Skip to main content

See also:

Fungi and microbe communities on cheese rind

Choosing cheese? Researchers study cheese to unlock secrets of how microbial communities form. If you're vegan, you can make nondairy cheese out of nuts such as almonds, soybeans, or other vegetable-based nondairy substitutions. But a new study from Harvard University looked at the bacteria and fungi that live on dairy-based cheese. the reason for studying cheese is to better understand how microbial communities form. The study, "Cheese Rind Communities Provide Tractable Systems for In Situ and In Vitro Studies of Microbial Diversity," is described in a July 17, 2104 paper in the journal Cell.

Fungi and microbe communities on cheese rind.
Fungi and microbe communities on cheese rind.
Photo by Justin Sullivan/Getty Images

Rachel Dutton's research at Harvard University, a Bauer Fellow at the Faculty of Arts and Sciences' Center for Systems Biology, looked at the microbial communities on cheese. Dutton and her lab study cheese in order to examine the bacteria and fungi that live on cheese.

After studying 137 varieties of cheese collected in 10 different countries, Dutton has been able to identify three general types of microbial communities that live on cheese, opening the door to using each as a "model" community for the study of whether and how various microbes and fungi compete or cooperate as they form communities, what molecules may be involved in the process and what mechanisms may be involved.

"We often use model organisms like E. coli or C. elegans because they can give us an understanding of the basic mechanisms and principles of how biology works," Dutton said, according to the July 17, 2014 news release, Choosing cheese. "The goal of this work was to identify something like a model organism, but for microbial communities – something we can bring into the lab and easily replicate and manipulate.

"The challenge in studying these communities is that many of the environments where they are found, such as the human body or the soil, are hard to replicate because they're so complicated," she continued, according to the news release. "Cheese seemed to offer a system…in which we knew exactly what these communities were growing on, so we thought we should be able to replicate that environment in the lab."

To understand what a model community might look like, Dutton and her lab first set out to identify dozens of naturally-occurring communities by collecting samples from the rinds of dozens of varieties of cheese around the world.

"We did some travelling in Europe and worked directly with a number of cheese-makers by having them send us samples or vising to collect samples, and in some cases we were able to collect samples from places like Formaggio Kitchen and other cheese shops," she said in the news release.

By sequencing those samples, Dutton was able to identify the type of bacteria and fungi in each, and found that while there was wide variation among different samples, the samples could be separated into one of three main types of communities.

"What we ended up finding is there are microbes which occur in all the areas where cheese is made," she said, according to the news release. "What was interesting is if you make the same type of cheese in France or in Vermont, they will have very similar communities.

What seems to be driving the type of community you find is the environment that the cheese-maker creates on the surface of the cheese, so you can make two cheeses that are very similar in two different places, or you can make two very different cheeses in the same place."

Working in the lab, Dutton and colleagues were able to isolate each species of microbe and fungi found in the samples and conduct tests aimed at reproducing the communities found on different cheeses. "In many environments, it is challenging to isolate all of the microbes, so we were surprised to find that we could culture all of the species present on cheese rinds. This gives us a great foundation for being able to study communities in the lab," says Julie Button, according to the news release. Button is a postdoctoral researcher in the Dutton lab.

"If we know a particular cheese has certain species, we can mix them together and try to recreate that community in the lab," Dutton said, according to the news release. "For example, we might try to simply put those species together at the same time in equal amounts to see if the community that forms is similar to that found in the sample."

The study was also aimed at understanding how various species of bacteria and fungi interact, and identified several instances in which certain bacteria halted fungal growth, and vice versa

"We are now working with chemists to characterize what the molecules are that different bacteria might use to kill a fungus," Dutton said. "It's also possible that there may be anti-microbials that may arise from this that are normally at play during the formation of a community."

While wider applications for understanding how bacterial communities form may eventually emerge, Dutton said, according to the news release, that there are still a number of fundamental questions to answer in the short term. "There are so many wide open questions in thinking about how microbial communities work, that future research could go in a number of different directions," she said, according to the news release. "Our goal is to understand some of these fundamental questions, such as: Are there certain principles that are operating as a community forms, and can we control those factors in the lab?

