UV-C and hydration state drive pulsed light-induced proteome damage in Bacillus pumilus spores

Introduction

Pulsed light (PL) is a non-thermal processing technology that inactivates microorganisms through high-intensity pulses of polychromatic light, including ultraviolet-C (UV-C). While the germicidal effect of PL has been widely studied, its impact on microbial proteomes remains poorly understood. Here, we investigate the proteomic response of Bacillus pumilus DSM492 (ATCC 27142) spores to PL treatment, comparing it to conventional UV-C 254 nm exposure.

Methods

B. pumilus spores were either suspended in water or sprayed onto a polystyrene surface and exposed to PL or UV-C at fluences achieving a 5-log and a > 7-log reduction in viability. Proteomic changes were analyzed using mass spectrometry to identify proteins with decreased abundance after treatment.

Results

PL treatment induced a significantly greater proteomic alteration compared to UV-C, particularly in spores suspended in water, where the number of proteins with decreased abundance was ~6-fold higher than in spores sprayed on a polystyrene surface. Proteomic analysis revealed that the effect of PL in water was primarily due to UV-C 254 nm, whereas on polystyrene, UV-C 254 nm had no significant impact. Furthermore, proteins most affected by PL were enriched in photosensitive amino acids such as tryptophan, histidine, tyrosine, cysteine, and methionine, suggesting oxidation and photoreactivity as key degradation mechanisms.

Discussion

Although the overall inactivation rate could not be directly correlated with proteome damage, we identified that core proteins involved in DNA and RNA protection and repair were specifically targeted by PL. These findings provide new insights into the molecular mechanisms underlying PL-mediated microbial inactivation and highlight the role of protein photodamage in spore susceptibility.

Sublethal damage and recovery of Staphylococcus aureus exposed to intense pulsed light: Implications for minimally processed foods

The growing demand for minimally processed foods has promoted the application of non-thermal sterilization technologies, such as intense pulsed light (IPL), to ensure food safety while preserving nutritional and sensory attributes. However, the potential for sublethal bacterial recovery after IPL treatment remains a major concern. In this study, IPL showed varying bactericidal capacities for Staphylococcus aureus in water and pork surfaces, respectively reducing by approximately 4 and 1.2 log CFU/mL after 2 applications at 4 cm, despite similar damage was observed by SEM. The differences in sterilization were speculated that sublethal S. aureus cells caused by IPL could be recovered within a nutritive environment such as pork. To elucidate the underlying mechanisms, intracellular redox enzyme activities and transcriptomic responses of sublethal S. aureus were analyzed. The results indicate that S. aureus repaired the damage caused by IPL mainly through three ways. Firstly, DNA damage was repaired by activating SOS response, restoring DNA double-strand breaks, and improving purine metabolism. Besides, S. aureus responded to oxidative damage by maintaining iron homeostasis, synthesizing biotin and clearing aldehyde metabolites. Meanwhile, amino acids, phosphate and ferric ions served as substrates and ATP for cell repair by amino acid metabolism. In conclusion, this study analyzed the recovery mechanism of sublethal S. aureus under IPL stimulus, and provides new insights for controlling pathogenic bacteria in minimally processed food industry.

Antibacterial Activity of Phloretin Against Vibrio parahaemolyticus and Its Application in Seafood

Although phloretin is widely utilized in the food industry as an additive, its effects on foodborne pathogens remain insufficiently investigated. This study aimed to evaluate the antimicrobial properties of phloretin (PHL) against Vibrio parahaemolyticus (V. parahaemolyticus) and to elucidate the potential mechanisms of action. After PHL treatment, alterations in the cell morphology, cell microstructure, and intracellular contents of V. parahaemolyticus were assessed. Scanning electron microscopy revealed substantial damage to cell integrity, subsequent to PHL treatment. A notable reduction in intracellular components, including proteins, ATP, and DNA, was observed in samples treated with PHL. PHL was shown to inhibit the activities of ATPase, β-galactosidase, and respiratory chain dehydrogenase in V. parahaemolyticus. Furthermore, it was demonstrated to elevate the intracellular levels of reactive oxygen species and promote cell death. After being applied to sea bass, shrimp, and oysters, PHL effectively inactivated V. parahaemolyticus in these seafoods. These findings demonstrate that PHL has potential for application in seafood to control V. parahaemolyticus.

