Listeria monocytogenes bacteria are present in the environment. They can contaminate foods and can multiply at refrigeration temperatures. Consumption of food contaminated with Listeria can cause illness in humans which is particularly serious in the very young, the old, pregnant women and the immunocompromised. In England and Wales from 2017 to 2022 there were 884 reported cases of listeriosis and 172 deaths. Foods associated with cases and outbreaks included ready to eat smoked fish such as smoked salmon, ready-to-eat sandwiches, cooked meats and soft cheeses.

The Foods Standards Agency provide advice on how you can reduce the risk of listeriosis including storage of foods and cooking certain foods like cold smoked fish.

Here at the Quadram Institute, Dr Matthew Gilmour’s research group study the microbial traits of Listeria that contribute to its significant risk as a foodborne contaminant and invasive human pathogen.

Listeria research has been a focus of research in Norwich for several decades. Back in the 1980s members of Dr Barbara Lund’s group, Susie George and David Mason, began studying Listeria at the Institute of Food Research in Norwich. Their work back then is proving key to current research in the Gilmour group and researchers globally. We caught up with Matt and Dr Ana Victoria Gutiérrez from the Gilmour group to find out more.

Tracking Listeria outbreaks in the 1980s

“I first started working with Listeria when I joined the Gilmour group in 2020. A colleague introduced me to Barbara. Barbara gave me a folder packed full of Listeria history,” explains Ana Victoria.

She continues, “The folder contained lots of newspaper clippings from the 1980s. This was the decade when listeria became known as a cause of outbreaks of disease and listeriosis. These clippings document key pieces of Listeria history from the UK and across the world, including newspapers from France and Germany.”

Matt adds, “Outbreaks were happening in the cheese industries in France and Switzerland. This was one of the first times the food industry was heavily impacted by the risk of Listeria monocytogenes. Barbara saw news of the Listeria outbreaks in scientific reports and newspapers and contacted scientists in other countries asking for these outbreak-associated bacterial strains for research.”

In response to researchers’ requests, a food industry company responded, “The significance of Listeria in food has unhappily been ignored, with yourselves as an exception, until the latest outbreak of Listeria.”

Reflecting on this Matt says, “That note is a major retailer becoming aware of what will become a prioritised food safety risk and is an early sign of the attention that food producers will continue to place on the importantance of Listeria.”

Samples from historical Listeria outbreaks across the globe

Along with passing on the newspaper history of Listeria, Barbara gave Ana Victoria access to the Listeria strains her group had collected and which Susie and David had carefully freeze-dried.

The historical listeria strains had been carefully looked after by Dr Sandra Stringer as the Institute of Food Research transitioned into the Quadram Institute. Sandra worked with Dr Barbara Lund at the Institute of Food Research in Norwich and is now a Laboratory Manager at the Quadram Institute.

Matt reflects on his initial observation of the Lund group’s strain collection, “The collection really astounded me. As a Canadian moving here, I first saw that one of the biggest components of the listeria collection are strains from an original Listeriosis outbreak in the early 1980s in Canada.”

“The 1980s outbreak in Canada was a significant outbreak related to coleslaw. It was one of the first public health events that revealed the significance of Listeria as a zoonotic organism, meaning that it can move between animals and humans. Researchers at the time suspected Listeria was moving from sheep manure onto cabbage while it was being grown in fields. People that ate foods like coleslaw made from that cabbage were getting Listeriosis.”

He continues, “Unfortunately, there was significant mortality with unborn babies in this coleslaw outbreak. The strains from the bacteria in this outbreak are part of the historical strain collection. Many of these strains were no longer present in Canada, so the collection of Barbara’s group was really quite valuable to re-establish a more complete collection.”

“The correspondence around the outbreak and subsequent research includes papers from Canadian researcher Jeff Farber who has been one of my mentors in life.”

Supermarket samples ahead of their time

Along with strains from disease outbreaks, and samples from the food industry, Barbara’s group collected samples from supermarket foods in the 1980s too.

Ana Victoria remarks, “The samples were collected even before I was born!”

Matt highlights, “For decades most people talked about Listeria in cheese, dairy products and meat products like ready to eat foods. Yet Barbara’s group also collected samples from buying raw chicken breast and salad leaves. That was very ahead of its time.”

Ana Victoria says, “They even swabbed the surfaces of eggs too, to see if there was Listeria there. Her group’s work is visionary.”

“The strains from the collection are very well organised so we know when and where each sample comes from”, she adds.

Strain collections enable genomics of Listeria

The Lund groups’ comprehensive collection of Listeria strains is proving key to current food safety research.

Ana reflects, “These historical strains are extremely important for our genomics research understanding Listeria. We’ve made high quality genome assemblies of these strains and we’re using these sequences to draw phylogenetic trees to study how Listeria has evolved over the decades. The collection allows us to turn back the molecular clock and go further back in time.”

She continues, “We are partnering with Canada and UK Health Security Agency (UKHSA) to build a reference collection of strains which includes some of the collection from Barbara’s team. This will be a benchmark strain collection for future research.”

Impact of cleaning on Listeria evolution

The strain collection has proved key to understanding how Listeria traits have evolved over time.

Matt explains, “From a microbiology perspective, it’s a been a treasure trove of Listeria, especially from the perspective of cleaning and disinfection in food production. We’ve got a chance to study isolates that are from an era where different biocides may have been used, so they are not resistant or tolerant to some of the chemicals that are commonly used now.”

Ana Victoria continues, “Historically ammonium compounds have been used for cleaning in food processing environments. Isolates from the collection from the 1980s do not have the genes that confer resistance to these disinfectants. In contrast, more contemporary isolates which we’ve acquired through UKHSA have genes that confer resistance to disinfectants. We are lucky to have a ‘before’ comparator and we are very grateful for these historical strains.”

Matt adds, “The collection gives us a chance to study Listeria in a more ancestral state.”

Co-culture experiments ahead of the time

Increasingly microbiology research is studying how bacteria live together as a community.

Matt describes how Barbara group’s research was ahead of its time in this way. “In the team’s lab books there were some co-culture experiments where you grow different bacteria together. We’re trying to do exactly the same things 40 years later but apply different microbiology skills including genomics,” he says.

Ana Victoria summarises, “Barbara’s group’s experiments are visionary because now in microbiology we’re thinking more about communities of microbes rather than single organisms. Barbara and her team were very ahead of the time.”

Future of Listeria research

Another feature of the group’s lab archives and collections are leaflets from conferences on Listeria, including the sixth International Symposium on Problems of Listeria and Listeriosis (ISOPOL) in 1970’s. Then the conference was a one-day event with seven speakers.

This autumn the Gilmour group and the Quadram Institute host the 21st ISOPOL in Norwich, bringing together global experts on Listeria. The four-day event covers the latest discoveries and opportunities to understand and control Listeria to improve food safety.

Ana Victoria concludes, “Thank you to Barbara and members of her group. She is a great guardian of Listeria history, strains and lab archives. Their work is key to current and future Listeria research.”