"Cheese is fascinating to me in its own right – it's somewhat surprising that, for a food that we've been eating for thousands of years, we don't have a complete understanding of the microorganisms that are present in this food." But now that Dutton has that understanding, does she still eat cheese? "I do," she said with a laugh. "But I'm very picky, because I like very good cheese now." You also may find noteworthy another work of research, "Conducting a Microbiome Study."

Pathogens also can grow in cheese

Contaminated Quargel cheese caused several deaths in 2009 and 2010. In 2009 and 2010 two different strains of the bacterium Listeria monocytogenes were found in traditional Austrian curd cheese known as 'Quargel'. Thirty-four people were infected, and a total of eight patients died. Experts from the Vetmeduni Vienna analyzed the genomes, according to other studies by different researchers.

The bacterial strains display distinct properties and entered the food chain independently. The results were published in the journal PLOS ONE, and increase the understanding of outbreaks and their prevention, explains the article published online February 26, 2014 in the journal, PLOS ONE, "Genome sequencing of Listeria monocytogenes “Quargel” listeriosis outbreak strains reveals two different strains with distinct in vitro virulence potential." Authors are Kathrin Rychli, Anneliese Müller, Andreas Zaiser, Dagmar Schoder, Franz Allerberger, Martin Wagner and Stephan Schmitz-Esser.

Pathogens in Cheese – Researchers Follow the Traces of Deadly Bacteria

If food products are not produced in a hygienic environment, consumers can face the threat of dangerous pathogens. This is exactly what happened in 2009 and 2010 when two different strains of Listeria monocytogenes were found in the traditional Austrian curd cheese known as “Quargel”.

Thirty-four people were infected, and a total of 8 patients died. Experts from the University of Veterinary Medicine, Vienna analyzed the genomes of the outbreak strains and were able to show that the strains displayed distinct properties and entered the food chain independently. The results were published in the journal PLOS ONE and will increase the understanding of outbreaks and their prevention.

Listeria monocytogenes can cause listeriosis

Listeria is a rod-shaped bacterium highly prevalent in the environment and generally not a threat to human health. One species however, Listeria monocytogenes, can cause listeriosis, a very dangerous disease.

This pathogen can be present in raw milk and soft cheeses, smoked fish, raw meat and ready-to-eat products. In Austria, health care providers are required to report all cases of listeriosis, which can be fatal particularly for patients with weakened immune systems. In 2009 and 2010, a dairy in Hartberg (Styria, Austria) produced Quargel cheese contaminated with Listeria monocytogenes leading to a multinational listeriosis outbreak in Austria, Germany and the Czech Republic, ultimately forcing the dairy to shut down.

Detective work: finding the source

“I’m happy to report that we see relatively few cases of listeriosis here in Austria. When an outbreak occurs though, the disease has among the highest mortality rate of all food-borne illnesses”, explains lead author Kathrin Rychli from the Institute for Milk Hygiene, Milk Technology and Food Science at the University of Veterinary Medicine -- Vienna, according to the March 21, 2014 news release, "Pathogens in cheese." The Institute's involved in investigating the causes of the outbreaks back in 2009 and 2010. The culprits: two distinct bacterial strains which had not recently evolved from a common ancestor, and therefore entered the food chain independently.

Genetics reveal the pathway

In their current study, the scientists sequenced and analysed the genomes of both strains, and assessed their virulence, the ability to infect cells. The samples were taken from listeriosis patients from the outbreak.

The first contamination event from June 2009 to January 2010 was attributed to one L. monocytogenes strain very effective at infecting epithelial cells of the intestine and liver cells. It contained additional four virulence genes, making it extremely invasive, and ultimately caused 14 cases resulting in 5 deaths.

A few months later in December 2009, the second L. monocytogenes strain emerged. It was particularly successful at infecting macrophages, important immune system cells. Over time, this highly infectious second strain replaced the first and by February 2010 had infected a total of 20 people, 3 of whom died. The average age of those taken ill was 70.

Highest level of operational hygiene essential

Listeria expert and co-author Stephan Schmitz-Esser emphasizes the importance of cleanliness in production: “It is absolutely essential that appropriate disinfectants are used properly, lots of salt, and that possible food for the bacteria be limited." Any products listeria is found in must be recalled immediately.

