Danish scientists uncover clue to Listeria’s toughness

Birgitte Kallipolitis, an associate professor in the Department of Biochemistry and Molecular Biology at the University of Southern Denmark, and colleagues report their findings in the journal Nucleic Acids Research.

listeriaListeria, a bacterium that sometimes occurs in unprocessed and processed foods, causes an infection called listeriosis. Most infections only result in mild symptoms such as fever, vomiting and diarrhea, which take about 3 days to pass and do not normally require treatment.

But in rare cases, listeriosis can spread to other parts of the body and cause severe conditions like meningitis, which is typically accompanied by severe headache, stiff neck and tremors.

Unborn babies can also contract a listeria infection from the mother via the placenta.

The US Centers for Disease Control and Prevention (CDC) estimate that around 1,600 Americans fall ill with listeriosis and 260 die from it every year. In Denmark – a country with a population some 60 times smaller than that of the US – over the last few weeks alone, 28 people have fallen ill and 13 have died from listeriosis caused by eating processed foods bought in supermarkets. Prof. Kallipolitis says Listeria is notoriously difficult to fight because it is extremely able to adapt to changes in its surroundings. This was the subject of their study, which reveals some important clues about how Listeria manages to retain the ability to invade cells while at the same time escape the attention of the immune system.

For their study, they examined what happens at the microbiological level when Listeria is exposed to some of the substances known to be challenging to bacteria – such as antibiotics, bile, salt, ethanol and acid, many of which it encounters in processed and unprocessed foods in the human body and also in disinfected environments.

Prof. Kallipolitis says, “We knew that Listeria can resist these substances, but we did not quite know how.” She and her colleagues found the bacterium uses various strategies to resist the substances.

“Generally speaking, Listeria must be described as extremely adaptable. It is constantly aware of its surroundings and if the environment changes around it. It reacts instantly and has a number of strategies to withstand threats,” she explains.

Listeria infects host cells by producing special proteins. In order for infection to be successful, the bacterium must keep the production of the proteins under a certain level – for above this level the host immune system becomes aware of it and attacks the pathogen.

The researchers discovered that when they exposed Listeria in the lab to the various anti-pathogen substances like bile, salt, ethanol and antibiotics, the bacterium started releasing special RNA molecules, as Prof. Kallipolitis explains:

“With these RNA molecules the bacteria can adjust how much or how little to produce of various proteins. For example it can downgrade the production of the protein LapB, which it uses to enter our cells. If this production is not downgraded, the bacterium will potentially be detected and fought by the immune system.”

The team also found that these same RNA molecules help Listeria keep watch on its cell walls in the face of danger. Antibiotics work by attacking the cell walls of bacteria. But when Listeria is exposed to antibiotics, it rapidly detects the assault on its cell walls and sets about repairing them.

The researchers observed that in the lab, Listeria only produces the special RNA molecules when exposed to one of the challenging substances. The bacterium did not produce them when there was no challenge.

Prof. Kallipolitis says this “reveals part of the mechanism behind Listeria’s extreme adaptability.”

The team now plans to find out if removing the RNA molecules renders Listeria harmless.

L. monocytogenes in a cheese processing facility: Learning from contamination scenarios over three years of sampling

The aim of this study was to analyze the changing patterns of Listeria monocytogenes contamination in a cheese processing facility manufacturing a wide range of ready-to-eat products. Characterization of L. monocytogenes isolates included genotyping by pulsed-field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST).

listeriaDisinfectant-susceptibility tests and the assessment of L. monocytogenes survival in fresh cheese were also conducted. During the sampling period between 2010 and 2013, a total of 1284 environmental samples were investigated. Overall occurrence rates of Listeria spp. and L. monocytogenes were 21.9% and 19.5%, respectively. Identical L. monocytogenes genotypes were found in the food processing environment (FPE), raw materials and in products. Interventions after the sampling events changed contamination scenarios substantially. The high diversity of globally, widely distributed L. monocytogenes genotypes was reduced by identifying the major sources of contamination.

