Researchers from the Institute of Milk Hygiene at Vetmeduni Viennashowed that two of the pathogen’s genes form a functional unit that ensures the bacteria’s survival.
They identified a novel stress survival islet 2 (SSI-2), predominantly present in L. monocytogenes ST121 strains, that increases the bacteria’s survival under alkaline and oxidative stress conditions, potentially supporting adaptation and persistence in food processing environments.
Sequence type almost exclusive to food processing environments
L. monocytogenes strains of ST121 are known to persist in food processing environments and increase the risk of contamination and listeriosis. However, the molecular mechanism of this niche-specific adaptation is unknown.
The key to the bacteria’s survival is its adaptability and persistence in stress situations.
Hypervariable, or easily changeable, regions of the genome contain genetic inserts that help Listeria survive.
“These inserts include a unit of gene sequences, the stress survival islet 1 (SSI-1), a ‘genomic island’ that helps the microorganisms to survive certain stress situations,” said first author Eva Harter.
“Depending on the specific bacterial strain, this region accommodates one of three different gene sequences whose function, with the exception of SSI-1, had so far not been known.”
The genes in the stress survival islet 1 give the bacteria a high tolerance toward acidic, bile, salt and gastric stresses and were characterized years ago.
However, this does not explain how it can survive hygiene standards in the food processing industry which involves alkaline and oxidative stress situations.
Expanding on previous knowledge
Researchers concentrated on two neighbouring gene sequences within the same hypervariable region and identified these as the genomic islet that acts as a lifesaver for certain L. monocytogenes strains.
Expression of the two genes and of the proteins which they code is increased during alkaline and oxidative stress so they must have a different function than the genes belonging to the SSI-1.
Harter said the team was able to assign a function to the two genes.
“The first gene is a transcriptional regulator, which in certain situations regulates the frequency and activity of the second protein. The second is a protease, an enzyme that breaks down other proteins. Proteases help bacteria break down unfunctional proteins that are created during stress situations.”
Most Listeria strains found in clinical isolates do not have SSI-2.
Kathrin Rychli, study director, said if the regulator is not active, then there is no protease.
“Without the protease, Listeria monocytogenes has a harder time compensating oxidative stress. The two genes therefore make up a functional unit, specifically the stress survival islet SSI-2.
“Knowing the genetic mechanism, allows you to think about new strategies for food safety.”
“The novel stress survival islet 2 (SSI-2), predominantly present in Listeria monocytogenes strains of ST121, is involved in alkaline and oxidative stress response”
Authors: Eva Harter, Eva Maria Wagner, Andreas Zaiser, Sabrina Halecker, Martin Wagner and Kathrin Rychli