Recalls are very expensive for producers, and we need to do everything we can to prevent them.” Austria-wide the Institute for Milk Hygiene, Milk Technology and Food Science at the University of Veterinary Medicine, Vienna offers effective Listeria monitoring and a range of molecular and microbiological examination methods for the food industry.

What are the symptoms of listeriosis?

Listeriosis generally manifests in healthy people with diarrhoea and stomach cramps, whereas the elderly, newborns and people with weakened immune systems are more susceptible. Listeriosis can result in septicaemia and meningitis.

In pregnant women it can lead to miscarriage or stillbirth. Therefore, experts recommend pregnant women to avoid raw milk, raw meat and raw fish products.

Why you shouldn't eat cheese or deli items such as hot dogs or cold cuts when pregnant, have a compromised immune system, or if you're an older adult

Because pregnancy affects your immune system, you and your unborn baby are more susceptible to the bacteria, viruses, and parasites that cause foodborne illness. Even if you don’t feel sick, some “bugs” like Listeria and Toxoplasma can infect your baby and cause serious health problems. Your baby is also sensitive to toxins from the food that you eat, such as mercury in certain kinds of fish.

Keep the government's checklist handy to help ensure that you and your unborn baby stay healthy and safe. The same rules apply to senior citizens with lowered immune systems due to advanced aging. And invest in a food thermometer to check the temperatures of cooked food.

Some foods to avoid when pregnant

Here are some foods to avoid during pregnancy, according to the Food Safety.gov site. You can look at the information at the article, "Checklist of Foods to Avoid During Pregnancy," but even if you're not pregnant, it's still a good idea to follow the same practice to lessen the chance that what you eat is contaminated with bacteria or parasites.

Don't eat these foods

Soft cheeses made from unpasteurized milk, including Brie, feta, Camembert, Roquefort, queso blanco, and queso fresco. Don't eat raw cookie dough or cake batter. The exception is if your mixing some almond milk with some oat bran, oat meal, raisins, and flaxseed bran and baking the mixture without adding any eggs. See, What are the symptoms of listeriosis?

Stay away from certain kinds of fish, such as shark, swordfish, king mackerel, and tilefish (golden or white snapper). And don't eat raw or undercooked FISH (sushi). Stay away from unpasteurized vegetable or fruit juice or cider (including fresh squeezed). Don't eat unpasteurized milk because it could contain bacteria such as Campylobacter, E. coli, Listeria, or Salmonella. If you drink milk, use the pasteurized variety.

Make your own salads. Don't eat store bought, deli, or packaged salads the restaurant has in the coolers because any salads made in a store, such as ham salad, chicken salad, and seafood salad could contain Listeria. Don't eat raw shellfish, such as oysters and clams. It could contain Vibrio bacteria.

Stay away from sprouts when pregnant

Don't eat raw or undercooked sprouts such as alfalfa, clover, mung bean, and radish. The sprouts could contain E. coli or Salmonella. This puts a damper on those who eat a lot of raw vegan foods. If you're sprouting your own indoors in special containers where you keep washing out the bacteria several times a day, it could be a different story from store-bought sprouts. But for good measure, don't eat these when you're pregnant. Eat other vegetables.

Don't eat ice cream when pregnant if the ice cream contains raw eggs

Why? There could be raw eggs in them, which is a no-no. Instead, make ice cream with a pasteurized egg product safer by adding the eggs to the amount of liquid called for in the recipe, then heating the mixture thoroughly. Don't eat undercooked fish. Cook the fish above 145 degrees F. Too many people sear the fish and leave the bacteria in the fish. So be careful when you eat fish. Not only do some fish contain excess mercury, but the fish may contain parasites or bacteria.

Don't eat hot dogs when pregnant because the hot dogs may contain Listeria

Don't eat deli items such as luncheon meats, cold cuts, fermented or dry sausage, and other deli-style meat and poultry because they, also could contain Listeria. When pregnant don't eat undercooked eggs, including pasteurized egg products. When you prepare eggs, cook the eggs until yolks are firm. Cook casseroles and other dishes containing eggs or egg products to 160° F.