Although susceptible to a broad range of disinfectants and cleaners, one dominant L. monocytogenes sequence type (ST) 5 could not be eradicated from drains and floors. Significantly, intense humidity and steam could be observed in all rooms and water residues were visible on floors due to increased cleaning strategies. This could explain the high L. monocytogenes contamination of the FPE (drains, shoes and floors) throughout the study (15.8%). The outcome of a challenge experiment in fresh cheese showed that L. monocytogenes could survive after 14 days of storage at insufficient cooling temperatures (8 and 16 °C). All efforts to reduce L. monocytogenes environmental contamination eventually led to a transition from dynamic to stable contamination scenarios. Consequently, implementation of systematic environmental monitoring via in-house systems should either aim for total avoidance of FPE colonization, or emphasize a first reduction of L. monocytogenes to sites where contamination of the processed product is unlikely. Drying of surfaces after cleaning is highly recommended to facilitate the L. monocytogenes eradication.

International Journal of Food Microbiology, Volume 189, 17 October 2014, Pages 98–105, DOI: 10.1016/j.ijfoodmicro.2014.08.001

I. Rückerla, M. Muhterem-Uyara, S. Muri-Klingera, K.-H. Wagnerc, M. Wagnera, B. Stessl


Listeria fears prompt meat recall in Sweden

Tulip Food Company, which sells its meat products in both Denmark and Sweden, announced a recall of its Danish deli meat products on Friday after finding traces of listeria in portions of meat sold in Sweden.

2eb1ea2e55248a2b0bd20c0697e907e16ac21e40bbd998668014044d2c666427The company specified that the discovered amounts were miniscule, but that it was exercising caution.

“Listeria can grow in a product and even a very small amount can turn into a risky amount before the meat expires,” Michael Larsen, quality manager at Tulip Food Company, said in a statement.

The recalled products are Shächter Luftikus, Shächter Bauernskinka, Tulip Kokt Skinka, and Tulip Rökt Skinka, all in 500 gramme packages produced between August 7th and August 20th.

Contaminated meat has killed 13 people and caused at least 24 infections in Denmark, The Local Denmark reported. 

13 dead: Listeria found in another Danish workplace

Another sandwich meat producer has been hit by the Listeria outbreak that has now claimed 13 lives in Denmark. The food product authorities Fødevarestyrelsen ordered the Delika plant near Hammel to close its doors – for 24 hours – so that cutting equipment and other listeriaequipment that came into contact with contaminated ‘rullepølse’ sandwich meat, which originated from Jørn A Rullepølser in Hedehusene near Copenhagen, can be thoroughly cleaned.

 “Delika Hammel wants to protect our customers and eliminate any possible risk,” the company said in a statement.

A wide range of Delika products were pulled from the shelves of shops last week. The meats were cut on machines that had been in contact with the infected products from Jørn A Rullepølser, and authorities feared cross-contamination could have occurred.

Belleisle Farms brand cole slaw recalled due to Listeria monocytogenes

Roy Pope and Sons is recalling Belleisle Farms brand cole slaw from the marketplace due to possible Listeria monocytogenes contamination. Consumers should not consume the recalled product described below.

belleisle.cole.slaw.listeriaRecalled products

Brand Name: Belleisle Farms        

Common Name: Garden Fresh Cole Slaw         

Size: 227 g     

Code(s) on Product: Best Before:


UPC: 0 33383 65260 3

This recall was triggered by Canadian Food Inspection Agency (CFIA) test results. The CFIA is conducting a food safety investigation, which may lead to the recall of other products. If other high-risk products are recalled, the CFIA will notify the public through updated Food Recall Warnings.

The CFIA is verifying that industry is removing recalled product from the marketplace.

More than Canada would admit: Denmark says ‘serious errors’ in handling of Listeria outbreak with 12 dead, another 12 sick

Denmark’s food safety watchdog made “serious mistakes” in its handling of a listeria outbreak linked to the death of 12 people, the country’s government has said.

