Catherine Donnelly is a research professor in microbiology, associate director of the Vermont Institute for Artisan Cheese (VIAC), and an expert on the subject of Listeria monocytogenes—a microorganism that has recently been in the news as the cause of several creamery closings. Culture recently caught up with Professor Donnelly to ask her about the nature of this pathogen as it relates to cheese.
culture: So what exactly is Listeria, and is it a serious concern for us cheese lovers?
CD: Listeria monocytogenes is a bacterium that is widely distributed in nature. It’s one that accounts for about 1,600 cases of listeriosis [a foodborne illness] each year, which is not a lot of cases compared to, say, salmonella, which causes about 2 million cases of foodborne illness every year. But of those cases of listeriosis, about 300 are fatal, compared to 400 for salmonella. So if you compare the percentage of deaths from Listeria, it is much higher. That makes it an incredibly serious organism by health standards.
Listeria poses a particular problem for cheesemakers because it can exist invisibly in creamery environments—in drains, on floors, or attached to stainless steel surfaces. And wherever it lives, Listeria often forms a biofilm—a capsulelike coating—to cover itself, making it very difficult to kill the organism with normal sanitizers. You’ve really got to disrupt the biofilm. The organism also can grow at refrigeration temperatures, whereas most other bacteria have adapted to our body temperature. So Listeria can grow and multiply at many different temperatures.
culture: What about heat? Can that kill Listeria?
CD: It is not heat resistant, so with proper cooking and pasteurization, you can kill Listeria. But even if someone is making cheese with pasteurized [heated] milk, that’s not necessarily a safeguard because the source of Listeria is most often in the cheesemaking environment, not in the milk. So cheeses made from either pasteurized or raw milk can be contaminated.
culture: That explains why the FDA has become more aggressive in its inspection of creameries. What are the safeguards in place, and are they working?
CD: The FDA has zero tolerance for Listeria. That means there can be no detectable Listeria present in any cheese samples. This kind of finished-product testing has a role, but it’s not the best way to ensure the absence of the pathogen because Listeria can contaminate cheese at very low levels that are often undetectable. So what you have to do instead is monitor the cheesemaking environment by sampling all the surfaces—the floors, walls, drains, the aging shelves or racks. Through this kind of surveillance you determine if it’s present. If you do have positive findings, you have to intervene right away with chemical sanitizers. Keeping the plant as dry as possible helps, too, since Listeria thrives in moist conditions.
culture: Are most cheesemakers aware of these practices and interventions?
CD: That’s one reason we created VIAC. Regulation is one way to control Listeria; education is another. We have a whole course in food-safety risk-reduction practices at VIAC. In the past we’ve also had grants that offer [us an] ability to go out and do an on-site risk-reduction program for cheesemakers where we spend a day, watch the make, and sample the milk, the whey, and the environment. With those findings we can make recommendations.
Where Listeria is of most concern to cheesemakers is in the category of soft-ripened cheeses. In a soft cheese product, Listeria can grow to extremely high levels, so the FDA is most concerned about this. They have been doing surveillance to try to examine the extent of the problem of contamination in soft cheese manufacturing plants specifically. Of the plants they have visited, 31 percent tested positive for Listeria. This is an unacceptably high incidence, so even though most cheesemakers may know about Listeria, more intervention needs to happen.
culture: Do you consider the FDA’s 60-day aging rule for cheese one of those safety interventions?
CD: We’ve been doing a lot of research regarding the part of the regulation that says either cheesemakers can use pasteurized milk for cheesemaking or they must hold the cheese for 60 days of aging if the milk used is raw. The 60-day aging rule was really intended to be applied to cheeses that as they age become hostile to microbial pathogens—like cheddar and hard-ripened ones. Now with all the artisan cheese being produced in the United States, cheesemakers must apply the 60-day rule to such cheeses as soft-ripened cheeses that were never designed to use aging to achieve safety. So in a Camembert, for example, holding that cheese for 60 days actually increases its health risk substantially. If you think about France, where they sell Camembert or Brie at 30 days, there’s a much lower risk of Listeria contamination and growth in their soft-ripened cheese. In fact, in France you can’t even sell an AOC Camembert beyond 59 days because the risk is considered to be so great. So even though the FDA applies the 60-day rule to raw milk soft-ripened cheeses, our research indicates that it’s not good practice to enforce that. The agency is, however, in the process of doing a soft-cheese reassessment. So the regulations are likely to change.
culture: Obviously Listeria has no national boundaries. Can you explain how European cheesemakers deal with the bacteria?
CD: There are tolerance limits in Europe. What they have established is a food-safety objective to keep Listeria as low as is feasibly possible in cheese products, which means no more than 100 Listeria per gram of product. If foods are kept at that level, foodborne illness can be substantially eliminated, and all the epidemiological studies have shown this to be true. But to achieve that objective they’re essentially doing what we do here in the United States—eliminating any environmental sources of contamination. So regardless of whether governments are working with the food-safety objective or zero tolerance policy, you come to the same method of prevention. Unfortunately, our liability laws in the United States are never going to allow us to move away from zero tolerance.
culture: What do you say to consumers who are seeing Listeria in the headlines and might be concerned about eating their favorite cheeses?
CD: First, people should know that cheese has a very, very good track record of food safety. Outbreaks are not the norm. Even with the 31 percent of facilities that the FDA found tested positive for Listeria, those were environmental findings—not in the finished cheese samples. When outbreaks have occurred, they generally involve products from unlicensed producers or from imports where there’s not sufficient oversight of the plant. Now the Food Safety Modernization Act gives the FDA the authority to work with foreign manufacturers to bring them into compliance.
It’s important to remember too that the majority of cases where listeriosis occurs is in people whose immune systems are compromised—persons undergoing chemo for cancer or those with diabetes, HIV/AIDS, the elderly, or pregnant women. People in those categories may choose to avoid soft-ripened cheeses in favor of hard cheeses. There are certain cheeses that don’t support the growth of Listeria—such as hard Italian cheeses, Swiss, and cheddar. They can be very comfortable choosing those.
The good news about Listeria is that back in the 1980s, when there were clustered outbreaks in America, the incidence was substantially higher than it is now—more like 5,000 cases reported and 450 deaths. So the fact is that we’ve made incredible progress in eliminating this pathogen.
Catherine Donnelly
Catherine Donnelly, Ph.D., is an expert on the microbiological safety of food and a professor in the Department of Nutrition and Food Sciences at the University of Vermont and the associate director at the Vermont Institute for Artisan Cheese.
