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.
"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."
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, he said. 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," Bhunia said.
Only extremely infective bacteria can invade the brain because most pathogens cannot get through the brain's protective layer, called the "blood brain barrier." Bhunia said these Listeria strains can enter the brain within 72 hours of when tainted food is eaten.
"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."