Survival of viable but nonculturable Cronobacter sakazakii in macrophages contributes to infections

Autocrine peptides inhibited the formation of VBNC state of Staphylococcus aureus

Viable but non-culturable (VBNC) Staphylococcus aureus cannot form colonies on a medium, causing a false negative result in culture-based detection, which is a potential hazard to human health. In this study, four peptides (PVSS.a-1, PVSS.a-2, PVSS.a-3, and PVSS.a-4) were identified in the suspension of S. aureus during the VBNC state induction. Notably, PVSS.a-1 and PVSS.a-2 prolonged the entry of S. aureus into the VBNC state in citric acid solution (pH 4.0) at 4℃ by 83 % and 103 %, respectively. Such a delaying effect indicates that S. aureus might be forced to enter the VBNC state under pressure, rather than actively. Microscopic observation and zeta-potential determination suggested that PVSS.a-1 and PVSS.a-2 improved the aggregation of S. aureus cells. Furthermore, the two peptides were demonstrated to enter cells by FITC-label localization detection, and changed internal structures and improved intracellular enzyme activities occurred in the two peptide-treated cells. Through the analysis of interactions with DNA and proteins of S. aureus, it was found that PVSS.a-1 and PVSS.a-2 might affect cellular processes, including cell division, transcription, translation, and material and energy metabolisms. These alterations improved the viability and culturability of S. aureus, thereby delaying VBNC formation. In summary, our study reveals how autocrine peptides delay VBNC formation of S. aureus, and provides a new insight into the real intention of bacteria to form VBNC state under adverse conditions.

Global transcriptional analysis for molecular responses of Alicyclobacillus acidoterrestris spores in drinking water after low- and medium-pressure ultraviolet irradiation

Ultraviolet (UV) irradiation can effectively disinfect water contaminated with pathogens. However, the biological mechanisms of inactivation by different types of UV irradiation are unknown. The present study investigated the inactivation mechanisms of Alicyclobacillus acidoterrestris spores in water by low-pressure UV (LPUV) and medium-pressure UV (MPUV) using a quasi-collimated beam apparatus. Global transcriptomic data obtained by RNA-seq revealed 291 shared differentially expressed genes (DEGs) that damaged DNA, reduced biofilm formation, and had other reactions. The individual downregulated DEGs (n = 123) mainly related to cell motility, membrane transport, and metabolism were induced by LPUV, and in turn contributed to energy-saving and metabolic activity inhibition, forcing bacteria into a viable but non-culturable (VBNC) state. The individual upregulated DEGs (n = 244) following MPUV treatment were mainly enriched in cell motility, membrane transport, metabolism, DNA replication and repair, and spore germination pathways. This results in high-energy consumption, severe damage to genetic material, and enhanced spore germination accelerated cell death. Additionally, hub genes in the protein-protein interaction network were mainly involved in transcription and translation. These findings contribute to the comprehensive understanding of the inactivation mechanisms of different types of UV irradiation, and will improve applications of UV disinfection in the treatment of water.

VBNC Cronobacter sakazakii survives in macrophages by resisting oxidative stress and evading recognition by macrophages

Survival in host macrophages is an effective strategy for pathogenic bacterial transmission and pathogenesis. Our previous study found that viable but non-culturable (VBNC) Cronobacter Sakazakii (C. sakazakii) can survive in macrophages, but its survival mechanism is not clear. In this study, we investigated the possible mechanisms of VBNC C. sakazakii survival in macrophages in terms of environmental tolerance within macrophages and evasion of macrophages recognition. The results revealed that VBNC C. sakazakii survived under oxidative conditions at a higher rate than the culturable C. sakazakii. Moreover, the stringent response gene (relA and spoT) and the antioxidant-related genes (sodA, katG, and trxA) were up-regulated, indicating that VBNC C. sakazakii may regulate antioxidation through stringent response. On the other hand, compared with culturable C. sakazakii, VBNC C. sakazakii caused reduced response (Toll-like receptor 4) in macrophages, which was attributed to the suppression of biosynthesis of the lipopolysaccharides (LPS). Furthermore, we found that ellagic acid can reduce the survival rate of bacteria in macrophages by improving the immune TLR4 recognition ability of macrophages. In conclusion, VBNC C. sakazakii may survive in macrophages by regulating oxidative tolerance through stringent response and altering LPS synthesis to evade TLR4 recognition by macrophages, which suggests the pathogenic risk of VBNC C. sakazakii.

