Sublethal Amounts of Origanum vulgare L. Essential Oil and Carvacrol Cause Injury and Changes in Membrane Fatty Acid of Salmonella Typhimurium Cultivated in a Meat Broth

Natural phenolic compounds: Antimicrobial properties, antimicrobial mechanisms, and potential utilization in the preservation of aquatic products

Natural antibacterials have stood out in the last decade due to the growing demand for reducing chemical preservatives in food. In particular, natural phenolic compounds are secondary metabolites produced by plants for numerous functions including antimicrobial defence. Polyphenol has significant antimicrobial activity, but its antimicrobial properties are affected by the cell structure difference of bacteria, the concentration, type, and extraction method of polyphenol, and the treatment time of bacteria exposed to polyphenol. Therefore, this paper analyzed the antibacterial activity and mechanism of polyphenol as an antimicrobial agent. However, there remained significant considerations, including the interaction of polyphenols and food matrix, environmental temperature, and the effect of color and odor of some polyphenols on sensory properties of aquatic products, and the additive amount of polyphenols. On this basis, the application strategies of polyphenols as the antimicrobial agent in aquatic products preservation were reviewed.

Chemical composition of Piper gaudichaudianum Kunth essential oil and evaluation of its antimicrobial and modulatory effects on antibiotic resistance, antibiofilm, and cell dimorphism inhibitory activities

Essential oils extracted from many plant species have different biological activities, among which microbial activity stands out. Species of the genus Piper have antimicrobial potential against different species of bacteria and fungi. In this sense, the present study aimed to determine the chemical composition of the essential oil from the leaves of Piper gaudichaudianum (EOPG), as well as to investigate their antimicrobial activity and their modulatory effect on the Norfloxacin resistance in the Staphylococcus aureus SA1199B strain overproducer of the NorA efflux pump. Furthermore, their inhibitory activities on the biofilm formation as well as on the cellular differentiation of C. albicans were evaluated. Gas chromatography analysis identified 24 compounds, such as hydrocarbon sesquiterpenes (54.8%) and oxygenated sesquiterpenes (28.5%). To investigate the antimicrobial potential of EOPG against S. aureus, E. coli, and C. albicans, a microdilution assay was performed, and no intrinsic antimicrobial activity was observed. On the other hand, the oil potentiated the activity of Norfloxacin against the SA1199B strain, indicating that EOPG could be used in association with Norfloxacin against S. aureus strains resistant to this antibiotic. EOPG also inhibited S. aureus biofilm formation, as evidenced by the crystal violet assay. In the dimorphism assay, EOPG was able to inhibit the cell differentiation process in C. albicans. Results indicate that EOPG could be used in association with Norfloxacin in the treatment of infections caused by resistant S. aureus strains overproducing the NorA efflux pump. Furthermore, its ability to inhibit the formation of hyphae by C. albicans suggests that EOPG could also be applied in the prevention and/or treatment of fungal infections.

Mechanism of Inhibiting the Growth and Aflatoxin B1 Biosynthesis of Aspergillus flavus by Phenyllactic Acid

Phenyllactic acid (PLA), a promising food preservative, is safe and effective against a broad spectrum of food-borne pathogens. However, its mechanisms against toxigenic fungi are still poorly understood. In this study, we applied physicochemical, morphological, metabolomics, and transcriptomics analyses to investigate the activity and mechanism of PLA inhibition of a typical food-contaminating mold, Aspergillus flavus. The results showed that PLA effectively inhibited the growth of A. flavus spores and reduced aflatoxin B₁ (AFB₁) production by downregulating key genes associated with AFB₁ biosynthesis. Propidium iodide staining and transmission electron microscopy analysis demonstrated a dose-dependent disruption of the integrity and morphology of the A. flavus spore cell membrane by PLA. Multi-omics analyses showed that subinhibitory concentrations of PLA induced significant changes in A. flavus spores at the transcriptional and metabolic levels, as 980 genes and 30 metabolites were differentially expressed. Moreover, KEGG pathway enrichment analysis indicated PLA-induced cell membrane damage, energy-metabolism disruption, and central-dogma abnormality in A. flavus spores. The results provided new insights into the anti-A. flavus and -AFB₁ mechanisms of PLA.