Beware of eggnog made with raw eggs

Homemade eggnog may be full of uncooked eggs which, in turn could be full of Salmonella. Instead, make eggnog with a pasteurized egg product or buy pasteurized eggnog. When you make eggnog or other egg-fortified beverages, cook to 160°F. Make your own eggnog with almond milk and various spices and flavorings such as cinnamon, natural vanilla flavoring, or other natural flavors you buy yourself where you can look at the ingredients you put in any beverages. You can buy the unsweetened varieties.

If you're making homemade ice cream, don't add uncooked eggs because they could contain Salmonella

Instead, make ice cream with a pasteurized egg product safer by adding the eggs to the amount of liquid called for in the recipe, then heating the mixture thoroughly. Be careful when you eat meat such as veal, lamb, ground meat, beef, and pork. Undercooked meat could be full of E. coli bacteria.

Cook beef, veal, and lamb steaks and roasts to 145° F. Cook pork to 160° F. Cook all ground meats to 160° F. Instead of eating meat spread or pate, realize that unpasteurized refrigerated pates or meat spreads may contain Listeria. You don't want a Listeria infection when pregnant. You could eat the canned versions, which usually are safe.

Don't eat undercooked poultry stuffing that has been heated inside the bird

Stay away from undercooked meat in poultry stuffing, including ground poultry in the stuffing because it may be undercooked. And undercooked meat may contain bacteria such as Campylobacter or Salmonella.

Cook your stuffing separately instead of putting undercooked or raw meat inside poultry because it won't cook to a high enough temperature there. Instead first cook poultry to 165° F. If the poultry is stuffed, cook the stuffing to 165° F.

Any type of stuffing should be cooked separately to a high enough heat before it is put on a plate near the meat

Don't stuff anything, including vegetables with stuffing that contains meat or fish unless you cook the stuffing first. An exception might be grains such as rice that cook inside a stuffed pepper, but it's better to cook the rice first, since an uncooked grain could break someone's tooth.

Be careful with smoked seafood when your pregnant, and even when you're not pregnant. Canned versions usually are safe. If you have smoked fish, cook the fish to 165 degrees F. Beware of the smoked fish in your refrigerator. It probably isn't safe enough to eat. Refrigerated versions of smoked fish usually are not safe, unless they have been cooked to 165° F.

Here are some articles, which also are helpful: Food Safety for Pregnant Women. It's a need-to-know guide to help you reduce your risk of foodborne illness. See, Food Safety for Moms-To-Be (FDA). Foodborne illness is a serious health risk for pregnant women and their unborn babies.

You also may be interested in articles such as Before You're Pregnant. Protect your unborn baby from methylmercury and toxoplasmosis. And take folic acid supplements, the suggestions explain. Also, you may wish to read the article, While You’re Pregnant. The idea is to stay away from foodborne bacteria. Protect your unborn baby from methylmercury, toxoplasmosis, listeria, and other foodborne illness.

What foods are safe if you're pregnant?

See the article, Safe Eats. It's a food-by-food guide to selecting, preparing, and handling foods safely throughout pregnancy. Also check out articles such as Protect Your Baby and Yourself From Listeriosis (USDA). If you are pregnant, you need to know what foods are safe to eat. Or take a look at the article, Highlights - Entertaining All Year. The goal is to keep yourself and your unborn baby safe from foodborne illness while entertaining.

Deli foods such as cheese and cold cuts or hot dogs may be off limits to pregnant women, according to some doctors, due to the possibility of listeria in those foods, even if the cheese and hot dogs are safe for young, healthy people who aren't pregnant or elderly

Both the pregnant and elderly as well as very young children may have weak immune systems. Are you pregnant, craving pizza, deli cold cuts, brie cheese, and worried about what foods might increase your risk of contracting listeriosis from certain cheeses or hot dogs?

Some doctors tell their patients not to eat cheese when they're pregnant, and women want to know why. It's because of the danger of contracting listeriosis, that's found in some cheeses. When you're pregnant, you're 20 times more likely to catch a bacteria or virus infection. And if you get listeria from eating certain cheeses, it could develop into a severe infection.

What's listeriosis?

Listeriosis is caused by a bacteria called "listeria monocytogenes." Most people don't even know they've contracted diarrhea from something they ate, but can't recall what it might be. It could be the cheese.

The problem is when you're pregnant your immune system is really compromised. It's just one more way your body prepares you not to fight off the foreign invader, your embryo -- you your baby can develop to full term.