EnvZ/OmpR Controls Protein Expression and Modifications in Cronobacter sakazakii for Virulence and Environmental Resilience

Cronobacter sakazakii is a notorious foodborne opportunistic pathogen, particularly affecting vulnerable populations such as premature infants, and poses significant public health challenges. This study aimed to elucidate the role of the envZ/ompR genes in environmental tolerance, pathogenicity, and protein regulation of C. sakazakii. An envZ/ompR knockout mutant was constructed and assessed for its impact on bacterial growth, virulence, environmental tolerance, and protein regulation. Results demonstrate that deletion of envZ/ompR genes leads to reduced growth rate and attenuated virulence in animal models. Additionally, the knockout strain exhibited compromised environmental tolerance, particularly in desiccation and oxidative stress conditions, along with impaired adhesion and invasion abilities in epithelial cells. Proteomic analysis revealed significant alterations in protein expression and phosphorylation patterns, highlighting potential compensatory mechanisms triggered by gene deletion. Furthermore, investigation into protein deamidation and glucose metabolism uncovered a link between envZ/ompR deletion and energy metabolism dysregulation. Interestingly, the downregulation of MalK and GrxC proteins was identified as contributing factors to altered desiccation tolerance and disrupted redox homeostasis, respectively, providing mechanistic insights into the phenotypic changes observed. Overall, this study enhances understanding of the multifaceted roles of envZ/ompR in C. sakazakii physiology and pathogenesis, shedding light on potential targets for therapeutic intervention and food safety strategies.

Cronobacter sakazakii Pyridoxal Kinase PdxY Mediated by TreR and pESA3 Is Essential for Vitamin B6 (PLP) Maintenance and Virulence

Cronobacter sakazakii is an opportunistic pathogen capable of causing severe infections, particularly in neonates. Despite the bacterium's strong pathogenicity, the pathogenicity of C. sakazakii is not yet well understood. Using a comparative proteomic profiling approach, we successfully identified pdxY, encoding a pyridoxal kinase involved in the recycling of pyridoxal 5'-phosphate (PLP), as a gene essential for the successful pathogenesis of C. sakazakii. Knocking out the pdxY gene resulted in slower growth and reduced virulence. Our study sheds light on the fundamental importance of pyridoxal kinase for the survival and virulence of C. sakazakii. The identification of pdxY as gene essential for successful pathogenesis provides a potential target for the development of new antibiotic treatments. IMPORTANCE The opportunistic pathogen Cronobacter sakazakii is known to cause severe infections, particularly in neonates, and can result in high mortality rates. In this study, we used a comparative proteomic profiling approach to identify genes essential for the successful pathogenesis of C. sakazakii. We successfully identified pdxY, encoding a pyridoxal kinase involved in the salvage pathway of pyridoxal 5'-phosphate (PLP), as a gene essential for the successful pathogenesis of C. sakazakii. Knocking out the pdxY gene resulted in impaired growth and reduced virulence. This study sheds light on the fundamental importance of pyridoxal kinase for the survival and virulence of C. sakazakii, which can be a potential target for the development of new antibiotic treatments. This study highlights the importance of comparative proteomic profiling in identifying virulence factors that can be targeted for the development of new antibiotics.