Sublethally injured microorganisms in food processing and preservation: Quantification, formation, detection, resuscitation and adaption

Sublethally injured state has been recognized as a survival strategy for microorganisms suffering from stressful environments. Injured cells fail to grow on selective media but can normally grow on nonselective media. Numerous microorganism species can form sublethal injury in various food matrices during processing and preservation with different techniques. Injury rate was commonly used to evaluate sublethal injury, but mathematical models for the quantification and interpretation of sublethally injured microbial cells still require further study. Injured cells can repair themselves and regain viability on selective media under favorable conditions when stress is removed. Conventional culture methods might underestimate microbial counts or present a false negative result due to the presence of injured cells. Although the structural and functional components may be affected, the injured cells pose a great threat to food safety. This work comprehensively reviewed the quantification, formation, detection, resuscitation and adaption of sublethally injured microbial cells. Food processing techniques, microbial species, strains and food matrix all significantly affect the formation of sublethally injured cells. Culture-based methods, molecular biological methods, fluorescent staining and infrared spectroscopy have been developed to detect the injured cells. Cell membrane is often repaired first during resuscitation of injured cells, meanwhile, temperature, pH, media and additives remarkably influence the resuscitation. The adaption of injured cells negatively affects the microbial inactivation during food processing.

Application of food-grade natural antimicrobials for the control of crop disease caused by phytopathogens

Phytopathogens can cause crop disease in agriculture, thus, synthetic pesticides are used to prevent disease. However, this type of pesticide has an adverse effect on human and environmental health. Consequently, it is important to develop natural pesticides for crop protection. The final goal of crop protection is enhancing the quality of fresh products for consumers, suggesting that crop safety is related with food safety. Several studies have investigated the effect of food-grade natural antimicrobials on phytopathogens in vitro and ex vivo. The objective of this review is to provide an overview of bactericidal effects when using natural antimicrobial compounds, including essential oils, plant extracts, and carboxylic acids, and their mechanisms of antibacterial action. However, more studies are needed to investigate the efficacy of natural antimicrobial compounds on phytopathogens in field conditions.

Antimicrobial effect and mechanism of non-antibiotic alkyl gallates against Pseudomonas fluorescens on the surface of Russian sturgeon (Acipenser gueldenstaedti)

Since Pseudomonas fluorescens is the main microorganism causing severe spoilage in refrigerated aquatic products, the searching for non-antibiotic antibacterial agents effective against it continues to receive increasing interest. This study aimed to investigate the antibacterial effects and mechanisms of alkyl gallic esters against P. fluorescens isolated from the Russian sturgeon (Acipenser gueldenstaedti), as well as the effectiveness in combination with chitosan films on the preservation of sturgeon meats at 4 °C. Our data shows that the alkyl chain length plays a significant role in eliciting their antibacterial activities and octyl gallate (GAC8) exhibited an outstanding inhibitory efficacy. GAC8 can rapidly enter into the membrane lipid bilayer portion to disorder the membrane, and further inhibit the growth of the P. fluorescens through interfering both tricarboxylic acid cycle related to energy supply and amino acid metabolism associated with cell membranes, suppressing oxygen consumption and disturbing the respiration chain. Moreover, the alteration in membrane fatty acids indicated that GAC8 could disrupt the composition of cell membrane fatty acids, rendering the bacteria more sensitive to the antibacterial. The SEM results also substantiate the damage of the structure of the bacterial membrane caused by GAC8. Additionally, the edible chitosan-based films incorporated with GAC8 showed the enhanced antibacterial efficacy to remarkably extend the shelf life of Russian sturgeon. Overall, our findings not only provide new insight into the mode of action of GAC8 against P. fluorescens but also demonstrate composite films containing GAC8, as a kind of safe and antibacterial material, have a great promise for application in food preservations.

