Combined effects of ultra-high hydrostatic pressure and mild heat on the inactivation of Bacillus subtilis

Synergistic effect of high-pressure thermal sterilization and muramidase on Bacillus subtilis spores: alterations in intrasporal components, inner membrane permeability, and structural integrity

The sporicidal mechanism of high-pressure thermal sterilization (HPTS) combined with muramidase against Bacillus subtilis spores was investigated. Results demonstrated that HPTS at 600 MPa/75°C with 0.3% muramidase achieved a 6.09 log reduction in Bacillus subtilis spores. The combined processing significantly increased the leakage of protein, nucleic acid, and dipicolinic acid, while significantly reducing Na+/K+-ATPase activity (P < 0.05). Scanning electron microscopy revealed notable morphological changes in spores after combined processing. A significant increase in propidium iodide (PI)-infiltrated spores indicated enhanced spore inner membrane permeability (P < 0.05). molecular composition analysis further showed disordered arrangement of fatty acid acyl chains, structural alterations in nucleic acids and proteins, and increased the peptidoglycan layer flexibility. These findings provided insights into the sporicidal mechanism of HPTS combined with muramidase.

Inactivation effects of combined thermosonication and potassium sorbate treatments on Bacillus subtilis spores

This study aimed to investigate the inactivation effect of combined TS (thermosonication) and PS (potassium sorbate) treatments on Bacillus subtilis spores. The inactivation effect and potential mechanisms were examined using plate counts, OD600 values, nucleic acid leakage, DPA (dipicolinic acid) leakage, flow cytometry, and FTIR (Fourier transform infrared spectroscopy). The results showed that, after TS + PS treatments, the integrity of the inner membrane was lost, the permeability of the inner membrane to water molecules was increased, and the intraspore substances leaked. Furthermore, the OD600 value was reduced, indicating that the spore core hydration was enhanced. Spores proportion with damaged inner membrane was significantly increased to 66%, the ordered secondary structure of the protein was changed into a disordered structure and nucleic acid was fragmented after TS + PS treatment. The results indicated that the combined TS and PS treatments may be a useful method for inactivating bacterial spores in food processing and sterilization.

Brij-58 supplementation enhances menaquinone-7 biosynthesis and secretion in Bacillus natto

Menaquinone-7 is a form of vitamin K2 that has been shown to have numerous healthy benefits. In this study, several surfactants were investigated to enhance the production of menaquinone-7 in Bacillus natto. Results showed that Brij-58 supplementation influenced the cell membrane via adsorption, and changed the interfacial tension of fermentation broth, while the changes in the state and the composition of the cell membrane enhanced the secretion and biosynthesis of menaquinone-7. The total production and secretion rate of menaquinone-7 increased by 48.0% and 56.2% respectively. During fermentation, the integrity of the cell membrane decreased by 82.9% while the permeability increased by 158% when the maximum secretory rate was reached. Furthermore, Brij-58 supplementation induced the stress response in bacteria, resulting in hyperpolarization of the membrane, and increased membrane ATPase activity. Finally, changes in fatty acid composition increased membrane fluidity by 30.1%. This study provided an effective strategy to enhance menaquinone-7 yield in Bacillus natto and revealed the mechanism of Brij-58 supplementation in menaquinone-7 production. KEY POINTS: • MK-7 yield in Bacillus natto was significantly increased by Brij-58 supplementation. • Brij-58 could be adsorbed on cell surface and change fermentation environment. • Brij-58 supplementation could affect the state and composition of the cell membrane.

High Pressure Thermal Sterilization and ε-Polylysine Synergistically Inactivate Bacillus subtilis Spores by Damaging the Inner Membrane

ABSTRACT

This study was conducted to determine the sterilization effect of a combination of high pressure thermal sterilization (HPTS) and ε-polylysine (ε-PL) on Bacillus subtilis spores. The spores were treated with HPTS (550 MPa at 25, 65, and 75°C) and ε-PL at 0.1 and 0.3%. HPTS and ε-PL synergistically decreased the number of surviving spores and increased the release of the intracellular components in the spore suspension, with the maximal effects from treatment with 550 MPa at 75°C plus 0.3% ε-PL. Maximum fluidity and permeability of the cell inner membrane were observed with 550 MPa at 75°C plus 0.3% ε-PL. Changes in membrane lipids were detected from 3,000 to 2,800 cm-1 by Fourier transform infrared spectroscopy. The results provide new insights into the mechanism by which HPTS and ε-PL synergistically sterilize B. subtilis spores.

HIGHLIGHTS

Synergistic Inactivation Mechanism of Combined Plasma-Activated Water and Mild Heat against Saccharomyces cerevisiae

ABSTRACT

This study aimed to elucidate the mechanism of synergistic inactivation of Saccharomyces cerevisiae by the combined use of plasma-activated water (PAW) and mild heat (40 to 50°C). A reduction of 4.40 log CFU/mL in S. cerevisiae was observed after the synergistic combination of PAW and mild heat at 50°C for 6 min, whereas the individual treatments of PAW at 25°C and mild heat at 50°C for 6 min resulted in a reduction of 0.27 and 1.92 log CFU/mL, respectively. The simultaneous application of PAW and mild heat caused significant increases in membrane permeability, resulting in the leakage of intracellular components (such as nucleic acids and proteins) and increased uptake of propidium iodide. The combined treatment of PAW and mild heat also resulted in significant increases in the intracellular levels of reactive oxygen species and disruption of mitochondrial membrane potential in S. cerevisiae cells. In summary, this study illustrates the potential of PAW treatment combined with mild heat to rapidly inactivate microorganisms in food products.

