could it prevent deadly outbreaks of Listeria in apple-processing facilities?
That is what my next guest is working on. Cynthia Haseltine is a microbiologist and an associate professor at Washington State University. She has spent decades studying so-called extremophiles, tiny organisms that have evolved to be able to live in various kinds of extreme environments. And she’s hoping that a substance derived from one of these microbes could be used by apple growers to prevent fire blight and Listeria.
Cynthia Haseltine, welcome to Think Out Loud.
Cynthia Haseltine: It’s great to be here.
Miller: I gave a five-word definition of this. What is an extremophile?
Haseltine: Extremophiles are any organism that can live outside what you might consider to be normal conditions. So, we live in a particular condition. We like our temperatures where we like them, but if it gets too hot for us, we can’t survive. If it gets too acid for us, we can’t survive. If we don’t have enough oxygen, we can’t survive. So there are other organisms that have evolved to survive in conditions that we, as humans, and most of the plants and animals that we think of cannot live.
Miller: You’re focusing on a particular kind of organism that, for a while I think, scientists thought were bacteria. But then not that long ago, they realized, wait, no, these things are different. So what are you focused on?
Haseltine: So there are three defined branches of life. There are the eukaryotes, which is us, and the whales that you were just talking about in the previous segment, and the trees, and things that we think of as animals. Those are the complex organisms on the planet. Then there are the bacterial organisms, which we can think of as small things that we might want to avoid. They might make us sick, things like E. coli.
And then there’s a third branch of life. And we, for a very long time, thought that that third branch were just really, really weird bacteria that lived in strange places. When we started to get genome sequence information back on those microbes, we discovered there was something completely different. They look like bacteria. They’re small. You have to have a microscope to see them, but they do a lot of things that are very different from bacteria. They tend to live in very strange places – boiling hot springs, deep sea thermal vents under the ocean, in salt crystals or in the Great Salt Lake in Utah, places like that.
Miller: They’re living inside salt crystals?
Haseltine: Yes, they can inhabit very salty environments. And historically, actually, these microbes that are called halophiles, the ones that like high concentrations of salt, they’re found in museum collections of tanned leather that have salt on them. You can actually scrape the salt off of the leather and isolate these halophiles from these salt crystals directly.
Miller: How did you come up with the idea of using something from an extremophile to fight an apple blight and a foodborne illness?
Haseltine: Well, ever since these extreme microbes were discovered, there’s always been a thought of how we could use them in biotechnology. How could we use them to make human lives better? So, archaea, this third branch of life, is famous in biotech for making very stable molecules that can survive extreme or extended conditions: low pHs, high pHs, low temperatures, high temperatures, salt or detergents, things like that. So everybody who works on archaea always has that going on in the back of their mind. How could this microbe help make human life better? How could we use it biotechnologically?
We were looking, I would say, about 10 years ago, as a side project in the lab … We were thinking, is there anything this microbe could do to help with the anti-microbial resistance epidemic that’s occurring across the planet? So microbes are becoming extensively resistant to our standard antibiotics. It’s making it harder to treat people that get infections in hospitals. So the thought was, is there any chance these microbes make something that could be used for that? Is there something we could extract from them that could help in this battle against antibacterial resistance? And we happened to find something that has turned out to be a very effective antimicrobial produced by our extremophile.
Miller: And one of the bacteria that you would like to use it against causes something known as fire blight. What does fire blight mean for commercial apple growers?
Haseltine: So fire blight is caused by a very specific microbe called Erwinia. It’s a bacterium, and fire blight disease is a really destructive disease for Washington apple growers. I’ll point out that Washington grows about 6 in 10 of all United States apples. Erwinia as a microbe also affects pear trees. And about 50% of all the U.S. pears are grown in the state of Washington as well, so it’s both apples and pears that this affects.
An infection in a tree by Erwinia causes the visual disease of fire blight. It can kill blossoms, shoots, branches of the tree. It can cause cankers of the tree, which are localized areas of dead tissue on the bark stems, the branches. And it can ruin fruit on affected parts of the tree. In really bad years, fire blight can even kill young trees or entire sections of orchards.