Sub-lethal Concentrations of Phytochemicals (Carvacrol and Oregano) Select for Reduced Susceptibility Mutants of Escherichia coli O23:H52

In vitro studies aimed at studying the mechanism of action of carvacrol and oregano as natural anti-bacterial agents to control multiple antibiotic-resistant avian pathogenic Escherichia coli (APEC) strain O23:H52 isolated from chicken were performed. Derivatives with increased minimum inhibitory concentrations (MIC) to the phytochemicals were selected after growing Escherichia coli (E. coli) strain O23:H52 at sub-lethal concentrations of carvacrol and oregano for a period of 60 days. Whole-genome sequencing (WGS) of two derivatives revealed a missense mutation in cadC and marR: the genes responsible for survival mechanisms and antibiotic resistance by efflux, respectively.

In vitro studies aimed at studying the mechanism of action of carvacrol and oregano as natural anti-bacterial agents to control multiple antibiotic-resistant avian pathogenic Escherichia coli (APEC) strain O23:H52 isolated from chicken were performed. Derivatives with increased minimum inhibitory concentrations (MIC) to the phytochemicals were selected after growing Escherichia coli (E. coli) strain O23:H52 at sub-lethal concentrations of carvacrol and oregano for a period of 60 days. Whole-genome sequencing (WGS) of two derivatives revealed a missense mutation in cadC and marR: the genes responsible for survival mechanisms and antibiotic resistance by efflux, respectively.

Successive exposure to Mentha piperita L. essential oil affects the culturability and induces membrane repair in a persister epidemic Salmonella Typhimurium PT4

This study had as aims to evaluate the effects of successive exposures to Mentha piperita L. essential oil (MPEO) on culturability and physiological functions of Salmonella Typhimurium PT4. S. Typhimurium PT4 cells (10⁸ log CFU/mL) were exposed to the same (1.25 μL/mL) or increasing MPEO concentrations (1.25-80 μL/mL) during 252 h. At each 36-h interval, the viable cell counts, and distinct cell functions were assessed using plate counting and flow cytometry, respectively. As the exposure time to the same MPEO concentration increased, the population of S. Typhimurium PT4 cells with damaged, permeabilized and depolarized membrane, and compromised efflux activity decreased. Otherwise, S. Typhimurium PT4 cells with damaged membrane physiological functions increased over the exposure to increasing concentrations of MPEO. Genomic analyses showed that the strain carries 17 genes associated with stress responses and the persistence of the tested strain among sources associated with poultry spanning more than 16 years and its virulence for humans. Therefore, successive exposure to a sublethal concentration of MPEO induced S. Typhimurium PT4 cells capable of maintaining the membrane integrity and its functions despite their non-culturable state.

Proteomic analysis and antibacterial resistance mechanisms of Salmonella Enteritidis submitted to the inhibitory effect of Origanum vulgare essential oil, thymol and carvacrol

