Enhanced inactivation of food-borne pathogens in ready-to-eat sliced ham by near-infrared heating combined with UV-C irradiation and mechanism of the synergistic bactericidal action

Acid adaptation increased the resistance of Escherichia coli O157:H7 in bok choy (Brassica rapa subsp. chinensis) juice to high-pressure processing

IMPORTANCE

Based on the U.S. Food and Drug Administration regulations, E. coli O157:H7 is a pertinent pathogen in high acid juices that needs to be inactivated during the pasteurization process. The results of this study suggest that the effect of acid adaptation should be considered in the selection of HPP parameters for E. coli O157:H7 inactivation to ensure that pasteurization objectives are achieved.

Combined UV-C Technologies to Improve Safety and Quality of Fish and Meat Products: A Systematic Review

This study aimed to identify the best UV-C combined treatments for ensuring the safety and quality of fish and meat products. A total of 4592 articles were screened in the relevant databases, and 16 were eligible studies. For fish, the most effective treatments to reduce Gram-negative and Gram-positive bacteria were UV-C at 0.5 J/cm² + non-thermal atmospheric plasma (NTAP) for 8 min (33.83%) and 1% Verdad N6 + 0.05 J/cm² + vacuum packaging (25.81%), respectively. An oxygen absorber with 0.102 J/cm² was the best combined treatment, reducing lipid oxidation (65.59%), protein oxidation (48.95), color (ΔE = 4.51), and hardness changes (18.61%), in addition to a shelf-life extension of at least 2 days. For meat products, Gram-negative bacteria were more reduced by nir-infrared heating (NIR-H; 200.36 µW/cm²/nm) combined with 0.13 J/cm² (70.82%) and 0.11 J/cm² (52.09%). While Gram-positive bacteria by 0.13 J/cm² with NIR-H (200.36 µW/cm²/nm), 1, 2, or 4 J/cm² with flash pasteurization (FP) during 1.5 or 3 s, and 2 J/cm² with FP for 0.75 s (58.89-67.77%). LAE (5%) + 0.5 J/cm² was promising for maintaining color and texture. UV-C combined technologies seem to be a cost-effective alternative to ensure safety with little to no quality changes in fish and meat products.

Effective control of microbial spoilage in soybeans by water-soluble ZnO nanoparticles

The microbial spoilage of soybeans during soaking process severely deteriorates the quality of soybean products and threatens human health. Herein, water-soluble aminated zinc oxide nanoparticles (ZnO NPs) were developed to effectively control the microbial spoilage in soybeans during soaking. ZnO NPs achieved significant inactivation of three dominant spoilage bacteria (bacillus cereus, bacillus megaterium and enterococcus faecium) isolated from the deteriorated soybeans, which could adhere to the bacterial surface and damage the cell wall/membrane, but also generate large amounts of reactive oxygen species (ROS). Compared to two commercial ZnO, water-soluble ZnO exhibited superior antibacterial properties due to producing more ROS and bacteria-adhered ability. After ZnO NPs treatment, the content of the residual Zn (51.1 mg/kg) in soybeans was the safety standards of Zn element in soybeans products for human). Therefore, the water-soluble ZnO NPs showed great potentials as efficient and safe antimicrobial agents for soybeans preservation during soaking process.

Inactivation of Shigella flexneri by 405-nm Light-Emitting Diode Treatment and Possible Mechanism of Action

Shigella flexneri, a common Gram-negative foodborne pathogen, is widely distributed in fresh-cut fruits and vegetables, unpasteurized milk, and food processing environments. The aims of this study were to evaluate the antibacterial effects of 405-nm light-emitting diode (LED) treatment on S. flexneri and to investigate the possible mechanism. The results showed that LED irradiation (360 min) reduced the number of S. flexneri in phosphate-buffered saline by 3.29 log colony-forming unit (CFU)/mL (initial bacterial count: 6.81 log CFU/mL). The cells in reconstituted infant formula, cells on fresh-cut carrot slices, and biofilm-associated cells on stainless steel surfaces were reduced by 1.83 log CFU/mL, 7.00 log CFU/cm², and 4.35 log CFU/cm² following LED treatment for 360, 120, and 120 min, respectively. LED treatment damaged both DNA and cell wall of S. flexneri and changed cell morphology and cell membrane permeability. In addition, LED treatment decreased total cell protein concentration of S. flexneri. These results indicated that 405-nm LED treatment effectively controlled S. flexneri contamination of foods and food contact surfaces and that the bacterial inactivation may be the result of damage to multiple cellular components. These findings highlight the potential of LED technology in controlling S. flexneri during food processing, storage, and preparation.