Right now, what growers are doing to combat fire blight, they rely on antibiotics and spraying copper. But again, we’re thinking about antimicrobial resistance, antibiotic resistance, and the Erwinia microbe is becoming increasingly more resistant to the treatments that the growers can use to battle it.
Miller: So if this work continues and the results remain promising, how would it be used? What’s the dream for what an orchardist, an apple grower, I don’t know, in Yakima, say, would do to their crop?
Haseltine: We’ve already moved into the field tests for this. It’s already being applied to trees at Washington State University Extension in Wenatchee. And the way it’s being applied is through a spray of the tree, it’s preventative. So as the trees blossom, the microbe moves in through the blossoms of the tree. This would be a preventative, helping the tree to battle an infection from occurring at the beginning. So it’s a spray.
Miller: The other bacteria that you’re hoping to use this compound against is Listeria, which can cause a potentially lethal foodborne illness. How big an issue is Listeria in the commercial apple world? I feel like I remember every now and we’ll hear about sliced turkey breast or something, cold cuts. I feel like I hear less about apples.
Haseltine: Listeria is a very serious post-harvest food safety risk because the microbe Listeria can grow on equipment that is kept cold in fruit packing facilities. So, if the grower has successfully prevented Erwinia from causing fire blight in their orchard, they harvest the apples, and send them to the packing facility. And it’s at that point that something else can sneak into the process. Listeria can be living within that packing line and can contaminate the apples.
A single case of Listeria infection can cost the industry more than $2.5 million. And the annual U.S. cost for Listeria contamination, according to recent USDA data, is around $4 billion dollars. It’s a very expensive problem in the United States, and not only is it expensive, people get sick. So, it has been leaked linked to deadly outbreaks. It can kill people, it can make people very sick if they contract Listeriosis from Listeria. So there’s a great interest in trying to eliminate Listeria from getting onto the produce as it’s being processed through the packing line. Because we have to wash our apples, we harvest our apples, we transport them, we wash our apples, we wax our apples, we sort our apples, we pack our apples and we get them out to the consumer. So we don’t want any Listeria contamination entering that process that could make people sick.
Miller: I want to go back to what you were saying earlier about this as a way to get around antibiotic resistance, which I think of as a kind of genetic mutation arms race. Some random mutation confers protection. Then that variant survives whatever drug we’re using at the moment and spreads. And then that variant has its own defenses against our best offense. Why wouldn’t that just happen in response to your new extremophile-derived compound?
Haseltine: Antibiotic resistance is often achieved through a single mutation by a microbe. It’s actually very easy for microbes to become antibiotic resistant, and you’ve hit right on that problem. Our new molecule, our newly identified compound is not an antibiotic. The nature of the compound suggests that it’s going to take multiple rare mutations for any microbe to become resistant to this. So while it’s not impossible that resistance will evolve over time for these microbes we’re interested in combating, it is a much longer time frame that our compound will be functional in the field.
Miller: And would it also kill the kind of bacteria that we want to have, say, in our guts, or would it only go after the stuff that we don’t like?
Haseltine: It’s not targeted to particular pathogens.
Miller: So it kills all bacteria?
Haseltine: Thus far, we have not found anything that it doesn’t kill in the bacterial world that we’ve tested.
Miller: Is that a problem?
Haseltine: It’s all about how it’s used. If we’re thinking about Listeria and a fruit packing line, when you think about how you’re going to clean stainless steel machinery, there could be detergents, there could be bleaches, there could be water flushing. This is the sort of thing that would go into that process. It would be a treatment of the packing line surface and then it would be rinsed away like any other sort of surface treatment like you might use in your house, like one of those antimicrobial sprays you might use to clean your home.
When it comes to using it in the field, we’re at the very beginning of these tests to figure out what the best use of this material is in the field for safety, for efficacy and for the biggest benefit to the growers that we can get. It also does affect a large number of other microbes of human health interest though.
Miller: Cynthia Haseltine, thanks so much.
Haseltine: Thank you for having me.
Miller: Cynthia Haseltine is a microbiologist at Washington State University.
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