Biological properties of natural products are an important research target and essential oils (EO) from aromatic plants with antimicrobial properties are well documented. However, their uses are limited, and the mechanisms underlying their antibacterial activity are still not well known. Therefore, our objective was to evaluate the antibacterial activities of Origanum vulgare EO, thymol and carvacrol against Salmonella Enteritidis ATCC 13076 strain, particularly regarding the bacterial proteic profile, enzymatic activities and DNA synthesis. Bacterial expressed proteins were evaluated using an untreated assay control and treatments with sublethal concentrations of oregano EO, carvacrol and thymol. The same protein extracts were also assayed for oxidative stress and energy metabolism enzyme activities, as well as effect on DNA synthesis. Protein expression outcomes revealed by 2D-SDS-PAGE, from antimicrobial actions, showed a stress response with differential expressions of chaperones and cellular protein synthesis mediated by the bacterial signaling system. In addition, Salmonella used a similar mechanism in defense against oxidative stress, for its survival. Thus, the antibacterial inhibitory activity of EO was preferentially associated with the presence of thymol and there was interference in protein regulation as well as DNA synthesis affected by these compounds. SIGNIFICANCE: Antimicrobial activity of essential oils (EO) is already known. In this way, the understanding of how this activity occurs is a fundamental part to provide the practical and rational use of these substances. In the current scenario, where the emergence of resistant bacteria or even multiresistant bacteria against conventional antimicrobials, the search for alternatives becomes essential, since the discovery of new inhibitory substances does not occur at the same speed. The anti-Salmonella action allied to the knowledge about the biological processes affected by O. vulgare EO contribute to these bioactive compounds being effectively used as agents in the safety and shelf life of food in a future product, packaging or process where the antibacterial activity is safe and best used.

Synergistic Combinations of Antibiotics with Cumin, Oregano and Rosewood Oils as a Strategy to Preserve the Antibiotic Repertoire

Background:

Formulations employing synergistic combinations of antibiotics with Essential Oils (EOs) could help preserve the antibiotic repertoire by improving their activity against resistant bacteria.

Objective:

Antimicrobial interactions between double and triple combinations of EOs, EO components and antibiotics were determined using the checkerboard method. The most active triple combinations were then assessed by a time-kill assay.

Methods:

Two synergistic EO-antibiotic combinations and eight additive EO-antibiotic combinations reduced the antibiotic minimum inhibitory concentration below clinical sensitivity breakpoints according to the checkerboard method. However, all the tested combinations were additive according to the time-kill assay; while the combinations completely killed S. aureus, E. coli and P. aeruginosa cells in 2 h. At least one EO compound from the combination alone completely killed the cells of test species.

Results:

Two synergistic EO-antibiotic combinations and eight additive EO-antibiotic combinations reduced the antibiotic minimum inhibitory concentration below clinical sensitivity breakpoints according to the checkerboard method. However, all the tested combinations were additive according to the time-kill assay; while the combinations completely killed S. aureus, E. coli and P. aeruginosa cells in 2 h. At least one EO compound from the combination alone completely killed the cells of test species.

Conclusion:

Positive interactions support the use of EOs or EO components to enhance antibiotic efficacy against antibiotic resistant bacteria. The EO-antibiotic combinations tested by the time kill assay were indifferent; therefore, the observed antimicrobial activity did not arise from synergistic mechanisms as indicated by the checkerboard method. Investigation of other synergistic combinations identified by the checkerboard method could reveal more promising candidates.

Efficacy of oregano and rosemary essential oils to affect morphology and membrane functions of noncultivable sessile cells of Salmonella Enteritidis 86 in biofilms formed on stainless steel

AIMS

This study evaluated the efficacy of essential oil from Origanum vulgare L. (oregano; OVEO) and Rosmarinus officinalis L. (rosemary; ROEO) to inactivate sessile cells of Salmonella enterica serovar Enteritidis 86 (SE86) in young and mature biofilms formed on stainless steel.

METHODS AND RESULTS

Ultrastructural alterations and damage in different physiological functions caused by OVEO and ROEO in noncultivable sessile cells of SE86 were investigated using scanning electron microscopy and flow cytometry. OVEO (2·5 μl ml^-1 ) and ROEO (40 μl ml^-1 ) were effective to eradicate young and mature biofilms formed by SE86 sessile cells on stainless steel surfaces; however, the efficacy varied with exposure time. OVEO and ROEO caused alterations in morphology of SE86 sessile cells, inducing the occurrence of bubbles or spots on cell surface. OVEO and ROEO compromised membrane polarization, permeability and efflux activity in noncultivable SE86 sessile cells. These findings show that OVEO and ROEO act by a multitarget mechanism on SE86 membrane functions.

