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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., Competing Interests: Declaration of Competing Interest The authors state that no conflict of interest exists., (Copyright © 2019 Elsevier B.V. All rights reserved.)

We investigated the responses and mechanisms of action of methicillin-resistant Staphylococcus aureus (MRSA) metabolism when exposed under sublethal concentrations of the synergistic antibacterial combination of nisin + oxacillin (¼ of maximum sublethal concentration) and sublethal concentrations of oxacillin only and nisin only. A total of 135 proteins were identified, showing an alteration in the expression of 85 proteins when treatment was compared with untreated bacteria (control). When the bacteria were treated using the combination, there was an increase in the expression of proteins related to resistance ( e.g ., beta-lactamase) and also in the ones involved in protein synthesis, and there was a decrease in the expression of proteins related to stress and alterations in proteins related to bacterial energy metabolism. Bacterial oxidative stress showed that the combination was able to induce oxidative stress ( p  < 0.05) and increase enzyme activities and lipid hydroperoxide levels compared with individual treatments. The analysis of cell ultrastructure showed damage in MRSA, especially on the bacterial wall and the plasma membrane, with cell lysis and death. Thus, the changes caused by these treatments affected different proteins related to the bacterial biological processes and signaling pathways such as cell division, structure, stress, regulation, bacterial resistance, protein synthesis, gene expression, energetic metabolism, and virulence. It was observed that synergism among antimicrobials has high potential in therapeutic use and may reduce the required amounts of antibacterial substances in addition to being effective on different targets in bacterial cells.

The search for new antimicrobial drugs has been necessary due to the increased bacterial resistance to antibiotics currently in use, and natural products play an important role in this field. The aim of this study was to evaluate the in vitro effect of cinnamaldehyde on S. epidermidis strains, biofilm set-up prevention, as well as its effect on pre-established biofilms. The minimum inhibitory concentration (MIC) ranged from 300 to 500 μg/mL, and the minimum bactericidal concentration (MBC) from 400 to 600 μg/mL. The biofilm inhibitory concentration and biofilm eradication concentration values were four-fold (clinical isolate) and eight-fold (ATCC strain) greater than the concentration required to inhibit planktonic growth. Sub-inhibitory concentrations of cinnamaldehyde attenuated biofilm formation of S. epidermidis strains on polystyrene microtiter plates. The combination of cinnamaldehyde and linezolid was able to inhibit S. epidermidis with a bactericidal effect. Further investigation of the mechanism of action of cinnamaldehyde revealed its effect on the cell membrane permeability, and confocal laser scanning microscopy (CLSM) images illustrated the impact of cinnamaldehyde in the detachment and killing of existing biofilms. Thereby, our data confirmed the ability of cinnamaldehyde to reduce bacterial planktonic growth of S. epidermidis, inhibiting biofilm formation and eradicating pre-formed biofilm., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

We aimed to investigate the antibacterial activities of carvacrol, thymol, eugenol, cinnamaldehyde, and lantibiotic nisin against standard bacterial strains of the milk pathogens Staphylococcus aureus ATCC 25923 and Listeria monocytogenes ATCC 15313 in cow milk. The minimum inhibitory concentrations (MIC) of these substances were recorded. The synergistic effects were also assessed in culture medium (time kill curve) and in a food model (cow milk) during the storage period (4 °C for 6 d) after inoculation with S. aureus and L. monocytogenes individually by combining nisin and the phenolic compounds at proportions of 1/4 + 1/4 the MIC (determined in a previous in vitro assay) in the culture medium and 1/4 + 1/4 of MIC in the food model. Inhibitory activities of nisin and the tested compounds, as well as synergism in the combinations, were found against both bacteria assayed. Bacteriostatic effects were found with all combinations and a significant difference in L. monocytogenes reduction was found compared with the control assays. Thus, the antibacterial activity of nisin combined with phenolic compounds was confirmed against these pathogenic bacteria that are important in the milk industry, or more broadly in food science, with potential applications for milk preservation., (Copyright © 2016 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.)