Expression of Virulence-Related Genes in Listeria monocytogenes Grown on Danish Hard Cheese as Affected by NaCl Content

Investigating Transcriptomic Induction of Resistance and/or Virulence in Listeria monocytogenes Cells Surviving Sublethal Antimicrobial Exposure

The potential transcriptomic induction of resistance and/or virulence in two L. monocytogenes strains belonging to the most frequent listeriosis-associated serovars (i.e., 1/2a and 4b), following their sublethal antimicrobial exposure, was studied through qPCR determination of the relative expression of 10 selected related genes (i.e., groEL, hly, iap, inlA, inlB, lisK, mdrD, mdrL, prfA, and sigB). To induce sublethal stress, three common antimicrobials (i.e., benzalkonium chloride, thymol, and ampicillin) were individually applied for 2 h at 37 °C against stationary phase cells of each strain, each at a sublethal concentration. In general, the expression of most of the studied genes remained either stable or was significantly downregulated following the antimicrobial exposure, with some strain-specific differences to be yet recorded. Thymol provoked downregulation of most of the studied genes, significantly limiting the expression of 6/10 and 4/10 genes in the strains of ser. 1/2a and ser. 4b, respectively, including those coding for the master regulators of stress response and virulence (SigB and PrfA, respectively), in both strains. At the same time, the two genes coding for the invasion internalin proteins (InlA and InlB), with crucial role in the onset of L. monocytogenes pathogenesis, were both importantly upregulated in ser. 4b strain. The results obtained increase our knowledge of the stress physiology of L. monocytogenes under certain sublethal antimicrobial conditions that could be encountered within the food chain and in clinical settings, and may assist in better and more effective mitigation strategies.

Effect of Temperatures Used in Food Storage on Duration of Heat Stress Induced Invasiveness of L. monocytogenes

The unpropitious conditions of the food processing environmenttrigger in Listeria monocytogenes stress response mechanisms that may affect the pathogen's virulence. To date, many studies have revealed that acid, osmotic, heat, cold and oxidative stress modify invasiveness of L. monocytogenes. Nonetheless, there is limited data on the duration of the stress effect on bacterial invasiveness. Since most food is stored at low or room temperatures we studied the impact of these temperatures on the duration of heat stress effect on invasiveness of 8 L. monocytogenes strains. Bacteria were heat-treated for 20 min at 54 °C and then incubated at 5 and 20 °C up to 14 days. A decrease in invasiveness over time was observed for bacteria not exposed to heating. It was found that heat shock significantly reduced the invasion capacity of all strains and the effect lasted between 7 and 14 days at both 5 and 20 °C. In conclusion, 20-min heating reduces invasion capacity of all L. monocytogenes strains; however, the stress effect is temporary and lasts between 7 and 14 days in the food storage conditions. The invasiveness of bacteria changes along with the incubation time and is temperature-dependent.

Complete genome sequence of bile-isolated Enterococcus avium strain 352

Background

Enterococcus avium is a Gram-positive pathogenic bacterium belonging to the family Enterobacteriaceae. E. avium can cause bacteremia, peritonitis, and intracranial suppurative infection. However, the mechanism of its pathogenesis and its adaptation to a special niche is still unclear.

Results

In this study, the E. avium strain 352 was isolated from human bile and whole genome sequencing was performed. The E. avium strain 352 consists of a circular 4,794,392 bp chromosome as well as an 87,705 bp plasmid. The GC content of the chromosome is 38.98%. There are 4905 and 99 protein coding sequences in the chromosome and the plasmid, respectively. The genome of the E. avium strain 352 contains number of genes reported to be associated with bile adaption, including bsh, sbcC, mutS, nifI, galU, and hupB. There are also several virulence-associated genes including esp, fss1, fss3, ecbA, bsh, lap, clpC, clpE, and clpP.

Conclusions

This study demonstrates the presence of various virulence factors of the E. avium strain 352, which has the potential to cause infections. Moreover, the genes involved in bile adaption might contribute to its ability to live in bile. Further comparative genomic studies would help to elucidate the evolution of pathogenesis of E. avium.

Use of Two-Dimensional Electrophoresis to Explore Foodborne Bacterial Pathogen Responses to Gastrointestinal Stress

Proteomics was applied here to study Listeria monocytogenes response to gastrointestinal stress. It separated extracted proteins by their isoelectric point (pI) in the first dimension followed by separation by molecular weight in the second dimension on a polyacrylamide gel. L. monocytogenes was grown in an appropriate culture medium after which it was transferred to a simulated cheese medium for 2 h. Bacteria were exposed to gastric stress using artificial saliva and gastric fluid for 5 min and 2 h, respectively. After each step samples were taken for protein extraction and a two-dimensional electrophoresis approach. Proteins were separated on 18 cm Immobiline DryStrip gels with a pH range of 4-7 and the protein pattern analyzed.

Global transcriptomic response of Listeria monocytogenes during growth on cantaloupe slices

Understanding a pathogen's response to food environments is imperative to develop effective control strategies as well as to elucidate the impact of foods on virulence potential. The purpose of this study was to assess transcriptional response of Listeria monocytogenes after growth in cantaloupe, as well as its impact on survival in synthetic gastric fluid (SGF). The transcriptional profiles of L. monocytogenes grown in cantaloupe or Brain Heart Infusion (BHI) under refrigeration were compared by a custom-designed microarray. A total of 286 and 175 genes were significantly up- and down-regulated, respectively, in L. monocytogenes grown in cantaloupe as compared to BHI (fold change ≥ 2.5 and adj. P < 0.05). The majority of upregulated genes belonged to functions related to amino acid and nucleotide metabolism, flagellar biosynthesis, and iron acquisition, while most downregulated genes belonged to carbohydrate metabolism. Notably, the branched chain amino acid (BCAA: leucine, isoleucine, valine) biosynthesis operon was shown to be highly upregulated as well as the purine and pyrimidine biosynthesis pathways. Transcript levels of several stress- and virulence-related genes were significantly altered, implying an impact of growth in cantaloupe on the virulence potential of L. monocytogenes. Enhanced survival of L. monocytogenes in SGF following growth in cantaloupe further demonstrated the impact of cantaloupe-associated growth on the pathogen's subsequent response to a host relevant stress.