CONCLUSIONS

ROEO and OVEO showed efficacy to eradicate SE86 sessile cells in preformed biofilms on stainless steel, displaying a time-dependent effect and multitarget action mode on bacterial cell membrane.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study provides for the first time the effects of OVEO and ROEO on morphology and physiological functions of noncultivable sessile cells of S. Enteritidis biofilms preformed on stainless steel surfaces.

Quantitative assessment of tolerance response to stress after exposure to oregano and rosemary essential oils, carvacrol and 1,8-cineole in Salmonella Enteritidis 86 and its isogenic deletion mutants ∆dps, ∆rpoS and ∆ompR

This study assessed the influence of rpoS, dps and ompR genes on the tolerance response of Salmonella Enteritidis 86 (SE86) to homologous and heterologous stressing agents after exposure to essential oils (EOs) from Origanum vulgare L. (oregano; OVEO) and Rosmarinus officinalis L. (rosemary; ROEO) and their major constituents (ICs), carvacrol (CAR) and 1,8-cineole (CIN), respectively, by modelling the log reduction over time. Minimum inhibitory concentration values of OVEO (1.25 μL/mL), CAR (0.62 μL/mL), ROEO (20 μL/mL) and CIN (10 μL/mL) against SE86 were always one-fold higher than those against ∆dps, ∆rpoS and ∆ompR mutants. Exposure to the same concentration of OVEO, CAR, ROEO or CIN caused higher reductions (up to 2.5 log CFU/mL) in ∆dps, ∆rpoS and ∆ompR mutants than in SE86 in chicken broth. In assays with homologous stressing agents, ompR, dps and rpoS influenced the tolerance to OEs or ICs. After adaptation to OVEO, CAR, ROEO and CIN, osmotolerance and acid tolerance of SE86 were influenced by rpoS gene, while thermotolerance of SE86 was influenced by ompR. Tolerance of SE86 to sodium hypochlorite after adaptation to OEs or ICs was influenced by rpoS and dps. These findings quantitatively describe for the first time the influence of rpoS, dps and ompR genes on the tolerance of Salmonella Enteritidis to OVEO, CAR, ROEO and CIN.

Susceptibility Distribution to Essential Oils of Salmonella enterica Strains Involved in Animal and Public Health and Comparison of the Typhimurium and Enteritidis Serotypes

To determine the distribution of the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of essential oils (EOs) of cinnamon (Cinnamomum zeylanicum), clove (Eugenia caryophyllata), oregano (Origanum vulgare), common thyme (Thymus vulgaris), and red thyme (Thymus zygis) against Salmonella enterica, double serial dilutions of each EO were challenged with 85 Salmonella strains belonging to 23 serotypes of animal origin. The results showed the bactericidal character of the EOs tested against S. enterica, highlighting the oregano with MIC₅₀ and MBC₅₀ of 3.12 × 10^-4 g/mL, and MIC₉₀ and MBC₉₀ of 6.25 × 10^-4 g/mL. When comparing the Salmonella Typhimurium and Salmonella Enteritidis serotypes susceptibility, we observed a significantly higher sensitivity of Typhimurium to clove and Enteritidis to cinnamon. In addition, Typhimurium isolates with significantly higher MIC and MBC values for all the EOs tested were found, suggesting the existence of a possible resistance profile. The results of this study provide relevant data for the potential of EOs as antibacterials, although they highlight the need to continue bacterial sensitivity distribution studies and consider the differences detected for future in vivo studies.

