Eliminating this food-poisoning bacterium from poultry is tricky—not least because rapid, precise tests are still unavailable. Researchers are looking at vaccines, probiotics, prebiotics and even essential oils as ways to reduce contamination on the farm.
Every year, food tainted with Salmonella or Campylobacter bacteria causes nearly 3 million illnesses in the US, according to the Centers for Disease Control and Prevention. Among those sickened by Salmonella, 26,500 will be hospitalized and 420 will die, accruing an estimated $365 million in direct medical costs. Though Campylobacter is less likely to lead to hospitalization or death, it’s still no fun, causing diarrhea, vomiting, nausea and, in some cases, long-term health problems. Preschoolers and the elderly are most at risk.
These pathogens can lurk in many different kinds of foods, but chicken and eggs are major sources. Researchers regularly find Salmonella or Campylobacter on chicken sold at grocery stores, with anywhere from 8 percent to 24 percent of packages testing positive. The law doesn’t ban the sale of raw chicken that’s contaminated like this — instead, it requires manufacturers to test a certain percentage of chicken coming off the production line, and as long as positive tests remain beneath a threshold, production can continue unchanged.
Part of the thinking is that raw chicken — unlike, say, lettuce — gets cooked, killing the microbes. But advocates for change find holes in that reasoning: If it’s so simple, they wonder, why do so many people get sick?
To learn more about the persistence of Salmonella or Campylobacter on poultry, Knowable Magazine spoke with Steven C. Ricke, a microbiologist at the University of Wisconsin–Madison and the author of a 2021 article about poultry safety in the Annual Review of Animal Biosciences. This conversation has been edited for length and clarity.
What are some of the challenges to eliminating these pathogens on the farm or in the processing facilities?
Campylobacter has an optimal growth temperature of about 42 degrees Celsius, which happens to be the body temperature of chickens. It’s pretty well adapted to poultry. And wild birds are carriers for Salmonella. So are house cats and cockroaches. It can get on the poultry feed when it’s stored. Mice and rats, they love grain. Salmonella can be airborne.
It’s a great survivor — so you can imagine the challenges.
And you can’t look at chickens going through a processing line and say, “There’s Salmonella there.” But sampling itself is a challenge. Ideally, for every product that goes out the door you’d like to be able to immediately run it through some kind of test. Is there any Salmonella there? Campylobacter? If yes, then you take other measures to decontaminate it. Right now, we don’t have tests that are fast enough or precise enough. There’s a lot of research being done, but we’re not there yet.
If we had faster, better and more precise diagnostic tools, other stuff would get easier. For example, in testing control measures, you don’t know how effective the measure was until you have really good diagnostic tools and can ask: How much did we lower the numbers when we applied this treatment?
We also need tools for rapid identification of Salmonella’s different serovars — the traditional typing system for this pathogen that is based on immunological assays. Identifying the serovar is important because not all Salmonella are equal. Salmonella serovars Typhimurium and Enteriditis — these are pathogens of major concern and have been the source of a lot of outbreaks. Other serovars, you would not assign much risk to them. Ideally, you’d have a sensor that would instantly tell you which Salmonella serovar was there and in what kind of amounts.
In your paper, you mention that to fight these pathogens the poultry industry uses probiotics (bacteria thought to be beneficial) and prebiotics (nutrients that promote growth of probiotics). You also mention essential oils. What’s the idea behind these things?
There’s been a growing concern about antibiotic resistance, so the poultry industry has been working to curtail routine antibiotic supplementation. The thing is, antibiotics had a benefit, so now there’s a lot of interest in trying to recoup some of that benefit with these other compounds. Essential oils, probiotics and prebiotics can be added to the feed or water, but they each have a different aim.
Essential oils have antimicrobial properties. They have the potential to kill foodborne pathogens — depending, of course, on the particular compound, the pathogen and the concentration.
Probiotics and prebiotics work, in different ways, to prevent pathogens from establishing in the gut in the first place.
Some people think of these products as more like snake oil.
There’s actually a lot of science behind it. We have worked with essential oils from the citrus industry and they can be very inhibitory towards pathogen growth and survival. In one study, orange oil reduced Salmonella by a detectable number, by a log or two, which in the industry would be important. I wouldn’t say we’ve achieved complete kills of the pathogens, but antibiotics haven’t done that either. Of course, some essential oils are inhibitory and some are not. In a best-case scenario, we want to reduce the pathogens below detection limits.
Probiotics used in the poultry industry include Bacillus, Bifidobacteria and Lactobacillus. It’s pretty much what’s in yogurt, but maybe different strains. The aim here is to help establish a healthy gut microbial population.
Prebiotics are non-digestible carbohydrates such as fructo-oligosaccharides, galacto-oligosaccharides and mannan-oligosaccharides. They’re essentially food for the good bacteria. There’s no evidence that I’m aware of that pathogens can use the prebiotics — that’s the beauty. It’s almost like you’re starving out the pathogens and you’re feeding the good bacteria at the same time.