Collective bacterial disinfection by opto-chemical treatment on mature biofilm in clinical endoscope

The present study proposes an innovative opto-chemical treatment using a basket-integrated optical device (BIOD) to disinfect mature bacterial biofilm on endoscope channels. A BIOD was designed to position an optical diffuser on the central axis of an endoscope channel and to distribute laser light concentrically to the bacterial biofilm on the channel surface. To apply thermal damage and oxidative stress to the bacterial biofilm, a low concentration of a crosslinking agent (glutaraldehyde ~0.5%) was combined with 808 nm infrared (IR) and 405 nm blue (BL) laser lights. The applied irradiances of IR and BL were 10 W/cm² and 1.6 W/cm² for Teflon channel model and 20 W/cm² and 3.2 W/cm² for a clinical model, respectively. Individual irradiation of either IR or BL for 180 s induced the maximum temperatures of 62 ± 2 °C and 53 ± 3 °C on the biofilm, respectively. The simultaneous opto-chemical treatment reduced a significant population of the bacterial biofilms (7.5-log₁₀ for Staphylococcus aureus and 7.1-log₁₀ for Pseudomonas aeruginosa), which were 2.9-fold and 3.9-fold higher than that of the standard treatment with 2% glutaraldehyde (GA) solution, respectively. The proposed opto-chemical disinfection method can help reduce multi-drug resistant bacteria and prevent cross-infection during the clinical usage of a flexible endoscope.

Comparison of measurement methods at determining the target sites injured by antimicrobials in Escherichia coli O157:H7 using metabolic inhibitors

Acquiring an understanding of the mechanisms underlying antimicrobial action is important for overcoming bacterial resistance to antimicrobials. This study evaluated three different methods (antimicrobial fixed broth dilution method, metabolic inhibitors fixed broth dilution method, and metabolic inhibitor fixed agar recovery method) for determining the target site of Escherichia coli O157:H7 by treatments with various antimicrobials (ethanol, ethylenediaminetetraacetic acid, polymyxin B, thymol, acetic acid, and citrus fruit extract). However, the results indicated only weak relationships between MIC values and mechanisms of antimicrobials known to cause damage or injury. In addition, the results of three measurement methods using metabolic inhibitors were not correlated. These results suggest that measurement methods using metabolic inhibitors alone may not be suitable for determining the target site injured by antimicrobials. Therefore, various measurement methods should be compared and analyzed to determine the damage or injury sites targeted by antimicrobials in pathogenic bacteria. Further studies are needed to compare and analyze the various measurement methods for determining the target site injured by antimicrobials in pathogenic bacteria.

Bactericidal and synergistic effects of X-ray irradiation and gallic acid against foodborne pathogens on lettuce

The objectives of this study were to evaluate the bactericidal effects of X-ray irradiation and gallic acid (GA) against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on lettuce leaves and in phosphate-buffered saline (PBS). Inoculated PBS and lettuce were exposed to X-rays (0.05, 0.1, and 0.15; 0.1, 0.2, and 0.3 kGy, respectively), and GA was applied to lettuce leaves as a solution and in PBS at concentrations of 0.5% (w/v). Combined treatment with 0.3 kGy and 0.5% GA reduced E. coli O157:H7, S. Typhimurium, and L. monocytogenes cell counts 5.41, 2.57, and 1.36 log CFU/cm2 on lettuce, respectively. Combined treatment with 0.15 kGy X-ray and 0.5% GA reduced counts for the same species by 6.54, 4.24, and 1.51 log CFU/mL in PBS. The combined treatments exerted a synergistic antibacterial effect against E. coli O157:H7 on lettuce, but not against S. Typhimurium or L. monocytogenes. In PBS, the synergistic effect was confirmed in both E. coli O157:H7 and S. Typhimurium cells. Mechanistic investigations indicated that the synergistic antibacterial effect was associated with intracellular reactive oxygen species (ROS) generation and bacterial cell membrane damage. Additionally, the X-ray and GA combination treatment did not adversely affect the color, total phenol content, and texture of lettuce. These findings demonstrate that treatment with X-ray radiation and GA can enhance the microbiological safety of fresh produce.

Outbreaks, Germination, and Inactivation of Bacillus cereus in Food Products: A Review

ABSTRACT

Bacillus cereus has been reported as a foodborne pathogen worldwide. Although food processing technologies to inactivate the pathogen have been developed for decades, foodborne outbreaks related to B. cereus have occurred. In the present review, foodborne outbreaks, germination, inactivation, and detection of B. cereus are discussed, along with inactivation mechanisms. B. cereus outbreaks from 2003 to 2016 are reported based on food commodity, number of cases, and consequent illnesses. Germination before sporicidal treatments is highlighted as an effective way to inactivate B. cereus, because the resistance of the pathogen increases significantly following sporulation. Several germinants used for B. cereus are listed, and their efficacies are compared. Finally, recently used interventions with sporicidal mechanisms are identified, and rapid detection methods that have been developed are discussed. Combining two or more interventions, known as the hurdle technology concept, is suggested to maximize the sporicidal effect. Further study is needed to ensure food safety and to understand germination mechanisms and sporicidal resistance of B. cereus.

