Scientists from the University of Veterinary Medicine, Vienna analysed the genomes of the outbreak strains and showed they displayed distinct properties and entered the food chain independently,
They said that the most likely explanation is the outbreak represents two overlapping incidents caused by separate contamination events in the same food processing plant.
In 2009 and 2010 two different strains of Listeria monocytogenes were found in the traditional Austrian curd cheese known as “Quargel”. 34 people were infected and eight patients died.
Scientists sequenced and analysed the genomes of both strains, and assessed their virulence with samples taken from listeriosis patients.
A dairy in Hartberg (Styria, Austria) produced contaminated Quargel cheese leading to a multinational listeriosis outbreak in Austria, Germany and the Czech Republic, forcing the dairy to shut down.
The institute was involved in investigating the causes of the outbreaks and said genomes of the two strains (QOC1 and QOC2) were highly similar, but displayed distinct properties.
“The QOC1 genome is approximately 74 kbp larger than the QOC2 genome. In addition, the strains harbour 93 (QOC1) and 45 (QOC2) genes encoding strain-specific proteins,” said the researchers.
“Consistently, QOC1 shows higher resistance to acidic, alkaline and gastric stress.”
Virulence of the two events
The first contamination event from June 2009 to January 2010 was attributed to one L. monocytogenesstrain very effective at infecting epithelial cells of the intestine and liver cells.
It contained four virulence genes, making it invasive, and caused 14 cases resulting in five deaths.
In December 2009, the second strain emerged. It was successful at infecting macrophages, important immune system cells.
The highly infectious second strain replaced the first and by February 2010 had infected 20 people, and caused three deaths. The average age of those taken ill was 70.
Stephan Schmitz-Esser, co-author, said cleanliness in production is vital.
“It is absolutely essential that appropriate disinfectants are used properly, lots of salt, and that possible food for the bacteria be limited. Any products listeria is found in must be recalled immediately. Recalls are very expensive for producers, and we need to do everything we can to prevent them.”
Source: PLoS ONE
Online ahead of print, DOI: 10.1371/journal.pone.0089964
“Genome Sequencing of Listeria monocytogenes “Quargel” Listeriosis Outbreak Strains Reveals Two Different Strains with Distinct In Vitro Virulence Potential”
Authors: Kathrin Rychli, Anneliese Müller, Andreas Zaiser, Dagmar Schoder, Franz Allerberger, Martin Wagner and Stephan Schmitz-Esser