The increased resistance of pathogenic microorganisms is frequently attributed to the extreme and inadequate use of antibiotics and transmission of resistance within and between individuals. To counter the emergence of resistant microorganisms, considerable resources have been invested in the search for new antimicrobials. Plants synthesize a diverse array of secondary metabolites (phytochemicals) known to be involved in defense mechanisms, and in the last few years it is recognized that some of these molecules have health beneficial effects, including antimicrobial properties. In this study, the mechanism of action of gallic (GA) and ferulic (FA) acids, a hydroxybenzoic acid and a hydroxycinnamic acid, was assessed on Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Listeria monocytogenes. The targets of antimicrobial action were studied using different bacterial physiological indices: minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), membrane permeabilization, intracellular potassium release, physicochemical surface properties, and surface charge. It was found that FA and GA had antimicrobial activity against the bacteria tested with MIC of 500 μg/mL for P. aeruginosa, 1500 μg/mL for E. coli, 1750 μg/mL for S. aureus, and 2000 μg/mL for L. monocytogenes with GA; 100 μg/mL for E. coli and P. aeruginosa, 1100 μg/mL and 1250 μg/mL for S. aureus and L. monocytogenes, respectively, with FA. The MBC for E. coli was 2500 μg/mL (FA) and 5000 (GA), for S. aureus was 5000 μg/mL (FA) and 5250 μg/mL (GA), for L. monocytogenes was 5300 μg/mL (FA) and 5500 μg/mL (GA), and 500 μg/mL for P. aeruginosa, with both phytochemicals. GA and FA led to irreversible changes in membrane properties (charge, intra and extracellular permeability, and physicochemical properties) through hydrophobicity changes, decrease of negative surface charge, and occurrence of local rupture or pore formation in the cell membranes with consequent leakage of essential intracellular constituents. The overall study emphasizes the potential of plant-derived molecules as a green and sustainable source of new broad spectrum antimicrobial products.

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Abstract:The majority of current infectious diseases are almost untreatable by conventional antibiotic therapy given the advent of multidrug-resistant bacteria. The degree of severity and the persistence of infections are worsened when microorganisms form biofilms. Therefore, efforts are being applied to develop new drugs not as vulnerable as the current ones to bacterial resistance mechanisms, and also able to target bacteria in biofilms. Natural products, especially those obtained from plants, have proven to be outstanding compounds with unique properties, making them perfect candidates for these much-needed therapeutics. This review presents the current knowledge on the potentialities of plant products as antibiotic adjuvants to restore the therapeutic activity of drugs. Further, the difficulties associated with the use of the existing antibiotics in the treatment of biofilm-related infections are described. To counteract the biofilm resistance problems, innovative strategies are suggested based on literature data. Among the proposed strategies, the use of phytochemicals to inhibit or eradicate biofilms is highlighted. An overview on the use of phytochemicals to interfere with bacterial quorum sensing (QS) signaling pathways and underlying phenotypes is provided. The use of phytochemicals as chelating agents and efflux pump inhibitors is also reviewed.

Aims:  The aim of the study was to evaluate the in vitro antibacterial activity of glucosinolates and their enzymatic hydrolysis product against bacteria isolated from the human intestinal tract. Methods and results:  Using a disc diffusion bioassay, different doses of intact glucosinolates and their corresponding hydrolysis products were tested. There were clear structure–activity and concentration differences with respect to the in vitro growth inhibition effects as well as differences in the sensitivities of the individual bacteria. The most effective glucosinolate hydrolysis products were the isothiocyanates; sulforaphane and benzyl isothiocyanate were the best inhibitors of growth. Indole‐3‐carbinol had some inhibitory effects against the Gram‐positive bacteria but had no effect, even at the highest dose, against the Gram‐negative bacteria. Indole‐3‐acetonitrile had some inhibitory activity against the Gram‐negative bacteria. Glucosinolates, nitriles and amines were ineffective at all the doses used. Conclusions:  Glucosinolate hydrolysis products and specifically the isothiocyanates SFN and BITC have significant antimicrobial activity against Gram‐positive and Gram‐negative bacteria, and might be useful in controlling human pathogens through the diet. Significance and Impact of the Study:  This the first major in vitro study demonstrating the potential of these natural dietary chemicals as an alternative to, or in combination with, current antibiotic‐based therapies for treating infectious diseases.

Bovine mastitis (BM) is a disease with high incidence worldwide and one of the most relevant bovine pathologies and the most costly to the dairy industry. BM is an inflammation of the udder and represents one of the most difficult veterinary diseases to control. Biofilm formation is considered a selective advantage for pathogens causing mastitis, facilitating bacterial persistence in the udder. In fact, recently some authors drew attention to the biofilm formation ability presented by several mastitis causing pathogens and to its possible relation with recurrent mastitis infections and with the increased resistance to antimicrobial agents and host immune defence system. Actually, up to now, several researchers reported the potential role of cells in this mode of growth in the previous facts mentioned. As a consequence of the presence of biofilms, the infection here focused is more difficult to treat and eradicate, making this problem a more relevant pressing issue. Thus, we believe that a deeper knowledge of these structures in mastitis can help to determine the best control strategy to be used in veterinary practice in order to reduce losses in the dairy industry and to ensure milk safety and quality. The aim of this paper was to review the existing research and consequently to provide an overview of the role of biofilms in BM infections.

Quorum sensing (QS) is an important regulatory mechanism in biofilm formation and differentiation. Interference with QS can affect biofilm development and antimicrobial susceptibility. This study evaluates the potential of selected phytochemical products to inhibit QS. Three isothiocyanates (allylisothiocyanate - AITC, benzylisothiocyanate - BITC and 2-phenylethylisothiocyanate - PEITC) and six phenolic products (gallic acid - GA, ferulic acid - FA, caffeic acid - CA, phloridzin - PHL, (-) epicatechin - EPI and oleuropein glucoside - OG) were tested. A disc diffusion assay based on pigment inhibition in Chromobacterium violaceum CV12472 was performed. In addition, the mechanisms of QS inhibition (QSI) based on the modulation of N-acyl homoserine lactone (AHLs) activity and synthesis by the phytochemicals were investigated. The cytotoxicity of each product was tested on a cell line of mouse lung fibroblasts. AITC, BITC and PEITC demonstrated a capacity for QSI by modulation of AHL activity and synthesis, interfering the with QS systems of C. violaceum CviI/CviR homologs of LuxI/LuxR systems. The cytotoxic assays demonstrated low effects on the metabolic viability of the fibroblast cell line only for FA, PHL and EPI.