When you're immune system is low, you also can easily catch the flu when pregnant

That's one reason why pregnant women are first in line to get the novel flu vaccination. But another nasty symptom for pregnant women is a listeria infection. You don't want the runs because the contractions from your colon can stimulate your uterus to go into labor.

That's why in the 1960s, most women arriving in a hospital to give birth not yet in labor were given enemas not only to clean them out before they get on the delivery table, but to induce labor contractions. So you don't want to contract listeria. See, What is listeriosis? and How does Listeria get into food?

Watch the pizza cravings. High risk groups for listeria also include older persons, cancer patients, people with diabetes, kidney disease, alcoholism, AIDS, those that are immune system compromised, or those taking corticosteroid treatments.

If you catch a bacterial or virus infection during pregnancy, the microbes will congregate and reproduce in your uterus because that's where there is the most blood pooled bringing oxygen to your developing fetus. It's not only cheeses some doctors tell pregnant women to avoid. It's also hot dogs. Here is a list of foods to avoid when pregnant because of the danger of possibly contracting listeriosis.

When you're pregnant also avoid hot dogs, luncheon meats such as bologna, salami, and deli meats or cold cuts unless you boil the deli meats until the bacteria are inactivated

Wash your hands after handling these foods for others. Wash hands often and always when touching those types of meats when preparing food for others. The specific types of cheeses to avoid are the soft cheeses such as brie, camembert, Mexican-style cheeses, and any cheeses made from raw milk.

Make sure any cheeses you eat, if you have to eat cheese when pregnant, come from pasteurized milk or are cooked and melted in a casserole that stays in a hot oven for a half hour to bake. Don't just heat up a pizza to melt the cheese.

To kill the bacteria, the casserole with the cheese on top has to bake for a long time such as more than a half hour. If you're pregnant or if you're a senior citizen with a weakened immune system, don't eat hot dogs or the juice from hot dogs or any other cold cuts, deli items, or cheese.

Don't eat hot dogs because not only the hot dog but the juice from the frank could transmit the bacteria

Wash up anything the hot dog juice spills on. Don't drink the hot-dog juice. Check the supermarkets in Sacramento to see what you can safely eat that doesn't raise your risk of contracting bacterial infections during pregnancy. Infections can spread to your newborn baby. See, Case Definitions for Infectious Conditions Under Public Health.

If you're pregnant and you contract listeriosis, it starts with the same symptoms as flu, plus nausea. You'll get headaches, fever, and lots of nausea. Then the worse happens. Your placenta gets infected, and your baby gets a brain infection or you have a miscarriage. You may wish to check out the CDC sites, Disease Listing: Listeriosis Additional Information | CDC DFBMD or Disease Listing: Listeriosis Technical Information | CDC DFBMD.

Don't take chances with food that usually carries listeria bacteria if you're pregnant or if you're a senior citizen

Read more about how to prevent this food-borne illness that really hits pregnant women hard at the Centers for Disease Control and Prevention (CDC). There's a link on listeriosis there. Be aware if you're pregnant. It's a disease you get from eating food that looks and smells perfectly edible.

According to the CDC, in the United States, an estimated 2,500 persons become seriously ill with listeriosis each year. Of these, 500 die. At increased risk are: * Pregnant women. If you're pregnant you may be about 20 times more likely than other healthy adults to get listeriosis. About one-third of listeriosis cases happen during pregnancy. See, Preventing Foodborne Illness: Listeriosis.

Also newborns can become infected with listeriosis. Newborns rather than just the pregnant women suffer the serious effects of infection when the mother contracts listeriosis during pregnancy. To protect your newborn and yourself at any stage of pregnancy, keep away from certain cheeses and certain processed meats. See, Disease Listing: Listeriosis General Information | CDC DFBMD.

Ask your doctor for a list of what foods to avoid when pregnant to protect your baby and yourself from any risk of infection from listeria bacteria. Check out the following CDC sites that answer frequently asked questions: How do you get listeriosis?, Can listeriosis be prevented?, How can you reduce your risk for listeriosis?, How do you know if you have listeriosis?, What should you do if you've eaten a food recalled because of Listeria contamination?, Can listeriosis be treated?, or What is the government doing about listeriosis? Or check out the CDC's Prevention Guidelines Titles.