Xuetong Fan

The objective of this work was to evaluate the effect of processing via ultrasonication on the physiochemical and antimicrobial properties of zein nanoemulsions (NE) loaded with essential oils (cinnamaldehyde [CA], D‐limonene [Lim], and their combination [CO]). NE consisting of 2% zein and 1% EO were prepared using probe sonication. Nanofilms (NF) were prepared from the subsequent NE using the solvent casting technique. Zeta potential, particle size, and polydispersity index (PDI) were monitored for 30 days at 4 and 22°C. Minimum inhibitory concentrations (MICs), minimal bactericidal concentrations (MBCs), and microbial reductions were also determined. Ultrasonication effectively reduced particle size, increased storage stability, and enhanced antimicrobial efficacy. SEM imaging revealed that the nanomaterial droplets ranged from 93.98 nm to 227.95 nm in diameter and remained stable after 1 month of storage at 4 or 22°C. In‐vitro results showed that NEs with CA and CO inhibited the growth of both Escherichia coli O157:H7 (EC) and Listeria monocytogenes (LM) at concentrations < 2000 ppm, whereas NEs with Lim had the least antimicrobial effect, with much higher MICs and MBCs than CA. NEs with CA exhibited bactericidal activity against EC and LM after 24 h of treatment. SEM imaging also showed that CA induced noticeable morphological changes in EC cells, indicating cell damage. Results confirm that essential oils, specifically cinnamaldehyde, can be loaded into zein‐based films and may be used as antimicrobial preservatives to enhance food safety. Practical Applications: In recent years there has been an increase in foodborne disease outbreaks associated with ready‐to‐eat produce. Two major bacterial pathogens responsible for these outbreaks are Listeria monocytogenes and Escherichia coli O157:H7. Several studies have shown that encapsulation of natural antimicrobials into edible nanoemulsion coatings and films can enhance their antimicrobial activity in food systems. The results of this study demonstrate that films and coatings made from zein‐based polymer loaded with cinnamaldehyde, limonene, and a combination of both essential oils effectively reduced populations of these pathogens. Our results also demonstrate that these nanoemulsions may be a promising tool to enhance the safety and quality of fresh produce.

Listeria monocytogenes contamination of fresh-cut produce poses a threat to consumers due to its high mortality rate in humans. Because it is widespread in the environment, as well as able to grow and survive in harsh conditions, such as low temperatures, it is difficult to control. A strain of L. monocytogenes , L008, with significantly faster growth than other strains, was identified in our laboratory during experiments with fresh-cut produce. The L008 strain had a consistently shorter lag phase and reached maximum spectrophotometric absorbance (optical density) faster than three other L. monocytogenes strains during growth curve studies at 4 and 8°C. The lag phase was also shorter when compared with two of the three strains at 12°C. A genetic comparison between L008 and the other three strains identified missense mutations in the fapR, mngB, pfkB, and uvrC genes of L008 that may account for the faster growth. These genes are involved in metabolism, cell membrane maintenance, and stress tolerance, which may allow L008 to adapt to lower temperatures better than the other strains. Accelerated growth at low temperatures may elevate the risk associated with this isolate and other L. monocytogenes strains with similar genetic variants. This study highlights the importance of including bacterial isolates that demonstrate a worst-case scenario for lag phase or more rapid growth in foodborne pathogen challenge studies. IMPORTANCE: Listeria monocytogenes is responsible for listeriosis in humans and has a relatively high incidence of hospitalization and mortality when compared to other foodborne pathogens. The psychrotrophic nature of L. monocytogenes allows it to grow at low temperatures, makes it difficult to control, and we identified an isolate with faster growth at 4, 8, and 12°C. A comparison of the isolate’s genome with three other L. monocytogenes strains found changes in genes involved in energy usage, cell membrane maintenance, and stress resistance. These genetic differences may allow the isolate to adjust to lower temperatures better than other strains and replicate more quickly. The ability to grow faster at low temperatures could increase the risk of foodborne illness from this isolate and other L. monocytogenes with similar genetic changes.

Gaseous chlorine dioxide (ClO₂) is a potent antimicrobial agent used to control microbial contamination in food and water. This study evaluates the bactericidal activity of gaseous ClO₂ released from a sodium chlorite (NaClO₂) pad against Campylobacter jejuni. Exposure to a low concentration (0.4 mg/L) of dissolved ClO₂ for 2 h resulted in a >93% reduction of C. jejuni, highlighting the bacterium’s extreme sensitivity to gaseous ClO₂. To elucidate the molecular mechanism of ClO₂-induced bactericidal action, transcriptomic analysis was conducted using RNA sequencing (RNA-seq). The results indicate that C. jejuni responds to ClO₂-induced oxidative stress by upregulating genes involved in reactive oxygen species (ROS) detoxification (sodB, ahpC, katA, msrP, and trxB), iron transport (ceuBCD, cfbpABC, and chuBCD), phosphate transport (pstSCAB), and DNA repair (rdgB and mutY). Reverse transcription-quantitative PCR (RT-qPCR) validated the increased expression of oxidative stress response genes but not general stress response genes (spoT, dnaK, and groES). These findings provide insights into the antimicrobial mechanism of ClO₂, demonstrating that oxidative damage to essential cellular components results in bacterial cell death.