Larry R. Beuchat’s research while affiliated with University of Georgia and other places

The influence of chemical diluents on the antimicrobial activity of plant essential oil (EO) vapors was evaluated. We first determined if vapors generated from 22 chemical diluents not containing EO had antimicrobial activities. Ethyl ether vapor retarded the growth of S. aureus. The minimal inhibitory concentrations (MICs) and the minimal lethal concentrations (MLCs) of cinnamon bark EO vapor, which was diluted in and generated from 21 diluents, against S. aureus and S. enterica were determined. Cinnamon bark EO vapor showed significantly (P ≤ 0.05) lower MICs against S. aureus when diluted in dimethyl sulfoxide (DMSO), ethanol, ethyl acetate, or jojoba oil, and against S. enterica when diluted in DMSO, ethanol, or jojoba oil, compared to those in other diluents. We compared antimicrobial activities of cinnamon bark EO vapor diluted in DMSO, ethanol, ethyl acetate, or jojoba oil against S. aureus and S. enterica on beef jerky as a food model. Antimicrobial activity was significantly (P ≤ 0.05) higher when vaporized from DMSO. These results indicate that antimicrobial activity of cinnamon bark EO vapor may vary significantly (P ≤ 0.05) depending on the type of diluent from which it is vaporized. These observations provide basic information when developing food and food-contact surface decontamination strategies using EO vapors.

We investigated the antimicrobial effects of essential oil (EO) vapors individually and in combination against Bacillus cereus. In an initial study, EO vapors were generated from 95 commercially available EOs and their antimicrobial activities against B. cereus were tested. Five EO vapors (citronella, lemongrass, may chang, oregano, and thyme thymol) showed strong inhibitory activity. Next, we determined the minimal inhibitory concentrations (MICs) and minimal lethal concentrations (MLCs) of these EOs against B. cereus cultured on nutrient agar; values were 0.0781 μL/mL and >5 μL/mL, respectively. The synergistic antimicrobial activity of combinations between EO vapors on B. cereus were evaluated using a modified checkerboard assay. Oregano and thyme thymol EO vapors showed greatest synergism (fractional inhibitory concentration index: 0.5000 at a 1:1 vol ratio). Finally, we tested for antimicrobial properties of oregano and thyme thymol EO vapors against B. cereus on iceberg lettuce individually and in combination. The growth of B. cereus was inhibited by 0.0049–0.0098 μL/mL of EO vapors at concentrations 8- to 16-fold lower than that of MICs on nutrient agar. However, a combination of oregano and thyme thymol EO vapors did not have a synergistic lethal effect. Observations provide basic information for the development of decontamination treatments to inhibit the growth of B. cereus and perhaps other spore-forming bacteria on foods and food-contact surfaces using EO vapors.

Aim: This study was done to determine the effects of temperature, pH, and sodium chloride (NaCl) on antimicrobial activity of magnesium oxide (MgO) nanoparticles (NPs) against Escherichia coli O157:H7. Methods and results: Culture conditions were established by varying the pH (5.0, 7.2, and 9.0), NaCl concentration (0.5, 2.0, 3.5, and 5.0%, w/v), and incubation temperatures (4, 12, 22, and 37°C). At each condition, the antimicrobial activities of MgO-NPs (0, 1, 2, and 4 mg/mL) against E. coli O157:H7 were measured. Four-way analysis of variance indicated interactions among all factors had a significant effect (P ≤ 0.05) on the antimicrobial activity of MgO-NPs. The concentration of MgO-NPs necessary to cause a 5-log reduction of E. coli O157:H7 under the most inhibitory conditions (37°C, pH 9.0, and 5.0% NaCl) was 0.50 mg/mL of MgO-NPs. Conclusion: The antimicrobial activity of the MgO-NPs increased significantly (P ≤ 0.05) with increased temperature, pH, and NaCl concentration in TSB. Significance and impact of the study: The influence of intrinsic and extrinsic factors on antimicrobial activity of MgO-NPs we found will contribute to the development of microbial decontamination strategies using MgO in the food industry.