Citrus fruit extracts with carvacrol and thymol eliminated 7-log acid-adapted Escherichia coli O157:H7, Salmonella typhimurium, and Listeria monocytogenes: A potential of effective natural antibacterial agents

Despite the widespread belief that citrus fruit extracts (CFEs) are microbiologically safe due to their acidity, limited bactericidal effect results in low applicability as antibacterial agent and outbreaks occurred by acid-adapted pathogens. Here, we examined the antibacterial effects of CFEs [lime (Citrus medica), lemon (Citrus limon), calamansi (Citrus microcarpa)] combined with essential oil components (EOCs; carvacrol and thymol) against non-acid-adapted/acid-adapted Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes under 22 °C for 5 min. CFEs (<20%) alone or small amounts of EOCs (2.0 mM; 0.032%) alone could not inactivate the target bacteria effectively. However, combined treatments exhibited marked synergy: CFE + EOCs eliminated all the bacteria (>6.9 log CFU/ml). Among the CFEs tested, the highest synergism was shown by calamansi, an exotic citrus fruit previously unrecognized as an antibacterial agent. Although acid-adaptation improved bacterial survival, calamansi (<20%) + EOCs (<0.032%) completely inactivated even the most resistant pathogen (E. coli O157:H7). Validation test also showed that all tested commercial juice products also eliminated acid-adapted pathogens when used with EOCs. Physicochemical analysis of tested CFEs (pH measurement and HPLC analysis of components) revealed that low pH and flavanone (hesperidin) did not contribute to the synergistic bactericidal effects. Rather, the high citric acid content is likely to contribute to the strong synergistic effect with EOCs by damaging susceptible bacterial membranes. Sensory scores for CFEs were not altered by addition of EOCs at concentrations up to 1.5 mM. This study provides new insight into the utility of CFEs with EOCs to improve not only the microbiological safety of food products containing CFEs but also their applicability as natural antibacterial complex.

Modification of membrane properties and fatty acids biosynthesis-related genes in Escherichia coli and Staphylococcus aureus: Implications for the antibacterial mechanism of naringenin

In this work, modifications of cell membrane fluidity, fatty acid composition and fatty acid biosynthesis-associated genes of Escherichia coli ATCC 25922 (E. coli) and Staphylococcus aureus ATCC 6538 (S. aureus), during growth in the presence of naringenin (NAR), one of the natural antibacterial components in citrus plants, was investigated. Compared to E. coli, the growth of S. aureus was significantly inhibited by NAR in low concentrations. Combination of gas chromatography-mass spectrometry with fluorescence polarization analysis revealed that E. coli and S. aureus cells increased membrane fluidity by altering the composition of membrane fatty acids after exposure to NAR. For example, E. coli cells produced more unsaturated fatty acids (from 18.5% to 43.3%) at the expense of both cyclopropane and saturated fatty acids after growth in the concentrations of NAR from 0 to 2.20mM. For S. aureus grown with NAR at 0 to 1.47mM, the relative proportions of anteiso-branched chain fatty acids increased from 37.2% to 54.4%, whereas iso-branched and straight chain fatty acids decreased from 30.0% and 33.1% to 21.6% and 23.7%, respectively. Real time q-PCR analysis showed that NAR at higher concentrations induced a significant down-regulation of fatty acid biosynthesis-associated genes in the bacteria, with the exception of an increased expression of fabA gene. The minimum inhibitory concentration (MIC) of NAR against these two bacteria was determined, and both of bacteria underwent morphological changes after exposure to 1.0 and 2.0 MIC.

Effects of adaptation to carvacrol on Staphylococcus aureus in the planktonic and biofilm phases

The effect of exposure to sub-minimum inhibitory concentrations of carvacrol, for either 3-10 days, on direct (carvacrol) or cross-protection (cinnamaldehyde, eugenol, antibiotics) and the influence on planktonic and biofilm growth of four Staphylococcus aureus strains were reported. The sequential exposure to carvacrol resulted in a direct protection that was more evident in two of the four strains after 10 days. No significant cross-protection against cinnamaldehyde, eugenol and antibiotics was detected. An adaptive response was associated with a prolonged lag phase, a lower yield of bacteria, a colony phenotype likely to be associated to small colony variants and an increase in biofilm production. Generally, the biofilm of the adapted strains was less susceptible to subMICs of carvacrol compared to the biofilms of non-adapted strains. In contrast, it was demonstrated that in the case of mature biofilms the susceptibility was similar. The exposure of S. aureus to carvacrol at concentrations above the MIC resulted in a very low mutation frequency.

