Background
Internalins are surface proteins that are utilized by
Listeria monocytogenes
to facilitate its invasion into human intestinal epithelial cells. The expression of a full-length InlA is one of essential virulence factors for
L. monocytogenes
to cross the intestinal barrier in order to invade epithelial cells.
Results
In this study, the gene sequences of inlA in 120
L. monocytogenes
isolates from food (n = 107) and humans (n = 13) were analyzed. Premature stop codon (PMSC) mutations in
inlA
were identified in 51 isolates (50 from food and 1 from human). Six mutation types of PMSCs were identified. Among the 51 isolates with PMSCs in
inlA
, there were 44 serogroup 1/2c, 3c isolates from food, of which seven belonged to serogroups 1/2a, 3a. A total of 153,382 SNPs in 2247 core genes from 42 genomes were identified and used to construct a phylogenetic tree. Serotype 1/2c isolates with inlA PMSC mutations were grouped together. Cell culture studies on 21 isolates showed that the invasion to Caco-2 cells was significantly reduced among isolates with inlA PMSC mutations compared to those without PMSC mutations (P < 0.01). The PMSC mutations in
inlA
correlated with the inability of the
L. monocytogenes
isolates to invade Caco-2 cells (Pearson’s coefficient 0.927,
P
< 0.01).
Conclusion
Overall, the study has revealed the reduced ability of
L. monocytogenes
to invade human intestinal epithelial cells in vitro was linked to the presence of PMSC mutations in
inlA
. Isolates with PMSC mutations shared the same genomic characteristics indicating the genetic basis on the potential virulence of
L. monocytogenes
invasion.
Electronic supplementary material
The online version of this article (10.1186/s13099-019-0307-8) contains supplementary material, which is available to authorized users.
Terrapins and turtles are known to transmit Salmonella to humans. However, little was known about the occurrence of this pathogen in soft-shelled terrapin that is a popular delicacy in Chinese and other East Asian cuisines. We isolated and characterized 82 (24.4%) isolates of Salmonella from 336 fecal samples of soft-shelled terrapins (51 of 172; 29.7%) and pet turtles (31 of 164; 18.9%) in Shanghai. Salmonella Thompson was the most common serotype (17.1%) among others. Many isolates (84.1%) were resistant to multiple antimicrobials (≥3). Molecular analysis of Salmonella Thompson and Salmonella Typhimurium using pulsed-field gel electrophoresis unveiled a close genetic relationship between several human and terrapin isolates. Our results highlight the risk associated with the handling and consumption of turtles and their role in the spread of Salmonella in the human salmonellosis.
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