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This study aimed to evaluate the antimicrobial efficacy of the combination of ɛ-polylysine (ɛ-PL) and carvacrol (Car) against foodborne pathogens, Escherichia coli and Staphylococcus aureus. The minimum inhibitory concentrations (MICs) of ɛ-PL (Car) against E. coli and S. aureus were 25 μg/mL (320 μg/mL) and 12.5 μg/mL (320 μg/mL), respectively. Checkerboard assays showed that the combination of ɛ-PL and Car exerted synergistic effects against E. coli and S. aureus with fraction inhibitory concentration index (FICI) of 0.375 and 0.5, respectively. It demonstrated that the combination of ɛ-PL and Car significantly inhibited the growth of the two strains compared to single treatment. Furthermore, the mode of action of ɛ-PL (6.25 μg/mL) or Car (80 μg/mL) in inhibiting E. coli and S. aureus was researched by assessing their changes with regard to cellular membrane integrity, membrane permeability, respiratory activity, and membrane structure. A combination of ɛ-PL and Car increased the damage to cell membranes and their permeability and led to the release of 260 nm absorbing materials, decreased respiratory-chain dehydrogenase activity compared with ɛ-PL or Car treatment alone. These results demonstrated that the combination of ɛ-PL and Car could be used as a new promising naturally sourced food preservative.

Recently, ready-to-eat vegetable salads have gained popularity worldwide. However, the microbial safety of ready-to-eat salads is a health concern, primarily due to Salmonella enteritidis contamination during the growing, harvesting, processing, and handling of produce. This study aimed to develop a bacteriophage-based strategy to control S. enteritidis growth in mixed-ingredient salads. A lytic Salmonella-specific phage SapYZU01 was isolated from a soil sample from a suburban vegetable field in Yangzhou, China. SapYZU01 exhibited characteristics such as a short latent period, a large burst size, and a lytic effect against 13 S. enteritidis strains isolated from various sources (human, pork, deli, chicken, and chicken meat). The SapYZU01 genome did not contain virulence or antibiotic resistance genes. SapYZU01 significantly decreased the viability of S. enteritidis cells in iceberg lettuce, chicken meat, and mixed-ingredient (lettuce+chicken) salads at 37 and 25 °C. Furthermore, bacterial counts in the salad decreased significantly (by 4.0 log colony-forming units (CFU)/g) at 25 °C upon treatment of contaminated lettuce with SapYZU01 at an MOI of 100 prior to salad preparation. Bacterial counts were decreased by 3.8 log CFU/g at 25 °C in (lettuce+chicken) salads treated with SapYZU01 at an MOI of 100 after the salad preparation. In contrast, treating cooked chicken meat with SapYZU01 at an MOI of 100 before mixing it with contaminated lettuce decreased the bacterial count of the salad by 1.2 log CFU/g at 25 °C. These findings indicate the potential application of SapYZU01 as a natural biocontrol agent against S. enteritidis in mixed-ingredient salads. Furthermore, the application of lytic bacteriophage SapYZU01 in mix-ingredient salads should considered the bacteriophage treatment method in addition to the bacteriophage concentration.

The applications of carvacrol are limited due to its poor stability and water solubility, and high volatility; however, ovalbumin can be used to encapsulate hydrophobic molecules, improve their aqueous solubility, and reduce their volatility. In this study, we fabricated ovalbumin-carvacrol nanoparticles (OCGns) under different pH (2, 5, 7, and 9) conditions using a gel embedding method and investigated their physicochemical and antibacterial properties. Rheological experiments revealed that the G' of ovalbumin gels (OGs) prepared under different pH conditions were OG-2 > OG-7 > OG-9 > OG-5. Carvacrol addition reduced the tight structure of ovalbumin and carvacrol under pH 5 and 7 conditions, with hardness first decreasing and then increasing, but increasing under pH 2 and 9 conditions. Fluorescence and infrared spectroscopy indicated complex formation, with carvacrol increasing the average diameter of nanoparticles prepared at pH 2, 5, 7, and 9. Encapsulation reached 89.34 and 91.86% at pH 2 and 9, respectively; however, inhibition experiments revealed that the minimum inhibitory concentration of OCGn-2 against Gram-positive Bacillus cereus and Salmonella (0.08 and 0.16 mg mL-1, respectively) was lower than that of OCGn-9 (both 0.28 mg mL-1). Moreover, OCGn-2 possessed a better dense gel structure and a higher stability, encapsulation rate, and antibacterial activity, suggesting that pH affects gel microstructure and thus the encapsulation efficiency and bacteriostatic properties of the prepared nanoparticles. These results contribute to our knowledge of the design and fabrication of polymeric nanoparticle delivery systems for bioactive compounds with beneficial properties.

