The dynamics of Listeria

A variety of factors play a role in whether food-borne bacteria actually make people sick, and a better understanding of the infection process from food-borne bacteria could certainly lead to ways to stop such illnesses from occurring, confirm US scientists at Purdue university.

In a comprehensive study of the virulence of one of the deadliest food-borne bacteria Listeria monocytogenes​, researchers report that how well the bacteria attach to cells does not alone determine the degree of illness.

The factors that determine if a person becomes ill and the degree of illness include the levels at which the pathogen attaches to intestinal cells, penetrates cell walls and then moves into other organs, said authors Arun Bhunia and Ziad Jaradat, both of the Department of Food Science at Purdue university​.

"I'm interested in understanding how the bacteria interacts with the intestinal cells,"​ said Bhunia, who is part of the department's Molecular Food Microbiology Laboratory. "If you eat food that contains these bacteria, the first place they react with cells is in the intestinal track. If we understand the initial interaction of​ Listeria in the gut, we might be able to prevent the binding and, therefore, the infection."

Listeria​ enters the body when a person eats contaminated food. Listeria​ then binds, or adheres, to intestinal cells. If it is a viable, disease-causing strain, it will penetrate the cell wall, causing infection. Once the bacteria have done this, the infected cells will move, or translocate, to another organ, usually the spleen or liver. Listeria​ strains also can cause encephalitis, or brain inflammation.

Bhunia and his co-author studied 25 strains of Listeria​, including some that specifically have caused outbreaks of human illness.

"This is the first comprehensive study in which this many strains of​ Listeria were tested for all three infection aspects - adhesion, invasion and translocation," Bhunia said. "We didn't find any direct relationship between adhesion and invasion, adhesion is needed but is not an indicator of infection."​

In the past two decades, four types of Listeria monocytogenes​ have been responsible for 90 per cent of the outbreaks of the illness they cause, listeriosis, according to the CDC.

The researchers introduced Listeria​ to human intestinal cells in laboratory dishes and to mice to determine how tightly the bacteria bound to cells, how quickly they invade cells and to what organ they spread.

The lack of direct relationship between the binding of the bacteria to the cell, like a key in a lock, and how highly invasive it is, means that both factors much be evaluated for each type of Listeria, said Bhunia. Though one type may bind tightly to the cells, it may not be able to enter the cell in a way to cause illness. Conversely some strains may not adhere to the cell as well but may be highly invasive and extremely harmful.

"We also found that the strains that had caused previous outbreaks in humans were highly invasive of cells and then translocated rapidly to the brain,"​ added Bhunia.

"This study gave us a good idea of how different Listeria strains bind to intestinal cells and how that relates to infection,"​ he said. "If we understand enough of the mechanism of bacterial adhesion to cells before it actually causes damage or becomes systemic, then maybe we could come up with a strategy to prevent the illness."​

Full findings are published in the June issue of Applied and Environmental Microbiology.​