Characterization of damage on Listeria innocua surviving to pulsed light: Effect on growth, DNA and proteome

Transcriptomic analysis reveals the inhibition mechanism of pulsed light on fungal growth and ochratoxin A biosynthesis in Aspergillus carbonarius

Pulsed light (PL) technology has a good effect on the control of fungi in postharvest fruit. In this present work, PL inhibited the growth of Aspergillus carbonarius in a dose-dependent manner, the mycelial growth decreased by 4.83 %, 13.91 % and 30.01 % at a fluence of 4.5 J·cm^-2 (PL5), 9 J·cm^-2 (PL10) and 13.5 J·cm² (PL15), respectively. When inoculated with PL15 treated A. carbonarius, the scab diameter of the pears, ergosterol and OTA content was reduced by 23.2 %, 27.9 % and 80.7 % after 7 days, respectively. The third-generation sequencing technique was applied to study the transcriptome response of A. carbonarius treated with PL. Compared with the blank control, a total number of 268 and 963 differentially expressed genes (DEGs) were discovered in the group of PL10 and PL15, respectively. To be specific, a large amount of DEGs involved in DNA metabolism were up-regulated, while most of DEGs related to cell integrity, energy and glucose metabolism, ochratoxin A (OTA) biosynthesis and transport were down-regulated. In addition, the stress response of A. carbonarius was imbalanced, including up-regulation of Catalase and PEX12 and down-regulation of taurine and subtaurine metabolism, alcohol dehydrogenase and glutathione metabolism. Meanwhile, the results of transmission electron microscopy, mycelium cellular leakage and DNA electrophoresis indicated that PL15 treatment caused mitochondrial swelling, the destroyed cell membrane permeability and imbalance of DNA metabolism. The expression of P450 and Hal involved in OTA biosynthesis pathway were down-regulated in PL treated samples detected by qRT-PCR. In conclusion, this study reveals the molecular mechanism of pulsed light on inhibiting the growth, development and toxin production of A. carbonarius.

Modelling the Inactivation and Possible Regrowth of Salmonella enterica Treated with Chlorophyllin-Chitosan Complex and Visible Light

The study focuses on predictive modelling of inactivation of Salmonella enterica after treatment with chlorophyllin-chitosan complex and visible light. Salmonella cells were incubated with chlorophyllin-chitosan complex (0.001% chlorophyllin and 0.1% chitosan) for different times (5-60 min) and then illuminated with visible light (λ=405 nm, H_e=38 J/cm²). Inactivation curves and post-treatment regrowth curves were built based on microbiological viability tests and data were fitted to ten inactivation and two regrowth models. The photoactivated complex reduced Salmonella population, which were unable to regrow. Weibull and Baranyi models were the best to describe the inactivation and regrowth kinetics respectively. In conclusion, data from the kinetic analysis and predictive modelling confirmed that photoactivated chlorophyllin-chitosan complex is a promising non-thermal approach for inactivation of Gram-negative pathogens, since no bacterial regrowth after treatment has been predicted.

Microbial lag phase can be indicative of, or independent from, cellular stress

Measures of microbial growth, used as indicators of cellular stress, are sometimes quantified at a single time-point. In reality, these measurements are compound representations of length of lag, exponential growth-rate, and other factors. Here, we investigate whether length of lag phase can act as a proxy for stress, using  a number of model systems (Aspergillus penicillioides; Bacillus subtilis; Escherichia coli; Eurotium amstelodami, E. echinulatum, E. halophilicum, and E. repens; Mrakia frigida; Saccharomyces cerevisiae; Xerochrysium xerophilum; Xeromyces bisporus) exposed to mechanistically distinct types of cellular stress including low water activity, other solute-induced stresses, and dehydration-rehydration cycles. Lag phase was neither proportional to germination rate for X. bisporus (FRR3443) in glycerol-supplemented media (r2 = 0.012), nor to exponential growth-rates for other microbes. In some cases, growth-rates varied greatly with stressor concentration even when lag remained constant. By contrast, there were strong correlations for B. subtilis in media supplemented with polyethylene-glycol 6000 or 600 (r2 = 0.925 and 0.961), and for other microbial species. We also  analysed data from independent studies of food-spoilage fungi under glycerol stress (Aspergillus aculeatinus and A. sclerotiicarbonarius); mesophilic/psychrotolerant bacteria under diverse, solute-induced stresses (Brochothrix thermosphacta, Enterococcus faecalis, Pseudomonas fluorescens, Salmonella typhimurium, Staphylococcus aureus); and fungal enzymes under acid-stress (Terfezia claveryi lipoxygenase and Agaricus bisporus tyrosinase). These datasets also exhibited diversity, with some strong- and moderate correlations between length of lag and exponential growth-rates; and sometimes none. In conclusion, lag phase is not  a reliable measure of stress because length of lag and growth-rate inhibition are sometimes highly correlated, and sometimes not at all.

A Proteogenomic Resource Enabling Integrated Analysis of Listeria Genotype-Proteotype-Phenotype Relationships

Listeria monocytogenes is an opportunistic foodborne pathogen responsible for listeriosis, a potentially fatal foodborne disease. Many different Listeria strains and serotypes exist, but a proteogenomic resource that bridges the gap in our molecular understanding of the relationships between the Listeria genotypes and phenotypes via proteotypes is still missing. Here, we devised a next-generation proteogenomics strategy that enables the community to rapidly proteotype Listeria strains and relate this information back to the genotype. Based on sequencing and de novo assembly of the two most commonly used Listeria model strains, EGD-e and ScottA, we established two comprehensive Listeria proteogenomic databases. A genome comparison established core- and strain-specific genes potentially responsible for virulence differences. Next, we established a DIA/SWATH-based proteotyping strategy, including a new and robust sample preparation workflow, that enables the reproducible, sensitive, and relative quantitative measurement of Listeria proteotypes. This reusable and publicly available DIA/SWATH library covers 70% of open reading frames of Listeria and represents the most extensive spectral library for Listeria proteotype analysis to date. We used these two new resources to investigate the Listeria proteotype in states mimicking the upper gastrointestinal passage. Exposure of Listeria to bile salts at 37 °C, which simulates conditions encountered in the duodenum, showed significant proteotype perturbations including an increase of FlaA, the structural protein of flagella. Given that Listeria is known to lose its flagella above 30 °C, this was an unexpected finding. The formation of flagella, which might have implications on infectivity, was validated by parallel reaction monitoring and light and scanning electron microscopy. flaA transcript levels did not change significantly upon exposure to bile salts at 37 °C, suggesting regulation at the post-transcriptional level. Together, these analyses provide a comprehensive proteogenomic resource and toolbox for the Listeria community enabling the analysis of Listeria genotype-proteotype-phenotype relationships.