While the majority of adults are able to avoid serious illness as a result of Listeria infection, some individuals – such as expectant mothers and people with weakened immune systems – are at high risk. A new study, however, suggests that gut bacteria may hold the key to preventing Listeria infection.

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Researchers have identified gut bacteria that could protect against Listeria infection.

Researchers from the Memorial Sloan Kettering Cancer Center in New York identified four species of gut bacteria that reduced the growth of the bacterium Listeria monocytogenes in mice, but they suggest that much of this beneficial bacteria may be eliminated by antibiotic use.

Study co-leader Simone Becattini and colleagues say that their findings indicate that colonizing patients’ guts with the four bacterial species though probiotics could help to stave off Listeria infection.

The results were recently published in The Journal of Experimental Medicine.

L. monocytogenes is the bacterium that causes listeriosis, a potentially fatal infection that affects around 1,600 people in the United States every year.

Symptoms of listeriosis may include headache, fever, diarrhea, muscle aches, and fatigue. In severe cases, the infection may lead to septicemia, meningitis, and even death.

Listeria infection is most commonly caused by consumption of contaminated dairy products such as soft cheeses, though infection has also been linked to deli meats, melon, and raw sprouts.

Most individuals do not become ill as a result of Listeria infection, but some groups are more vulnerable.

According to the Centers for Disease Control and Prevention (CDC), pregnant women and patients with cancer – whose immune system is weakened by chemotherapy – are 10 times more likely to develop Listeria infection than the general population, and their symptoms are often more severe.

The new study suggests that a lack of protective gut bacteria may play a role in greater susceptibility to Listeria infection among these populations.

Becattini and colleagues came to their findings by analyzing the gut microbiomes – that is, the population of microorganisms in the gut – in a group of mice treated with antibiotics, as well as mice treated with chemotherapy drugs, and comparing them with those of control mice.

On introducing L. monocytogenes to the rodents, the researchers found that those treated with antibiotics showed greater vulnerability to Listeria infection.

They found that the drugs boosted the bacterium’s ability to colonize the gastrointestinal tract and reach the circulatory system, which caused the mice to die.

Mice treated with chemotherapy drugs also experienced an increased susceptibility to Listeria infection, and this was exacerbated when they were also treated with antibiotics.

On further investigation, the researchers identified four species of gut bacteria – all of which were part of the Clostridiales family – that appeared to protect against Listeria infection.

The researchers then tested these probiotic bacteria on L. monocytogenes in the laboratory, and they found that they were able to reduce the pathogen’s growth.

Next, the researchers transferred the probiotic bacteria into germ-free mice – that is, mice that possess no microorganisms – and exposed them to L. monocytogenes.

They found that the probiotic bacteria protected the rodents from Listeria infection by reducing the ability of L. monocytogenes to colonize the gastrointestinal tract.

“Thus, augmenting colonization resistance functions in immunocompromised patients by introducing these protective bacterial species might represent a novel clinical approach to prevent L. monocytogenes infection,” says Becattini.

Our results also raise the possibility that in other at-risk categories for listeriosis, such as infants or pregnant women, disruptions to the gut microbiome could be a contributing factor to susceptibility.

Pregnant women in their third trimester, the phase in which susceptibility to Listeria is known to be highest, show an altered microbiome, with a marked reduction in Clostridiales species.”

Simone Becattini

While further studies are needed to confirm whether or not these findings apply to humans, the current research does advance the possibility of probiotics for the prevention of Listeria infection.

Learn how omega-3 fatty acids might weaken Listeria.