Microbial Photoinactivation by Visible Light Results in Limited Loss of Membrane Integrity

Interest in visible light irradiation as a microbial inactivation method has widely increased due to multiple possible applications. Resistance development is considered unlikely, because of the multi-target mechanism, based on the induction of reactive oxygen species by wavelength specific photosensitizers. However, the affected targets are still not completely identified. We investigated membrane integrity with the fluorescence staining kit LIVE/DEAD® BacLight™ on a Gram positive and a Gram negative bacterial species, irradiating Staphylococcus carnosus and Pseudomonas fluorescens with 405 nm and 450 nm. To exclude the generation of viable but nonculturable (VBNC) bacterial cells, we applied an ATP test, measuring the loss of vitality. Pronounced uptake of propidium iodide was only observed in Pseudomonas fluorescens at 405 nm. Transmission electron micrographs revealed no obvious differences between irradiated samples and controls, especially no indication of an increased bacterial cell lysis could be observed. Based on our results and previous literature, we suggest that visible light photoinactivation does not lead to rapid bacterial cell lysis or disruption. However, functional loss of membrane integrity due to depolarization or inactivation of membrane proteins may occur. Decomposition of the bacterial envelope following cell death might be responsible for observations of intracellular component leakage.

UV dose effects on the revival characteristics of microorganisms in darkness after UV disinfection: Evidence from a pilot study

Ultraviolet (UV) disinfection during water supply treatment aims to reduce the number of bacteria. Although UV disinfection is effective at inactivating most microorganisms, some microbe species may be entirely impervious. A pilot study was conducted to compare the quantity and community component of bacteria in surface water collected from filtration effluent before UV disinfection with different doses of UV, and those 1 and 2 days afterwards, in darkness. The aim was to elucidate the relationship between the UV dose and the total revived microorganisms in darkness after UV disinfection. In the filtration effluent samples, Gammaproteobacteria, Bacilli, Actinobacteria, and Alphaproteobacteria were the predominant classes. After storage in the dark at a constant temperature of 19 °C, the UV-disinfected samples showed a considerable increase in Bacilli, while Gammaproteobacteria remained the predominant population. Genera such as Exiguobacterium, Citrobacter, Acinetobacter, and Pseudomonas presented a selective advantage in terms of revival in darkness after UV disinfection, irrespective of the UV dose and storage time. The lowest rate of microbial revival (5% day^-1) was noted at a UV dose of 266.10 mJ m^-2 (with an average UV illumination time of 124.4 s and an average intensity of 86.61 W m^-2). Our results suggest that higher UV intensity and lower illumination time are key factors in minimizing the revival of microorganisms in darkness.

Microbes associated with fresh produce: Sources, types and methods to reduce spoilage and contamination

Global food security remains one of the most important challenges that needs to be addressed to ensure the increasing demand for food of the fast growing human population is satisfied. Fruits and vegetables comprise an essential component of a healthy balanced diet as they are the major source of both macro- and micronutrients. They are particularly important for communities in developing countries whose nutrition often relies solely on a plant-based diet. Recent advances in agriculture and food processing technologies have facilitated production of fresh, nutritious and safe food for consumers. However, despite the development of sophisticated chemical and physical methods of food and equipment disinfection, fresh-cut produce and fruit juice industry still faces significant economic losses due to microbial spoilage. Furthermore, fresh produce remains an important source of pathogens that have been causing outbreaks of human illness worldwide. This chapter characterizes common spoilage and human pathogenic microorganisms associated with fresh-cut produce and fruit juice products, and discusses the methods and technology that have been developed and utilized over the years to combat them. Substantial attention is given to highlight advantages and disadvantages of using these methods to reduce microbial spoilage and their efficacy to eliminate human pathogenic microbes associated with consumption of fresh-cut produce and fruit juice products.

Pulse UV light effect on microbial biomolecules and organic pollutants degradation in aqueous solutions

This study present assessed the effect of UV pulsed light (PL) on microbial and organic pollutants using two spiral lamps were used, i.e., PL1 and PL2 lamps, with wavelength cut-offs of 190 and 240 nm, respectively. Overall, our study demonstrated that pulsed UV light impacts several microbial biomolecules and degrades polycyclic aromatic hydrocarbons (PAHs) in aqueous solution. In microbial inactivation by PL2, temporary changes of bacterial cellular components, specifically proteins, were observed, but the compositional changes of bacteria that were exposed to PL1 were permanent due to ozonolysis. PL1 irradiation caused greater deactivation of the bacteria than PL2 irradiation due to the generation of ozone. The higher efficacy of PL1 in terms of membrane disruption, reduction of respiration rate, and reduction of growth rate was due to the production of ozone during the irradiation period. The bacteria that were irradiated with both PL lamps regrew due to photoreactivation, such as an enzymatic DNA-repair mechanism. The PAH degradation kinetics indicate that higher molecular weights degraded faster than those with lower molecular weights. For both lamps, the degradation of naphthalene and fluorene was first order, whereas second order for pyrene and anthracene. Any effect of ozonolysis on the PAH degradation rates was not apparent, which indicated that photolysis was the primary degradation pathway. PAH solutions treated with both pulsed UV lamps did not result in a toxicity effect on the bacteria.

