This is a scanning electron micrograph of Listeria monocytogenes.

Listeria bacterium found in food, which can infect people and cause temporary gastro-intestinal distress, is a serious health risk for pregnant women and for people with compromised immune systems. According to a dissertation from Umea University in Sweden, the bacterium, which sometimes causes the lethal illness listeriosis, reacts to light by activating defense mechanisms.

Listeria monocytogenes, named after the British surgeon Joseph Lister, is ubiquitous in nature but can sometimes spread to food, especially to unpasteurized dairy products and charcuterie. The Listeria bacterium can grow in food stored in the fridge, and if contaminated food is consumed without being properly heated, the bacterium can cause infection.

Researchers have now discovered a new property in Listeria; namely that the bacterium activates protective mechanisms when exposed to light. This discovery can, in future, be used by the food industry to prevent the spread of Listeria.

In the dissertation, doctoral student Christopher Andersson also describes the discovery of two new molecules that combat the pathogenicity of the Listeria bacterium. The researchers also studied how the molecules can be used to prevent the bacterium from causing disease.

For healthy individuals, the Listeria bacterium usually causes no extreme harm apart from a few days of stomach problems. For individuals with a compromised immune system or for pregnant women, however, the bacterium can be very dangerous. If a bacterial infection spreads to the brain it can progress to listeriosis, which has a mortality rate of 20-30 percent. If a pregnant woman is infected, the bacteria can spread to the fetus and cause miscarriage.

"Hopefully, this new knowledge on how light and these small molecules affect the bacterium can, in future, be used to prevent the spread of Listeria and help treat listeriosis," says Andersson, a doctoral student at the Department of Molecular Biology at Umea University and author of the dissertation. 