ITALY-G8-G5-AGRICULTURE-FARMFood minister, Dan Jørgensen, has blasted the food authorities, Fødevarestyrelsen, over its handling of the Listeria outbreak that has claimed the lives of 12 people in Denmark over the past year.

A Fødevarestyrelsen report has showed there were serious errors in its handling of the case and concluded that it should have carried out its investigation into the source of the outbreak, Jørn A Rullepølser, more quickly and effectively.

“When it is proved there is a direct connection between the food products and deaths, the authorities should immediately launch a thorough investigation of the specific company,” Jørgensen said in a press release. “That hasn’t happened quickly enough, which is lamentable.”

Goats killed by Listeria in Ohio, family says city water system to blame

A grandmother in Crooksville said two of her grandson’s prized goats were killed by Listeria and she blames the city’s water system for it.

listeria.goatKim Burkhart said her veterinarian told her the goats came down with the disease from contaminated soil. There’s a Crooksville pump station on the edge of her property. She said the soil samples showed high levels of E coli that she said is coming from a leak at the pump station.

“We hear story upon story upon story and we’re tired of stories,” Burkhart said. “All we want is, if it’s (the city’s) situation, we want it fixed.”

Her grandson was raising the goats as part of a 4H project. The boy had hoped to show one of the goats at the Perry County Fair. “When he actually saw they had died, it was devastating to him,” she said. “We didn’t know what to say to him. We all cried.”

Danish food minister wants answers in Listeria case

The Copenhagen Post reports that food minister, Dan Jørgensen, has announced that he wants an explanation regarding the listeria outbreak that has claimed 12 lives in Denmark since September 2013.

rullepølseYesterday, the food product authorities Fødevarestyrelsen closed down the suspected supplier of the bad sandwich meat responsible for the infection, Jørn A Rullepølser, but now it has emerged that Listeria was first found in the company’s products in May.

Three weeks after the May findings, Jørn A Rullepølser was cleared, despite the fact that five people at the time had been infected with the bacteria.

12 dead, 8 sick from Listeria in Denmark

Since September last year, 12 people have died of Listeria and a further eight have been infected after consuming ‘rullepølse’ sandwich meat.

rullepølseThe food product authorities Fødevarestyrelsen suspect that the bad meat originated from Jørn A Rullepølser in Hedehusene near Copenhagen and have closed down the producer.

The 20 infected patients consist of eleven women and nine men all aged 43-89 and all hailing from various parts of Denmark, according to the national serum institute Statens Seruminstitut.

“From September 2013 until today, 20 patients have been registered suffering from listeriosis, which is an aspect of the outbreak,” Statens Seruminstitut wrote in a press release.

“Most cases have however occurred recently. In June, July, and August, 15 cases have been registered alone.”

Similar to previous cases, the 12 people who died also suffered from other serious illnesses and their deaths cannot completely be attributed to a listeria infection, Statens Seruminstitut stated.

Wyoming researchers discover substance that increases Listeria monocytogenes survival

University of Wyoming researchers have discovered a substance that greatly increases the survival of Listeria monocytogenes, a foodborne bacterial pathogen that contaminates processed meat and milk products, as well as fresh produce.

listeriaResearchers hope the discovery will lead to the development of techniques to better combat the pathogen and to improve food safety.

Mark Gomelsky, a professor in UW’s Department of Molecular Biology, and other researchers discovered and characterized a substance, called exopolysaccharide (EPS), that Listeria secretes on its cell surface under certain conditions. The EPS coats bacterial cells and makes them form aggregates or clumps, which are strongly protected from commonly used disinfectants and desiccation (extreme drying).

“We think that EPS plays a significant role in survival of Listeria in the environment, during food storage, processing and transportation,” Gomelsky says. “Listeria rarely causes serious disease in healthy individuals but, in immune-compromised people, elderly and pregnant women, it can be deadly, causing as much as 20 percent to 25 percent mortality.”

Gomelsky is a senior writer of a paper, titled “Cyclic di-GMP-Dependent Signaling Pathways in the Pathogenic Firmicute Listeria monocytogenes,” that was published in Public Library of Science (PLoS) Pathogens Thursday.