We developed a short-time treatment to inactivate Xanthomonas campestris and Salmonella enterica on cabbage, chili pepper, and radish seeds by a combination of chlorine dioxide (ClO2) gas and heat. ClO2 gas was evaporated from a mixed solution which contained hydrochloric acid (1 N) and sodium chlorite (100,000 ppm) (ClO2 solution). To produce ClO2 gas (3,000 ppm) in an 1.8 L airtight container, a standard curve indicating the amount of ClO2 solution and time needed to produce desired concentrations was constructed (y = 5758.3x; R² = 0.9981). The influence of simultaneous treatment with ClO2 gas (3,000 ppm) and heat (70 °C at 85% relative humidity [RH]) on seed germination rates was determined. It was found that the germination rates of cabbage, chili pepper, and radish seeds were not significantly (P > 0.05) changed after 30, 20, and 60 min of treatment, respectively. The antimicrobial effects of the combined treatment against X. campestris and S. enterica on cabbage, chili pepper, and radish seeds were determined. When 3,000 ppm of ClO2 gas were applied at 70 °C and 85% RH for 10 min, the populations of X. campestris (initial populations: 6.2–6.4 log CFU/g) on seeds were reduced by > 4.4 log CFU/g regardless of seed type. The initial populations of S. enterica (6.3–6.5 log CFU/g) on cabbage and chili pepper seeds treated for 10 min was reduced by > 5.3 log CFU/g. However, populations on radish seeds decreased only by 2.5 log CFU/g. The lower lethality against S. enterica on radish seeds, revealed by field-emission scanning microscopy to have a rougher surface than do cabbage and chili pepper seeds, is attributed in part to inhibited access of ClO2 gas to cells in protected areas. Our findings provide useful information when developing more efficacious methods to decontaminate vegetable seeds using a combination of ClO2 gas and heat.

Aim: This study was done to develop a seed decontamination treatment for organic seeds against plant pathogens (Acidovorax citrulli and Xanthomonas campestris) using essential oil (EO) vapors without affecting the seeds' germination rate. Methods and results: By using a diffusion assay and determining minimum inhibitory and lethal concentrations, we screened two EO vapors which were most inhibitory to A. citrulli (cinnamon bark and garlic EO vapors) and X. campestris (onion and garlic EO vapors). After 48 h of exposure to EO vapors at 25°C and 43% or 85% relative humidity (RH), no significant decrease (P > 0.05) in germination rates was observed compared with those of control seeds. It was observed that EO vapor treatment at 25°C and 43% or 85% RH for 48 h caused significant population reductions (P ≤ 0.05) (ca. 0.3 - 2.6 log CFU/g) compared to those of untreated seeds. Conclusion: Applications of EO vapors showed significant (P ≤ 0.05) antimicrobial effects against A. citrulli and X. campestris on both laboratory mediums and plant seeds without decreasing germination rate of seeds. Significance and impact of the study: This study provides useful information for the development of natural seed sterilization treatments using EO vapors.

The objectives of this study were to screen essential oil (EO) vapors for inhibitory activities against Staphylococcus aureus on a laboratory medium and to validate their antibacterial effects on dried black peppercorns (Piper nigrum). Among 84 commercially available EOs, using a vapor disc assay, vapors from caraway seed, carrot seed, cinnamon bark, citronella, kanuka, lemongrass, may chang, oregano, and thyme thymol EOs were found to be more inhibitory to S. aureus. The minimal inhibitory concentration (MIC) and minimal lethal concentration (MLC) of nine EO vapors from these plant materials were measured on a laboratory medium. Citronella, lemongrass, and oregano EO vapors had the lowest MIC and MLC values, at 0.0781 and 0.1563 μL/mL, respectively. Using a checkerboard assay, partial synergism in inhibitory activity of a combination of citronella and lemongrass EO vapors was observed (fractional inhibitory concentration index = 0.6250). When tested against S. aureus on black peppercorns, citronella, lemongrass, and oregano EO vapors, both individually and in combination, showed a significant antibacterial effect (P ≤ 0.05). Notably, a combination of citronella and lemongrass EO vapors (1:4 volume ratio) showed significant synergistic activity, reducing the S. aureus population by ca. ≥ 1.3–1.8 log CFU/g more than treatment with single EOs (P ≤ 0.05). The results of this study provide important basic information for developing technologies to control S. aureus on the surfaces of dried foods using EO vapors.

