Xiaojie Qin

Salmonella Typhimurium monophasic variant (STm) has emerged as a novel predominant serovar in global foodborne Salmonella outbreaks. In our previous study, the two-component regulator OmpR was significantly upregulated in STm under disinfectant stress. However, effects of ompR deletion on stress tolerance and pathogenicity of this pathogen remained unclear. Therefore, this study aimed to systematically investigate the function of ompR in the adaptation of STm to food/food processing environments (e.g., disinfectants, thermal treatment, and high-salt preservation) and virulence. An isogenic ΔompR mutant was constructed using homologous recombination, and its phenotypic changes relative to the wild-type strain were evaluated, including bacterial growth, motility, stress resistance (thermal, osmotic, acid, and oxidative stress), and in vivo virulence in ICR mice. The results revealed that the deletion of ompR significantly decreased bacterial tolerance to multiple tested stresses, including thermal (50 °C, 55 °C, 60 °C), osmotic (10%, 20%, 30% NaCl), acid (pH = 3, 4, 5), and oxidative (1 mM, 2 mM, 5 mM H₂O₂) stresses. Additionally, the ΔompR mutant exhibited markedly reduced swimming and swarming motility compared to the wild-type strain. In the ICR mice infection model, the ΔompR mutant showed significantly attenuated virulence (100% survival rate within 7 days post-infection), with significantly lower bacterial loads in the liver, spleen and intestine. Transcriptome analysis showed that compared with the wild-type strain, the ΔompR mutant strain exhibited significant upregulation of fimbriae-related genes and downregulation of flagellar biosynthesis and virulence-related genes, thereby affecting bacterial adhesion, biofilm formation, flagellar assembly and virulence expression. Collectively, these findings suggest that ompR contributes to the adaptation of STm to food/food processing environments and virulence, and this gene may provide potential targets for future research to control STm contamination in food production systems.

Food microbial contamination results in serious food safety issues and numerous food loss and waste, presenting one of the most significant challenges facing the global food system. Photodynamic inactivation (PDI) technology, which combines light and photosensitizers (PS) to provide antimicrobial effects, is an ideal nonthermal antimicrobial technique for the food industry. This review provides a comprehensive overview of PDI technology, beginning with the fundamental photoactivation principles of PS and the pathways of photoinduced reactive oxygen species (ROS) generation. PS is the most critical factor affecting PDI efficiency, which is categorized into three types: organic, metal oxide‐, and carbon‐based. This review systemically summarizes the photophysical properties, in vitro PDI performances, potential enhancement strategies, and the advantages and limitations of each type of PS. Furthermore, the antimicrobial mechanisms of the PDI technologies are analyzed at both microscopic and molecular levels. Finally, the current applications of PDI in various food systems are discussed, along with the associated challenges and opportunities. Overall, this review offers crucial insights into optimizing and advancing PDI technology, highlighting key challenges and suggesting future research directions to enhance the effectiveness and scalability of PDI for diverse food applications.

Listeria monocytogenes is one of the most important foodborne pathogens that can cause invasive listeriosis. In this study, the virulence levels of 26 strains of L. monocytogenes isolated from food and clinical samples in Shanghai, China, between 2020 and 2022 were analyzed. There were significant differences among isolates in terms of their mortality rate in Galleria mellonella, cytotoxicity to JEG-3 cells, hemolytic activity, and expression of important virulence genes. Compared with other STs, both the ST121 (food source) and ST1930 (clinic source) strains exhibited higher G. mellonella mortality. The 48 h mortality in G. mellonella of lineage II strains was significantly higher than that in lineage I. Compared with other STs, ST1930, ST3, ST5, and ST1032 exhibited higher cytotoxicity to JEG-3 cells. Based on the classification of sources (food and clinical strains) and serogroups (II a, II b, and II c), there were no significant differences observed in terms of G. mellonella mortality, cytotoxicity, and hemolytic activity. In addition, ST121 exhibited significantly higher hly, inlA, inlB, prfA, plcA, and plcB gene expression compared with other STs. A gray relation analysis showed a high correlation between the toxicity of G. mellonella and the expression of the hly and inlB genes; in addition, L. monocytogenes may have a consistent virulence mechanism involving hemolysis activity and cytotoxicity. Through the integration of in vivo and in vitro infection models with information on the expression of virulence factor genes, the differences in virulence between strains or subtypes can be better understood.

Cronobacter sakazakii may encounter subinhibitory concentrations of ethanol stress over its lifecycle. The bacterial tolerance to homologous or heterologous stressing agents may alter as a result of ethanol adaptive responses. Therefore, the tolerance of ethanol-exposed and control cells to subsequent lethal stresses was evaluated in the current work. It was discovered that sublethal ethanol exposure increased the susceptibility to lethal ethanol stress in C. sakazakii as determined by the Weibull model. Furthermore, sublethal ethanol concentration exposure in C. sakazakii did not lead to any cross-tolerance against other stressors such as benzalkonium chloride (BC, 120 mg/L), heat (55 ℃), cold (4 ℃), simulated gastric fluid (SGF, pH 3.0), osmotic stress (sorbitol, 0.75 g/mL), and desiccation stress. Analysis of zeta potential, scanning electron microscope (SEM), and Surface-enhanced Raman spectroscopy (SERS) spectra revealed that cellular injury and changes in cellular chemical composition may contribute to the reduced resistance of C. sakazakii after ethanol exposure. Furthermore, sublethal ethanol exposure resulted in an elevated proportion of unsaturated fatty acids (USFA), while reducing the proportion of saturated fatty acids (SFA) and the ratio of SFA/USFA. The developed inactivation models can serve as a valuable source of data to support quantitative microbial risk assessment. Moreover, a better understanding of the response of C. sakazakii to sublethal ethanol exposure may provide valuable insights into the prevention and control of C. sakazakii.

Disinfectants such as benzalkonium chloride (BC) extensively used in animal farms and food-processing industries contribute to the development of adaptive- and cross-resistance in foodborne pathogens, posing a serious threat to food safety and human health. The purpose of this study is to explore whether continuous exposure of Salmonella enterica serovar 1,4, [5],12: i:- (S. 1,4, [5],12: i:-) to sublethal concentrations of BC could result in acquired resistance to this agent and other environmental stresses (e.g. antibiotics, heat, and acid). BC tolerance increased in all tested strains after exposure to gradually increasing concentrations of BC, with increases in minimum inhibitory concentrations between two and six-fold. The survival rate of BC-adapted strains was significantly (P < 0.05) higher than that of their wild-type (non-adapted) counterparts in lethal concentrations of BC. In addition, significant reductions (P < 0.05) in zeta potential were observed in BC-adapted strains compared to wild-type ones, indicating that reduction in cell surface charge was a cause of adaptative resistance. More importantly, two BC-adapted strains exhibited increased antibiotic resistance to levofloxacin, ceftazidime, and tigecycline, while gene mutations (gyrA, parC) and antibiotic efflux-related genes (acrB, mdsA, mdsB) were detected by genomic sequencing analysis. Moreover, the tolerance of BC-adapted strains to heat (50, 55, and 60 °C) and acid (pH 2.0, 2.5) was strain-dependent and condition-dependent. Repeated exposure to sublethal concentrations of BC could result in the emergence of BC- and antibiotic-resistant S. 1,4, [5],12: i:- strains.