Researchers have detected a highly virulent new form of pathogenic Listeria monocytogenes.

It was identified as the cause of serious diseases in sheep in a remote area of the Chinese province Jiangsu.

An international research group led by the Institute of Medical Microbiology at the Justus Liebig University Giessen (JLU) said it had found the most virulent representatives of this bacterial species to date. The work was published in the journal Nature Communications.

In people the bacterium Listeria monocytogenes can cause an infection known as listeriosis. Contaminated food is a frequent source of the infections. Raw and processed foods can be contaminated, especially dairy products, meat, seafood and ready-to-eat products such as pre-packaged salads. Elderly people, those with a weakened immune system and pregnant women are particularly at risk of serious infections and life-threatening complications.

Listeria virulence
Professor Trinad Chakraborty, director of the Institute of Medical Microbiology at the JLU and research scientist at the German Center for Infection Research (DZIF), said the finding highlights the need for international collaboration

“Only by combining resources and expertise can we rapidly identify newly emerging threats to food safety from highly virulent strains worldwide,” according to the professor.

After decoding the genome sequence of these bacteria, scientists were able to determine the genetic basis for hypervirulence and identify the factors that enhance the ability of this Listeria strain to cause severe septic diseases.

“These isolates are unique in the sense that they combine the virulence characteristics of various highly pathogenic Listeria species that infect animals or humans into a single strain. Since listeriosis is a foodborne infection, measures to identify such highly virulent strains are extremely urgent,” said Chakraborty.

Examining the isolates
The foodborne pathogen Listeria monocytogenes has four evolutionarily distinct lineages. The team characterized isolates from severe ovine listeriosis outbreaks that represent a hybrid sub-lineage of the major lineage II (HSL-II) and serotype 4h. HSL-II isolates share core virulence factors found in isolates of other highly virulent Listeria monocytogenes lineages I and II, implying an association with human cases of listeriosis.

Isolates harbored the Listeria monocytogenes Pathogenicity Island (LIPI)-1 and a truncated LIPI-2 locus, encoding sphingomyelinase (SmcL), a virulence factor required for invasion and bacterial translocation from the gut, and other non-contiguous chromosomal segments from another pathogenic species, Listeria ivanovii. HSL-II isolates exhibit a unique wall teichoic acid (WTA) structure essential for resistance to antimicrobial peptides, bacterial invasion, and virulence.

Researchers studied isolates obtained during five years from different sources that harbor distinct genes from the species Listeria ivanovii. Isolates came from separate listeriosis outbreaks on goat farms in 2011, 2012, and 2015, in a remote region of Jiangsu Province, China.

Multi-locus sequence type (MLST) analysis revealed three isolates are members of an unidentified sequence type (ST) and clonal complex (CC). The isolates have been assigned as ST626 and CC33 by the Listeria MLST Pasteur database, and are the only ones in either category.

Scientists from the State Key Laboratory of Zoonosis of the University of Yangzhou in Jiangsu, China, the Laboratory of Food Microbiology of the Swiss Federal Institute of Technology (ETH) Zurich, Switzerland, and JLU, Germany were involved in the study.

Work at the JLU was funded by the EU ERA-NET PROANTILIS program. The study was also supported by the German Center for Infection Research.

In April 2018, a new bacterial species was described by investigators from the Costa Rican Institute of Technology and WHO-collaborating center on Listeria at the Pasteur Institute. The species, called Listeria costaricensis, is non-pathogenic and was isolated from water collected at an industrial drainage area in the Costa Rican province of Alajuela, after three years of sample collection and research.

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