Cinnamaldehyde, Carvacrol and Organic Acids Affect Gene Expression of Selected Oxidative Stress and Inflammation Markers in IPEC-J2 Cells Exposed to Salmonella typhimurium

Essential oils and organic acids are used as feed additives to improve health status and reduce colonization with pathogens. Although bactericidal in vitro, concentrations achieved in the animal gut are probably not lethal to pathogens. The aim of this study was to investigate the effects of cinnamaldehyde, carvacrol and cinnamic, lactic and propionic acids on the ability of Salmonella typhimurium ATCC 14028 (ST) to invade intestinal epithelial cells (IPEC-J2) and on the expression levels of immune related genes in the cells. The minimum inhibitory concentration (MIC) and non-inhibitory concentration (NIC) were determined and influence on the invasion capacity of ST was investigated. The structure of fimbriae and flagella was analysed by electron microscopy, and expression levels of HSP70, IkBa, IL-8 and IL-10 in the IPEC-J2 cells were carried out by q-PCR. Cinnamaldehyde, carvacrol and cinnamic and propionic acids inhibited ST invasion but not cell viability, bacterial viability and motility or the development of flagella. Propionic acid and cinnamaldehyde in combination with cinnamic acid caused structural impairment of fimbriae. Cinnamaldehyde up-regulated expression of HSP70 irrespective of the presence of organic acids or ST; exposure to carvacrol induced HSP70 only in the presence of propionic acid and ST. © 2016 The Authors. Phytotherapy Research published by John Wiley & Sons Ltd.

Synergism between carvacrol or thymol increases the antimicrobial efficacy of soy sauce with no sensory impact

Here, we examined the antimicrobial effects of soy sauce containing essential oils (EOs) against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes at 22°C and 4°C. To screen a variety of combined effects, soy sauce was mixed with six different EOs (carvacrol, thymol, eugenol, trans-cinnamaldehyde, β-resorcylic acid, and vanillin), each at a concentration of 1mM for 10 min. None of the oils showed bactericidal activity when used alone. Soy sauce combined with carvacrol and thymol induced the greatest antibacterial activity against all tested bacteria; therefore, these oils were further tested at 0.25, 0.5, and 1mM (0.0039%, 0.0078%, and 0.0157%) for 1, 5, and 10 min at 4°C and 22°C. In addition, sensory evaluation of soy sauce containing each EO at 0.25, 0.5, 1, and 2mM was performed using the nine point hedonic test. Carvacrol or thymol (1mM) eliminated all the test bacteria (initial population, 7.0-7.5logCFU/ml) in 1-5 min at 22°C and within 10 min at 4°C. L. monocytogenes was slightly more tolerant at 4°C, which may be attributable to the ability of the cell membrane to adapt to low temperatures. The sensory scores for soy sauce containing EOs were not significantly different from that of soy sauce without EOs (P>0.05). The stability of EO efficacy in soy sauce was also verified. These results suggest that carvacrol and thymol act synergistically with other factors present in soy sauce to increase antimicrobial activity against major foodborne pathogens at both 4°C and 22°C. The synergism may be attributable to the combination of factors (mainly high salt concentration and low pH imparted by organic acids) present in soy sauce and the membrane attacking properties of carvacrol and thymol. This method will facilitate the production of microbiologically safe soy sauce, soy sauce-based marinades, and various marinated foods.

