A team of scientists from the University of Veterinary Medicine, Vienna have been analysing the genetics of two different strains of Listeria monocytogenes involved in a multinational outbreak of infection linked to a curd cheese.
The outbreak, in 2009 and 2010, affected 34 people in Austria, Germany and the Czech Republic, eight of whom died. It was associated with the consumption of Quargel curd cheese produced at a dairy in the town of Hartberg in Austria, which later ceased operations. Two distinct strains of Listeria monocytogenes serovar 1/2a were found to be involved in the outbreak, which consisted of two separate clusters of infection, but both strains were present in the contaminated cheese.
The genomes of both strains were sequenced and then analysed and found to be very similar, but they also showed some important differences. The first strain was found to contain genes for a number of virulence factors, making it very invasive for epithelial cells, while the second was able to infect important immune cells called macrophages and was extremely virulent. Both strains were responsible for deaths among those infected. The first strain was also found to be more resistant to acid stress, making it more likely to survive passage through the stomach.
The researchers conclude that the two strains were distinct and did not stem from a common ancestor, meaning that they probably entered the food chain independently. They point out that their findings illustrate the importance of good hygiene and cleanliness in food production.
The study is published in the journal PLOS ONE and can be found in full here.