JMM Editor's Choice: hypervirulence in Listeria

Posted on November 5, 2019   by Microbiology Society

The Journal of Medical Microbiology (JMM) is a journal published by the Microbiology Society, focused on providing comprehensive coverage of medical, dental and veterinary microbiology and infectious diseases, including bacteriology, virology, mycology and parasitology. This month, Dr Rikke Meyer has selected an outstanding paper from the October issue to highlight as Editor's Choice. The paper, titled 'Evidence of hypervirulence in Listeria monocytogenes clonal complex 14' discusses how different isolates of the bacterium Listeria are capable of causing more severe disease.

Listeria monocytogenes is a foodborne pathogen that causes listeriosis in humans and animals, resulting in a number of clinical outcomes, including bacteremia, central nervous system infections, maternal-neonatal infections, mastitis, and gastroenteritis. L. monocytogenes isolates belong to four phylogenetic lineages, which are sub-divided into clonal complexes and sequence types. There are great differences in virulence and clinical outcome of infections caused by different L. monocytogenes strains. Being able to pinpoint why different clonal complexes are more prevalent in human infections, or why they lead to a specific clinical outcome, is an important step towards managing the transmission and treatment of these pathogens.
In this study, Cardenas-Alvarez and colleagues compared the virulence potential of L. monocytogenes stains isolated from humans, ruminants, or non-clinical sources using the larvae Gallaria mellonella as a model organism. They chose this model due to its immunological similarities with mammals.
Their research found that isolates from clonal complex 14 (CC14) and isolates capable of maternal-neonatal infections were hypervirulent. The bacteria increased in numbers during the early phase of the infection, indicating that they grow and disseminate in the host without being detected by the immune system. While most cases of listeriosis in humans is caused by strains from Lineage I, the hypervirulent strains of CC14 belong to Lineage II. And, like other members of Lineage II, the strains from CC14 can be found in the environment and diverse sources of food and feed. Discovery of hypervirulent clonal complexes from this lineage is an important basis for identifying the underlying genetic features responsible for virulence and different clinical outcomes.

Evidence of hypervirulence in Listeria monocytogenes clonal complex 14

Listeria monocytogenes is a leading cause of severe foodborne disease. Currently, all L. monocytogenes isolates are considered equally virulent by surveillance programmes, however recent epidemiological data shows certain subgroups of isolates with a higher frequency in listeriosis cases, which could be attributed to increased invasiveness and virulence.

Here, differences in virulence among L. monocytogenes subgroups were evaluated in an insect model. Findings showed that isolates belonging to specific subgroups are hypervirulent, as they need a lower bacterial concentration to cause disease and do not elicit significant host cell damage early in an infection, indicating potential to evade host immune responses.

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Maria X. Cardenas-Alvarez
Sahar Malekmohammadi
North Dakota State University Department of Microbiology