Antibiotic-resistant bacteria: prevalence in food and inactivation by food-compatible compounds and plant extracts

Foodborne antibiotic-resistant pathogenic bacteria such as Campylobacter jejuni, Bacillus cereus, Clostridium perfringens, Escherichia coli, Salmonella enterica, Staphylococcus aureus, Vibrio cholerae, and Vibrio parahemolyticus can adversely affect animal and human health, but a better understanding of the factors involved in their pathogenesis is needed. To help meet this need, this overview surveys and interprets much of our current knowledge of antibiotic (multidrug)-resistant bacteria in the food chain and the implications for microbial food safety and animal and human health. Topics covered include the origin and prevalence of resistant bacteria in the food chain (dairy, meat, poultry, seafood, and herbal products, produce, and eggs), their inactivation by different classes of compounds and plant extracts and by the use of chlorine and physicochemical methods (heat, UV light, pulsed electric fields, and high pressure), the synergistic antimicrobial effects of combinations of natural antimicrobials with medicinal antibiotics, and mechanisms of antimicrobial activities and resistant effects. Possible areas for future research are suggested. Plant-derived and other safe natural antimicrobial compounds have the potential to control the prevalence of both susceptible and resistant pathogens in various environments. The collated information and suggested research will hopefully contribute to a better understanding of approaches that could be used to minimize the presence of resistant pathogens in animal feed and human food, thus reducing adverse effects, improving microbial food safety, and helping to prevent or treat animal and human infections.

Tolerance response of multidrug-resistant Salmonella enterica strains to habituation to Origanum vulgare L. essential oil

Multidrug-resistant Salmonella enterica isolates from human outbreaks or from poultry origin were investigated for their ability to develop direct-tolerance or cross-tolerance to sodium chloride, potassium chloride, lactic acid, acetic acid, and ciprofloxacin after habituation in subinhibitory amounts ( of the minimum inhibitory concentration - (MIC) and of the minimum inhibitory concentration - MIC) of Origanum vulgare L. essential oil (OVEO) at different time intervals. The habituation of S. enterica to OVEO did not induce direct-tolerance or cross-tolerance in the tested strains, as assessed by the modulation of MIC values. However, cells habituated to OVEO maintained or increased susceptibility to the tested antimicrobials agents, with up to fourfold double dilution decrease from previously determined MIC values. This study reports for the first time the non-inductive effect of OVEO on the acquisition of direct-tolerance or cross-tolerance in multidrug-resistant S. enterica strains to antimicrobial agents that are largely used in food preservation, as well as to CIP, the therapeutic drug of salmonellosis.

Chemistry and multibeneficial bioactivities of carvacrol (4-isopropyl-2-methylphenol), a component of essential oils produced by aromatic plants and spices

Aromatic plants produce organic compounds that may be involved in the defense of plants against phytopathogenic insects, bacteria, fungi, and viruses. One of these compounds, called carvacrol, which is found in high concentrations in essential oils such as oregano, has been reported to exhibit numerous bioactivities in cells and animals. This integrated overview surveys and interprets our present knowledge of the chemistry and analysis of carvacrol and its beneficial bioactivities. These activities include its antioxidative properties in food (e.g., lard, sunflower oil) and in vivo and the inhibition of foodborne and human antibiotic-susceptible and antibiotic-resistant pathogenic bacteria, viruses, pathogenic fungi and parasites, and insects in vitro and in human foods (e.g., apple juice, eggs, leafy greens, meat and poultry products, milk, oysters) and food animal feeds and wastes. Also covered are inhibitions of microbial and fungal toxin production and the anti-inflammatory, analgesic, antiarthritic, antiallergic, anticarcinogenic, antidiabetic, cardioprotective, gastroprotective, hepatoprotective, and neuroprotective properties of carvacrol as well as metabolic, synergistic, and mechanistic aspects. Areas for future research are also suggested. The collated information and suggested research might contribute to a better understanding of agronomical, biosynthetic, chemical, physiological, and cellular mechanisms of the described health-promoting effects of carvacrol, and facilitate and guide further studies needed to optimize the use of carvacrol as a multifunctional food in pure and encapsulated forms, in edible antimicrobial films, and in combination with plant-derived and medical antibiotics to help prevent or treat animal and human diseases.