Enterobacter hormaechei is a foodborne pathogen responsible for neonatal sepsis in humans and respiratory disease in animals. In this work, a new virulent phage (P.A-5) infecting E. hormaechei was isolated from domestic sewage samples and characterized. Transmission electron microscopy revealed that P.A-5 belonged to the family Myoviridae having a head size of 77.53 nm and a tail length of 72.24 nm. The burst size was 262 PFU/cell after a latent period of 20 min. Phage P.A-5 was able to survive in a pH range of 4–9 and resist temperatures up to 55 °C for 60 min. The genome sequence of P.A-5 had homology most similar to that of Shigellae phage MK-13 (GenBank: MK509462.1). Pork artificially contaminated with E. hormaechei was used as a model to evaluate the potential of P.A-5. The results clearly showed that P.A-5 treatment can completely inhibit E. hormaechei growth in pork within 8 h, indicating the potential use of P.A-5 as a biocontrol agent for E. hormaechei.

In this study, carvacrol (Car) is first embedded in β-cyclodextrin (β-CD) by the freeze-drying method to form the β-cyclodextrin-Carvacrol inclusion compound (β-CD-Car), and then β-CD-Car liposomes (β-CD-Car-Lip) and β-CD-Car liposomes coated with e-polylysine (e-PL/β-CD-Car-Lip) with different concentrations (0–20 mg/mL) of β-CD-Car were prepared. The liposomes were characterized, and their physicochemical properties, in vitro release characteristics, and antibacterial activities were analyzed. Results showed that the fabricated liposomes β-CD-Car-Lip and e-PL/β-CD-Car-Lip were nanosized and spherical, and the latter had a larger particle size (152.21 ± 14.48 to 520.45 ± 30.69 nm) and greater encapsulation efficiency (69.23 ± 0.95 to 73.25 ± 0.65%). The in vitro release study results showed that the rate of release from e-PL-coated liposomes was much lower than that from uncoated liposomes. Inhibition of bacterial growth experiments revealed that the minimum inhibitory concentration of e-PL/β-CD-Car-Lip against Escherichia coli and Staphylococcus aureus (0.025 and 0.05 mg/mL, respectively) was twice as low as that of β-CD-Car-Lip and nearly 12 times lower than that of Car at the same concentration. The e-polylysine coating increased the diameter, encapsulation efficiency (EE), release time, and antibacterial activity of β-CD-Car-Lip, and the fabricated complex liposome could be applicable as an additive in the design of antibacterial packaging.

Pickle is a type of mildly lactic acid fermented vegetable and is a traditional dish favored in China, Japan, and Korea. Corruption of spoilage bacteria and accumulation of nitrite during vegetable fermentation are common problems that affect the pickle industry and consumer health. In this work, cucumber juice was used as a vegetable model to study the dominant mesophilic aerobic bacteria (MAB) producing nitrite during pickle fermentation. Virulent phages infecting the dominant MABs combined with Lactobacillus plantarum M6 were used to control these bacteria. Enterobacter cloacae and Pseudomonas fluorescens are the dominant MABs in the fermentation of cucumber juice containing 4% or 8% NaCl, with isolation percentages reaching 30.6% and 23.1%, respectively. Virulent phages PspYZU5415 and EcpYZU01 were isolated using P. fluorescens J5415 and E. cloacae J01 as the host bacteria, respectively. These two phages show a broad host range and strong lytic activity, and their genomes contain no toxins and antibiotic resistance genes. PspYZU5415 and EcpYZU01 were combined into a cocktail (designated as Phage MIX) that effectively inhibits the growth of E. cloacae and P. fluorescens in cucumber juice with different salt concentrations. PhageMIX combined with L. plantarum M6 decreased the counts of P. mendocina and E. cloacae to undetectable levels at 48 h during the fermentation of cucumber juice artificially contaminated with P. mendocina and E. cloacae. In addition, nitrite content increased to 11.3 mg/L at 20 h and then degraded completely at 36 h. By contrast, P. mendocina and E. cloacae remained in the groups without PhageMIX during fermentation (0–48 h). Nitrite content rapidly increased to 65.7 mg/L at 12 h and then decreased to 21.6 mg/L at 48 h in the control group. This study suggests that PhageMIX combined with lactic acid bacterial strains can be used as an ecological starter for controlling the dominant MABs P. mendocina and E. cloacae and for reducing nitrate production during the early stage of pickle fermentation.