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

The effect of pulsed light treatment on the lag phase and the maximum specific growth rate of Listeria innocua was determined in culture media at 7 °C. Fluences of 0.175, 0.350 and 0.525 J/cm² were tested. The lag phase of the survivors increased as fluence did, showing significant differences for all the doses; an 8.7-fold increase was observed at 0.525 J/cm². Pulsed light decreased the maximum specific growth rate by 38% at the same fluence. Both parameters were also determined by time-lapse microscopy at 25 °C in survivors to 0.525 J/cm², with an increase of 13-fold of the lag phase and a 45% decrease of the maximum specific growth rate. The higher the fluence, the higher the variability of both parameters was. To characterize pulsed light damage on L. innocua, the formation of dimers on DNA was assessed, and a proteomic study was undertaken. In cells treated with 0.525 J/cm², cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts were detected at 5:1 ratio. Pulsed light induced the expression of three proteins, among them the general stress protein Ctc. Furthermore, treated cells showed an up-regulation of proteins related to metabolism of nucleotides and fatty acids, as well as with translation processes, whereas flagellin and some glucose metabolism proteins were down-regulated. Differences in the proteome of the survivors could contribute to explain the mechanisms of adaptation of L. innocua after pulsed light treatment.

ATP-synthesis capacity of pulsed light-exposed bacteria

The ability of four different bacteria to synthesize new ATP upon exposure to different doses of pulsed-light (PL) irradiation was investigated. The bacterial cells were PL treated on a gel surface, resuspended in phosphate buffered saline (PBS) and subsequently incubated in Tryptic Soy Broth (TSB) at 37°C. Cellular ATP levels were monitored during a 2h incubation period and compared to the respective colony count data. Although PL affected ATP production in a dose dependent manner, the results showed that bacteria, which had rendered unculturable after PL exposure, are still capable of generating significant quantities of ATP. Escherichia coli and Listeria innocua proved to be more resistant to PL than Salmonella enterica and Staphylococcus aureus, which was supported by the colony count data and the ATP synthesis capacity. These findings underline that bacteria undetectable by culture-based methods may still show cellular activity and synthesize new ATP.

Recent findings in pulsed light disinfection

Nonthermal disinfection technologies are gaining increasing interest in the field of minimally processed food in order to improve the microbial safety or to extend the shelf life. Especially fresh-cut produce or meat and fish products are vulnerable to microbial spoilage, but, due to their sensitivity, they require gentle preservation measures. The application of intense light pulses of a broad spectral range comprising ultraviolet, visible and near infrared irradiation is currently investigated as a potentially suitable technology to reduce microbial loads on different food surfaces or in beverages. Considerable research has been performed within the last two decades, in which the impact of various process parameters or microbial responses as well as the suitability of pulsed light (PL) for food applications has been examined. This review summarizes the outcome of the latest studies dealing with the treatment of various foods including the impact of PL on food properties as well as recent findings about the microbicidal action and relevant process parameters.

Impact of pulsed light on cellular activity of Salmonella enterica

AIMS

The objective of this study was a comprehensive characterization of physiological changes of Salmonella enterica induced by intense broad spectrum pulsed light (PL). After exposing the bacteria to this nonthermal decontamination technology on a gel surface, multiple viability parameters beyond culturability were assessed.

METHODS AND RESULTS

By applying flow cytometry, a luciferin-luciferase bioluminescence assay and a microplate assay to measure the current redox activity, the impact of pulsed light on the membrane potential, membrane integrity, esterase activity, efflux pump activity, expression of the green fluorescent protein (GFP), respiration activity and ATP-content of Salm. enterica ATCC BAA-1045 was determined. These culture-independent methods for assessing the bacterial activity were compared to the ability to grow on tryptic soy agar. It is shown that this strain is rather sensitive to PL considering colony count reductions, while on the other hand unculturable bacteria still exhibit significant cellular energetic functions. However, this residual activity after PL exposure significantly decreases during sample storage in buffer for 24 h. This study also shows that the GFP expression of PL-treated cells which have rendered unculturable is severely reduced.

CONCLUSIONS

This study reveals that although not all cellular functions of Salm. enterica are immediately shut down after PL exposure, the synthesis of new GFP is strongly reduced and affected to a similar extent as the culturability.

SIGNIFICANCE AND IMPACT OF THE STUDY

It is shown for the first time, that even there is significant bacterial activity measurable after PL exposure, it is likely that nongrowing pathogenic bacteria like Salm. enterica are unable to express proteins, which is of great importance regarding their pathogenicity.