The biochemical characteristics of viable but nonculturable state Yersinia enterocolitica induced by lactic acid stress and its presence in food systems

The viable but nonculturable (VBNC) state is adopted by many foodborne pathogenic bacteria to survive in adverse conditions. This study found that lactic acid, a widely used food preservative, can induce Yersinia enterocolitica to enter a VBNC state. Y. enterocolitica treated with 2 mg/mL lactic acid completely lost culturability within 20 min, and 10.137 ± 1.693 % of the cells entered a VBNC state. VBNC state cells could be recovered (resuscitated) in tryptic soy broth (TSB), 5 % (v/v) Tween80-TSB, and 2 mg/mL sodium pyruvate-TSB. In the VBNC state of Y. enterocolitica induced by lactic acid, the intracellular adenosine triphosphate (ATP) concentration and various enzyme activities were decreased, and the reactive oxygen species (ROS) level was elevated, compared with uninduced cells. The VBNC state cells were significantly more resistant to heat and simulated gastric fluid than uninduced cells, but their ability to survive in a high-osmotic-pressure environment was significantly less than that of uninduced cells. The VBNC state cells induced by lactic acid changed from long rod-like to short rod-like, with small vacuoles at the cell edges; the genetic material was loosened and the density of cytoplasm was increased. The VBNC state cells had decreased ability to adhere to and invade Caco-2 (human colorectal adenocarcinoma) cells. The transcription levels of genes related to adhesion, invasion, motility, and resistance to adverse environmental stress were downregulated in VBNC state cells relative to uninduced cells. In meat-based broth, all nine tested strains of Y. enterocolitica entered the VBNC state after lactic acid treatment; among these strains, only VBNC state cells of Y. enterocolitica CMCC 52207 and Isolate 36 could not be recovered. Therefore, this study is a wake-up call for food safety problems caused by VBNC state pathogens induced by lactic acid.

Quantitative Detection of Viable but Nonculturable Cronobacter sakazakii Using Photosensitive Nucleic Acid Dye PMA Combined with Isothermal Amplification LAMP in Raw Milk

An accurate method that rapidly detects the number of viable but nonculturable (VBNC) Cronobacter sakazakii was developed by combining propidium bromide with quantitative LAMP (PMA-QLAMP). The gyrB gene was the target for primers design. The optimal PMA treatment conditions were determined to eliminate the DNA amplification of 10⁸ CFU/mL of dead C. sakazakii without affecting any viable C. sakazakii DNA amplification. Compared with the DNA of 24 strains of common non-C. sakazakii strains found in raw milk and dairy products, the DNA of only six C. sakazakii strains from different sources was amplified using PMA-QLAMP. The ability of PMA-QLAMP to quantitatively detect non-dead C. sakazakii in a 10% powdered infant formula (PIF) solution was limited to 4.3 × 10² CFU/mL and above concentrations. Pasteurizing 10⁶ CFU/mL viable C. sakazakii yielded the maximum ratio of the VBNC C. sakazakii. PMA-QLAMP-based detection indicated that, although approximately 13% of 60 samples were positive for viable C. sakazakii, the C. sakazakii titers in these positive samples were low, and none entered the VBNC state under pasteurization. PMA-QLAMP showed potential as a specific and reliable method for detecting VBNC-C. sakazakii in pasteurized raw milk, thereby providing an early warning system that indicates potential contamination of PIF.

Insights into the mechanisms of Cronobacter sakazakii virulence

Cronobacter species have adapted to survive harsh conditions, particularly in the food manufacture environment, and can cause life-threatening infections in susceptible hosts. These opportunistic pathogens employ a multitude of mechanisms to aid their virulence throughout three key stages: environmental persistence, infection strategy, and systemic persistence in the human host. Environmental persistence is aided by the formation of biofilms, development of subpopulations, and high tolerance to environmental stressors. Successful infection in the human host involves several mechanisms such as protein secretion, motility, quorum sensing, colonisation, and translocation. Survival inside the host is achieved via competitive acquisition and utilization of minerals and metabolites respectively, coupled with host immune system evasion and antimicrobial resistance (AMR) mechanisms. Across the globe, Cronobacter sakazakii is associated with often fatal systemic infections in populations including neonates, infants, the elderly and the immunocompromised. By providing insight into the mechanisms of virulence utilised by this pathogen across these three stages, this review identifies current gaps in the literature. Further research into these virulence mechanisms is required to inform novel mitigation measures to improve global food safety with regards to this food-borne pathogen.