NEWS Jörgen Johansson, professor of molecular biology at Umeå University, has studied the Listeria bacteria for 25 years. He is following the outbreak at a top restaurant in Stockholm this week with great interest. We ask him five curious questions:
Listeria can grow on agar plates at incredibly low temperatures.
ImageEva-Maria DiehlFor a healthy individual, it takes an incredible number of bacteria – several billion! – to get really sick
“The outbreak is serious because it has affected so many healthy people in a short period of time. Normally, people who have some form of weakened immune system are affected, such as pregnant women, the elderly and cancer patients. It is also remarkable that it has happened so quickly, from some of those involved, getting the infection to becoming seriously ill."
Jörgen Johansson is professor at the Department of Molecular Biology at Umeå University.
Imageprivat“Listeria is a bit like Dr. Jekyll and Mr. Hyde, as it can live in the soil and multiply without any problems. Sometimes, however, it can enter the food chain, usually through dairy production or in fishery products, and then become an extremely aggressive and "successful" pathogen that can have a mortality rate of up to 30 percent.”
“Listeria is probably one of the bacteria that we know best about how it causes disease and how it interacts with the human cell. The bacterium enters the body by eating food contaminated with Listeria and enters the intestinal epithelial layer.”
“For a healthy individual, it takes an incredible number of bacteria – several billion! – to get really sick. In those cases, the bacteria make their way from the epithelial cells into the bloodstream where they usually spread to the liver and spleen and multiply. In a healthy individual, the immune system takes care of the bacteria, but if you have a weakened immune system, the bacteria can spread into the blood and cause sepsis or get into the brain and cause meningitis. In pregnant women, the bacteria can enter through the amniotic sac to the fetus. The later stages of the infection have a mortality rate of up to 30 percent.”
“Listeria can grow at incredibly low temperatures. In my lab, for example, we have grown the bacteria on agar plates that we have placed on ice. Even if you buy a food with few Listeria bacteria, its ability to grow at low temperatures means that after a week or so you can have enormous amounts of bacteria in the food. In addition, it can withstand very high salt concentrations. I usually use Paris as an example where I was a postdoctoral fellow at the Pasteur Institute. Just a few hundred meters away, on certain days you can buy cheese at a food market that is made from unpasteurized cheese. Even if there are few bacteria when you buy it, there can be an incredible number of bacteria if you take it home and eat it after a few weeks.”
“Listeria is still (mostly) susceptible to antibiotics. However, it can be good to be prepared for the fact that it will at some point become antibiotic resistant, which is true for all pathogenic bacteria.”
Listeria infection is caused by the bacterium Listeria monocytogenes, which is widely found in nature. The bacterium is found in soil, water, and the intestines of many mammals, including humans. Humans are primarily infected by consuming contaminated food. The bacterium causes the infection listeriosis in humans. In Sweden, between 70 and just over 100 cases are reported annually.