HIGHLIGHTS

Synergistic bactericidal effect and mechanism of X-ray irradiation and citric acid combination against food-borne pathogens on spinach leaves

In this study, we investigated the antimicrobial activity of the X-ray irradiation and citric acid (CA) combination against Escherichia coli O157:H7 and Listeria monocytogenes on the surface of spinach leaves and elucidated the mechanisms underlying their synergistic interaction. Upon treatment with 0.3 kGy X-ray irradiation and 1% CA combination, the cell counts of E. coli O157:H7 and L. monocytogenes reduced by 4.23 and 3.69 log CFU/mL on spinach leaves, respectively. The synergistic reduction in the cell counts of E. coli O157:H7 and L. monocytogenes by the combination treatment was 0.95 and 1.14 log units, respectively. The X-ray and CA combination exerts its antimicrobial effect by damaging the bacterial cell membrane and enhancing the generation of intracellular reactive oxygen species in the pathogens. The enhanced bactericidal effect of the combination treatment may not be due to the loss of intracellular enzyme activity. We also evaluated the effect of the combination treatment on the quality attributes of spinach leaves. The combination treatment did not result in adverse changes in color and texture of spinach leaves. These results demonstrate the potential of citric acid and X-ray irradiation combination for decontaminating foodborne pathogens on fresh produce.

Inactivation of Bacillus cereus Spores on Stainless Steel by Combined Superheated Steam and UV-C Irradiation Treatment

Bacillus cereus spore contamination on food contact surfaces is of great concern in the food industry. Thus, in the present study, superheated steam (SHS) was used alone or combined with UV-C irradiation for inactivation of B. cereus spores inoculated on stainless steel coupons. Temperatures higher than 250°C were needed to effectively inactivate B. cereus spores by SHS treatment alone, while a synergistic bactericidal effect resulted from the sequential treatment of SHS before or after UV-C irradiation. The increased dipicolinic acid ratio obtained by the combined treatment had a significant role in the synergistic bactericidal effect. Therefore, the combined treatment of SHS and UV-C could be used effectively to inactivate B. cereus on stainless steel. It is recommended to use hurdle technology with reduced energy consumption to ensure microbiological safety on food contact surfaces.

Antibacterial effect and mechanisms of action of 460-470 nm light-emitting diode against Listeria monocytogenes and Pseudomonas fluorescens on the surface of packaged sliced cheese

The aim of this study was to investigate the antibacterial effect of 460-470 nm light-emitting diode (LED_460-470nm) illumination against pathogens and spoilage bacteria on the surface of agar media and packaged sliced cheese. LED_460-470nm illumination highly inhibited the growth of Listeria monocytogenes and Pseudomonas fluorescens on agar media covered with oriented polypropylene (OPP) film (thickness, 0.03 mm). When sliced cheeses inoculated with L. monocytogenes or P. fluorescens and packaged with OPP film were illuminated by an LED_{460-470 nm} at 4 or 25 °C, reduction levels of L. monocytogenes and P. fluorescens on packaged slice cheese were higher at 4 °C than at 25 °C. There were no significant differences in color between non-illuminated and illuminated sliced cheese after storage for 7 d at 4 °C. LED_{460-470 nm} illumination at 4 °C for 4 d caused cellular injury of L. monocytogenes and P. fluorescens related to RNA, protein, and peptidoglycan metabolism, and a disruption of the cell membrane and loss of cytoplasmic components were observed from TEM results. These results suggest that LED_{460-470 nm} illumination, in combination with refrigeration temperatures, may be applied to extend the shelf-life of packaged slice cheese and minimize the risk of foodborne disease, without causing color deterioration.

