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Abstract
The facultative intracellular pathogen Listeria monocytogenes is of major veterinary importance in small ruminants. Nevertheless, details of L. monocytogenes interactions with cells of small ruminants are not fully established. To study the potential of L. monocytogenes to infect sheep cells, we used the finite sheep kidney cell line (shKEC), which was infected with the wild-type L. monocytogenes strain EGDe. The invasion efficiency was 0.015 ± 0.004%. The invasion factor InlB was critically important for invasion, and inlB gene deletion almost prevented L. monocytogenes invasion into shKEC cells. Comparison of the potential of phylogenetically defined InlB isoforms to restore the invasive phenotype of the EGDeΔinlB strain demonstrated that although all InlB isoforms restored invasion of the EGDeΔinlB strain into shKEC cells, the InlB isoforms typical of highly virulent ruminant strains of the clonal complexes CC1 and CC7 were more efficient than isoforms typical of CC2 and CC9 strains (which are less virulent toward ruminants) in supporting invasion. Listeria monocytogenes effectively multiplied with a doubling of time in about 90 min after they entered the sheep cells. Intracellular bacteria moved using the well-known actin polymerization mechanism. Cell-to-cell spreading was restricted to the infection of a few tens of neighboring cells for 7 days. Overall, the obtained results demonstrated that (i) InlB is required for invasion into sheep cells, (ii) InlB isoforms might be important for hypervirulence of certain clonal groups toward ruminants, and (iii) L. monocytogenes effectively multiplies in ovine cells once entered.
Highlights
The Gram-positive bacterium Listeria monocytogenes causes a serious foodborne disease, listeriosis, in humans and farm animals (Vázquez-Boland et al, 2001; Nightingale et al, 2004b; Allerberger and Wagner, 2010; Walland et al, 2015)
To resolve the question of whether L. monocytogenes could move from cell to cell, we studied infected sheep kidney cell line (shKEC) monolayers by using light microscopy
Factors that improve active invasion or intracellular multiplication in cells of a certain mammalian species will increase the susceptibility of this species to L. monocytogenes infection
Summary
The Gram-positive bacterium Listeria monocytogenes causes a serious foodborne disease, listeriosis, in humans and farm animals (Vázquez-Boland et al, 2001; Nightingale et al, 2004b; Allerberger and Wagner, 2010; Walland et al, 2015). Mass testing showed that approximately 2% of domestic ruminants are Listeria positive in the EU, suggesting that asymptomatic carriage is often among ruminants (European Food Safety Authority and European Centre for Disease Prevention and Control (EFSA and ECDC), 2019). Both individual animal resistance and strain virulence are responsible for the selective development of the disease in a part of the infected flock. The clonal complexes CC7 and CC37, which are associated with abortion in ruminants, are described as intermediate virulence clones in humans (Charlier et al, 2017; Papić et al, 2019)
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