Study and development of systems to evaluate the antimicrobial effect of extracts derived from agri-food by-products

Recently, consumers have increased the demand for natural food additives for their beneficial properties as opposite to the negative health effects of the synthetic counterpart, which are associated with cancer, allergies and gastrointestinal tract disorders. As a result, food industries started researching plant and herbal extracts derived from agri-food by-products, in order to use them as food additives reducing the environmental impact. Indeed, these extracts are a rich, low-price source of bioactive compounds and their reutilization in a circular economy with the aim of zero waste approach, thanks to the valorisation of by-products, can promote the synthetic additives replacing. Moreover, these extracts show a potential antimicrobial and antioxidant power due to the presence of phenolic compounds. This research project aimed first to screen the antimicrobial effects of vegetable extracts, including tomato peel waste and olive mill wastewater derived from different agri-food by-products, by experimentally determining minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) with microtiter assays, both on Gram-positive and -negative strains (Listeria innocua, Staphylococcus xylosus and Escherichia coli). Secondly, the project aimed to set-up and develop a new assay through flow cytometric analyses specific for Listeria innocua, in which the antimicrobial effect of the phenolic compounds from olive mill wastewater was tested, to evaluate the physiological states of bacterial cells depending on the tested concentration. Meanwhile, the sub-lethal injury was valued on L. innocua to quantify the damage in cells after the end of treatment. The results obtained in this project showed that the tested tomato peel waste extracts are promising antimicrobial agents, exerting a certain inhibitory activity against both Gram-positive and Gram-negative microorganisms, which are essential for the widespread adoption of such products as alternatives to synthetic additives. The results of the second part of this work provide novel frameworks to enable the assessment of the antimicrobial properties of olive mill wastewater extract. In fact, the development of this combined method, which includes traditional microbiological methods and flow cytometer analyses, allows to observe the viability and the damage induced in bacterial cells, that cannot be evaluated in plates. Moreover, the damage provoked by the phenols of OMWW was not sub-lethal. Therefore, the presence of phenolic compounds in these extracts makes them potentially usable as natural additives in food, as they exhibit biological activities, in particular antimicrobial properties.