A Novel Mathematical Model for Studying Antimicrobial Interactions Against Campylobacter jejuni

The aim of this study is to investigate the antimicrobial synergistic effect against Campylobacter jejuni, a leading foodborne pathogen that causes human gastroenteritis, by cinnamon oil, encapsulated curcumin, and zinc oxide nanoparticles (ZnO NPs). We compared three approaches to study the antimicrobial interactions, including the time-killing method, the fractional inhibitory concentration index (FICI) method, and a mathematical concentration-effect model. Isobologram analysis was performed to evaluate the synergy in different combinations, and a median-effect equation was applied to identify the combinations of synergistic effects at median, bacteriostatic, and bactericidal reduction levels. The time-killing method overestimated the synergistic interaction between antimicrobials, while the FICI method failed to detect an existing synergistic phenomenon. This lack of accuracy and sensitivity was mainly due to combining antimicrobials without a deep understanding of their concentration-effect relationships. Our results showed that each antimicrobial had a unique concentration-effect curve. Specifically, encapsulated curcumin showed a sharp sigmoidal curve unlike cinnamon oil and ZnO NPs. A mathematical model was applied to study the interaction between antimicrobials with a different shape of concentration-effect curve. We observed an additive effect of cinnamon oil/ZnO NPs and synergistic interactions of other binary combinations (cinnamon oil/encapsulated curcumin and ZnO NPs/encapsulated curcumin). The tertiary combination of cinnamon oil/ZnO NPs/encapsulated curcumin at IC₂₅ (additive line <1-log CFU/mL) presented the greatest synergistic effect by reducing the bacterial population over 8-log CFU/mL. This mathematical model provided an alternative strategy to develop a new antimicrobial strategy.

Optimization of broth recovery for repair of heat-injured Salmonella enterica serovar Typhimurium and Escherichia coli O157:H7

AIMS

The purpose of this research was to determine optimum conditions for broth recovery of heat-injured Salmonella Typhimurium and Escherichia coli O157:H7.

METHODS AND RESULTS

Exposure to 55°C for 15 and 25 min, respectively, induced cellular injury to those pathogens. Comparison was made with the commonly used overlay method using selective medium for recovering sublethally injured cells of S. Typhimurium. For E. coli O157:H7, phenol red agar base with 1% sorbitol was used. After cell suspensions were heated at 55°C for selected time intervals, microbes were 10-fold diluted with brain heart infusion (BHI), tryptic soy broth (TSB) and TSB with 0·6% yeast extract (TSBYE) and incubated at 37°C for up to 3 h. At hourly intervals, diluents were plated onto selective medium for recovery. Simultaneously, diluents were plated onto tryptic soy agar (TSA) for recovery of sublethally injured cells. For overlays, diluents were plated onto TSA and overlaid with selective agar after a resuscitation interval. Broth recovery conditions for S. Typhimurium and E. coli O157:H7 were determined to be 1 h in any of the following broth media: BHI, TSB or TSBYE. When liquid resuscitation was applied to sublethally injured cells in food samples (milk), 1 h was also sufficient time for recovery.

CONCLUSIONS

The broth recovery method is a convenient alternative to conventional recovery methods.

SIGNIFICANCE AND IMPACT OF THE STUDY

Cells sublethally injured by control interventions might not grow on selective medium because they have no resistance to several selective compounds. However, injured cells can recuperate and multiply under conditions sufficient for recovery. To repair and detect heat-injured cells, the overlay method is commonly used but this method has some limitations. This study confirms the effectiveness of liquid resuscitation method on recovery of injured cells. The broth recovery can replace the overlay method due to greater convenience and timesaving.

Antibacterial activity of Staphylococcus aureus biofilm under combined exposure of glutaraldehyde, near-infrared light, and 405-nm laser

Healthcare-associated infections have increasingly become problematic in the endoscopic procedures resulting in several severe diseases such as carbapenem-resistant Enterobacteriaceae (CRE)-related infections, pneumonia, and bacteremia. Especially, some bacterial strains are resistant to traditional antimicrobials. Therefore, the necessity of developing new antibiotics or management to deal with bacterial infections has been increasing. The current study combined a low concentration of glutaraldehyde (GTA) with near-infrared (NIR) light and 405-nm laser to entail antibacterial activity on Staphylococcus aureus biofilm. MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay and colony forming unit (CFU) counting were used to quantify the viable cells while fluorescent and scanning electron microscopic images were used to qualitatively evaluate the cell membrane integrity and structural deformation, respectively. Practically, S. aureus biofilm was highly susceptible (7% cell viability and 6.8-log CFU/cm2 bacterial reduction for MTT assay and CFU analysis, respectively) to the combination of GTA (0.1%), NIR light (270 J/cm2), and 405-nm laser (288 J/cm2) exposure. GTA could form either DNA-protein or protein-protein crosslinks to inhibit DNA and protein synthesis. The NIR light induced the thermal damage on protein/enzymes while 405-nm laser could induce reactive oxygen species (ROS) to damage the bacterial membrane. Thus, the proposed technique may be a feasible modality for endoscope cleaning to prevent any secondary infection in the healthcare industry.