Survival and growth of Salmonella and Shiga toxin-producing Escherichia coli (STEC) were investigated in kombucha prepared from four brands of commercially available kombucha kits intended for use by home brewers. Changes in populations of the indigenous microbiota responsible for fermentation of kombucha were also determined. An initial population of Salmonella (6.77 log CFU/mL) decreased to below the detection limit (0.30 log CFU/mL) within 10 days in kombucha prepared from two of the test brands. Populations of 1.85 and 1.20 log CFU/mL were detected in two brands fermented for 14 days. An initial STEC population of 7.02 log CFU/mL decreased to <0.30 log CFU/mL in two brands within 14 days; 0.77 and 0.87 log CFU/mL were detected in kombucha prepared from the other two brands. Salmonella and STEC increased within 1 day in three brands of base tea used to prepare kombucha and were stable throughout 14 days of incubation. Both pathogens steadily declined in base tea prepared from one brand of kombucha kit. Inactivation of the pathogens occurred as the pH of the kombucha decreased, but a clear correlation between rates of inactivation among different brands of kits and decrease in pH was not evident. Growth and peak populations of mesophilic aerobic microorganisms, yeasts, lactic acid bacteria, and acetic acid bacteria varied depending on the kombucha kit brand. No strong evidence was found of a correlation between the behavior of Salmonella or STEC and that of any of these groups of indigenous microbiota. Results of this study show that survival of Salmonella and STEC in kombucha and base tea used to prepare kombucha is dependent on inherent differences in commercially available kombucha kits intended for use in home settings. Strict application of hygienic practices is essential for preventing contamination with Salmonella or STEC and reducing the risk of illness associated the consumption of kombucha. Highlights:

This study was done to develop a method to inactivate Escherichia coli O157:H7 on radish and cabbage seeds using simultaneous treatments with gaseous chlorine dioxide (ClO2) and heat at high relative humidity (RH) without decreasing seeds’ viability. Gaseous ClO2 was spontaneously vaporized from a solution containing hydrochloric acid (HCl, 1 N) and sodium chlorite (NaClO2, 100,000 ppm). Using a sealed container (1.8 L), an equation (y = 5687×, R² = 0.9948) based on the amount of gaseous ClO2 generated from HCl–NaClO2 solution at 60 °C and 85% RH was developed. When radish or cabbage seeds were exposed to gaseous ClO2 at concentrations up to 3000 ppm for 120 min, germination rates did not significantly decrease (P > 0.05). When seeds inoculated with E. coli O157:H7 were treated with 2000 or 3000 ppm of gaseous ClO2 in an atmosphere with 85% RH at 60 °C, populations (6.8–6.9 log CFU/g) on both types of seeds were decreased to below the detection limit for enrichment (−0.5 log CFU/g) within 90 min. This study provides useful information for developing a decontamination method to control E. coli O157:H7 and perhaps other foodborne pathogens on plant seeds by simultaneous treatment with gaseous ClO2 and heat at high RH.

The objective of this study was to identify combinations of plant essential oils (EOs) that have synergistic antilisterial effects in raw organic tomato juice. Among the EOs approved for addition to organic foods in the Republic of Korea, twenty-four EOs were screened for antilisterial activity using a well diffusion assay. Six EOs (cinnamon bark, lemon, lemongrass, oregano, peppermint, and thyme thymol) showed relatively strong inhibitory activities (diameter of zone of inhibition ≥ 35.0 mm) against Listeria monocytogenes. The minimal inhibitory concentrations (MICs) and minimal lethal concentrations (MLCs) of these EOs against L. monocytogenes in a laboratory medium were determined. Oregano and thyme thymol EOs had the lowest MIC (0.3125 μL/mL), and cinnamon bark, oregano, and thyme thymol EOs had the lowest MLC (0.6250 μL/mL). The synergistic antilisterial activities of combinations of EOs in a laboratory medium were determined using a modified checkerboard assay. Combinations of cinnamon bark and lemongrass EOs, cinnamon bark and thyme thymol EOs, and lemongrass and thyme thymol EOs showed synergistic antilisterial activities (fractional inhibitory concentration index = 0.5). A combination of cinnamon bark and thyme thymol EOs was tested for synergistic antilisterial activity in tomato juice incubated at 10°C and 25°C for up to 48 h. The number of L. monocytogenes in juice treated with combined EOs at a total concentration of 0.3125 μL/mL decreased significantly (P ≤ 0.05) compared to numbers in juice treated with 0.3125 μL/mL of either cinnamon bark EO or thyme thymol EO alone. These observations show the potential for using combinations of EOs for controlling L. monocytogenes in juice released from cut or bruised raw organic tomatoes intended for consumption in food service or home settings.