HIGHLIGHTS

Flow Cytometric Assessment of the Morphological and Physiological Changes of Listeria monocytogenes and Escherichia coli in Response to Natural Antimicrobial Exposure

Essential oils (EOs) or their components represent one of the most promising natural, safe, and feasible alternatives to prevent the growth of food-borne pathogens like Listeria monocytogenes and Escherichia coli in food matrices. Although antimicrobial properties of EOs and their components are well-documented, limited and fragmented information is available on the changes induced by these compounds, even at sub-lethal concentrations, in the physiological properties of microbial cells. The aim of this study was to explore the morpho-physiological changes of L. monocytogenes Scott A and E. coli MG 1655 induced after 1 h exposure to different sub-lethal and lethal concentrations of citral, carvacrol, (E)-2-hexenal, and thyme EO. For this purpose, different cell viability parameters such as membrane integrity, esterase activity, and cytoplasmic cell membrane potential were measured by flow cytometry. Flow cytometric data revealed specific response patterns in relation to the strain, the natural antimicrobial and its concentrations. Both the target microbial strains showed an increased cell membrane permeabilization without a loss of esterase activity and cell membrane potential with increasing citral, carvacrol and thyme EO concentrations. By contrast, (E)-2-hexenal did not significantly affect the measured physiological properties of L. monocytogenes Scott A and E. coli MG 1655. The used approach allowed identifying the most effective natural antimicrobials in relation to the microbial target.

Contribution of Fluorescence Techniques in Determining the Efficiency of the Non-thermal Plasma Treatment

We have recently developed a non-thermal plasma (NTP) equipment intended to sterilize fragile medical devices and maintain the sterile state of items downstream the treatment. With traditional counts on agar plate a six log reduction of Staphylococcus aureus viability was obtained within 120 min of O₂, Ar, or N₂ NTP treatments. However to determine the best NTP process, we studied the different physiological states of S. aureus by flow cytometry (FC) and confocal laser scanning microscopy (CLSM) focusing on the esterasic activity and membrane integrity of the bacteria. Two fluorochromes, 5-(and-6)-carboxy-2^',7^'-dichlorofluorescein diacetate and propidium iodide were used in order to distinguish three sub-populations: metabolically active, permeabilized, and damaged bacteria that can be in the viable but nonculturable state. FC and CLSM highlight that O₂ and Ar NTP treatments were the most attractive processes. Indeed, a 5 min of Ar NTP generated a high destruction of the structure of bacteria and a 120 min of O₂ NTP treatment led to the higher decrease of the total damaged bacteria population. SEM observations showed that in presence of clusters, bacteria of upper layers are easily altered compared to bacteria in the deeper layers. In conclusion, the plate counting method is not sufficient by itself to determine the best NTP treatment. FC and CLSM represent attractive indicator techniques to select the most efficient gas NTP treatment generating the lowest proportion of viable bacteria and the most debris.

Mechanisms of Nα-lauroyl arginate ethyl ester against Penicillium digitatum and Pectobacterium carotovorum subsp. carotovorum

The purpose of this study was to investigate the antimicrobial activity and mechanisms of Nα-lauroyl arginate ethyl ester (LAE) against Penicillium digitatum and Pectobacterium carotovorum subsp. carotovorum. The minim inhibitory concentrations of LAE against P. digitatum and P. carotovorum were found to be 400 and 25 μg/ml, respectively. Loss of intracellular protein and nucleic acid increased significantly, and membrane permeability reached 76.28, 54.29 and 85.20%, respectively, when 400 μg/ml of LAE was applied to the hyphae and spores of P. digitatum and to P. carotovorum. Flow cytometry showed that LAE reduced the membrane potential, and the depolarization ratios of P. digitatum and P. carotovorum were 98.19 and 97.25% (P < 0.05), respectively. Transmission electron microscopy photos revealed that LAE caused a rough surface, irregular cellular organelles, protoplast shrinkage, intracytoplasmic coagulation and empty cavities in all three cell types. These results showed that LAE had notable ability to damage the structure of fungal and bacterial cells, making it a possible alternative chemical for use in the preservation of fruits and vegetables.

Recent Advances on Multi-Parameter Flow Cytometry to Characterize Antimicrobial Treatments

The investigation on antimicrobial mechanisms is a challenging and crucial issue in the fields of food or clinical microbiology, as it constitutes a prerequisite to the development of new antimicrobial processes or compounds, as well as to anticipate phenomenon of microbial resistance. Nowadays it is accepted that a cells population exposed to a stress can cause the appearance of different cell populations and in particular sub-lethally compromised cells which could be defined as viable but non-culturable (VBNC). Recent advances on flow cytometry (FCM) and especially on multi-parameter flow cytometry (MP-FCM) provide the opportunity to obtain high-speed information at real time on damage at single-cell level. This review gathers MP-FCM methodologies based on individual and simultaneous staining of microbial cells employed to investigate their physiological state following different physical and chemical antimicrobial treatments. Special attention will be paid to recent studies exploiting the possibility to corroborate MP-FCM results with additional techniques (plate counting, microscopy, spectroscopy, molecular biology techniques, membrane modeling) in order to elucidate the antimicrobial mechanism of action of a given antimicrobial treatment or compound. The combination of MP-FCM methodologies with these additional methods is namely a promising and increasingly used approach to give further insight in differences in microbial sub-population evolutions in response to antimicrobial treatments.